Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho

Guy W. Adema Roy M. Breckenridge Kenneth F. Sprenke

Idaho Geological Survey University of Idaho Technical Report 07-2 Moscow, Idaho 83844-3014 ISBN 1-55756-514-6 Contents

Abstract ...... 1 Introduction ...... 1 Geologic Setting ...... 2 Basement Rocks ...... 2 Tertiary Geology ...... 3 Quaternary Geology ...... 4 Previous Geophysical Investigations ...... 7 Seismic Surveys ...... 7 Gravity Surveys ...... 9 Gravity Data Collection ...... 9 Purves Data ...... 9 Cady and Meyer Data ...... 21 New Observations ...... 21 Gravity Data Reduction ...... 23 Instrument Calibration Correction ...... 23 Tidal and Instrument Drift Corrections ...... 23 Terrain Corrections ...... 24 Latitude and Elevation Corrections ...... 25 Bouguer Corrections ...... 25 Correlation of Data Sets ...... 25 Removal of Regional Trend ...... 26 Gravity Data Modeling ...... 27 Gravity Model Interpretation ...... 30 Conclusion ...... 34 Acknowledgments ...... 34 References ...... 34

Figures

Figure 1. Location of the Rathdrum Prairie ...... 2 Figure 2 Maximum extent of the Cordilleran ice sheet ...... 4 Figure 3. Maximum terminal extent of the Lake Pend Oreille lobe ...... 5 Figure 4. Locations of geophysical work performed on the Rathdrum Prairie ...... 7 Figure 5. Seismic reflection profile...... 8 Figure 6. Locations of eight gravity measurements ...... 26 Figure 7. Bouguer anomaly map of the Rathdrum Prairie ...... 27 Figure 8. Bouguer gravity map showing regional trend ...... 28 Figure 9. Residual Bouguer gravity map of the Rathdrum Prairie ...... 29 Figure 10a. The Idaho Road (Washington) profile...... 30 Figure 10b. The Corbin Road profile...... 31 Figure 10c. The Idaho Road (Idaho) profile...... 31 Figure 10d. The Idaho Highway 41 profile...... 32 Figure 10e. The Hayden Avenue profile...... 32

Tables

Table 1. Principal Gravity Station Data ...... 10-20 Table 2. Principal Facts for Primary Benchmark ...... 22-23 Table 3. Digital Elevation Models ...... 25 Table 4. Modeled Aquifer Characteristics ...... 33

iii Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho

Guy W. Adema Roy M. Breckenridge1 Kenneth F. Sprenke

ABSTRACT the Coeur d’Alene Mountains to the east. The aquifer is identified as the sole-source water supply for the greater The Rathdrum Prairie overlies part of a regional Coeur d’Alene and Spokane metropolitan areas. Our ground-water source, known as the Rathdrum Prairie- study will focus on the 615-square-km part in Idaho, the Spokane Valley aquifer, that covers 1,055 square km in Rathdrum Prairie aquifer. Spokane County, Washington, and Kootenai and Bonner counties, Idaho. The aquifer is considered a sole-source In 1978, the aquifer was designated “sole source,” water supply for the greater Coeur d’Alene and Spokane which qualified it for protection under the 1974 metropolitan areas. The 615-square-km part in Idaho is Federal Safe Drinking Water Act. Since then, the called the Rathdrum Prairie aquifer. It extends from Lake area’s rapid growth in both population and irrigation Pend Oreille south and west to the Idaho-Washington demand continues to strain the aquifer’s capability. state line. The aquifer occupies a glacially scoured Consequently, to answer civic concerns about the aquifer trough filled with highly permeable, coarse-grained, requires reconstructing the broader geologic history of catastrophically deposited glacial outwash. the region. This must begin with a clear understanding of the subsurface geology of the Rathdrum Prairie; Five geologic cross-sections of the valley have been however, developing an accurate predictive model of created using 630 gravity measurements, 146 of which its structure has been difficult because of its complex were collected specifically for our study to complement history. Nonetheless, knowing what the limits are for existing data. The data were modeled in 2¾ dimensions, this critical aquifer depends on scientific analysis that and the regional trend caused by crustal thickening measures its actual extent, recharge potential, and future to the east has been removed. The models present a expectations. Determining the depth and geometry of generally smooth valley floor with an incised channel the bedrock surface is the first step. Our study brings in the ancestral, subsurface valley in the western half together new geophysical information and an improved of the prairie. The bedrock-sediment interface appears interpretation technique to more fully analyze the basin to be slightly sloped to the east with the deepest point geometry and its influence on hydrologic assumptions. between Idaho Road (Idaho) and Hayden Avenue where sediments may extend over 350 m below the surface. The Rathdrum Prairie aquifer is primarily composed Westward, near the state line, the sediments appear to of valley-fill deposits of glaciofluvial origin. Extensive thin to a thickness of 216 m. gravels, deposited by catastrophic floods from Glacial Lake Missoula, fill the ancestral valley of the Rathdrum INTRODUCTION River (Breckenridge, 1989). The Missoula Floods, as these events are known today, were caused by the periodic The Rathdrum Prairie-Spokane Valley aquifer (Figure 1) impoundments and sudden releases of water from Glacial is a valley-fill aquifer that extends from the southern end Lake Missoula, which was reestablished many times of Lake Pend Oreille, south to Coeur d’Alene Lake, and during the Pleistocene when the Pend Oreille lobe of the west to Spokane. The valley is bound by Mount Spokane Cordilleran ice sheet repeatedly blocked the Clark Fork to the north, the Mica Peak uplands to the south, and River (Bretz and others, 1956; Bretz, 1959). The latest 1Idaho Geological Survey, University of Idaho, Moscow, ID 83844 2Department of Geological Sciences, University of Idaho, Moscow, ID 83844 Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Adema, Breckenridge, and Sprenke Adema, Breckenridge, and Sprenke

Figure 1. Location of the Rathdrum Prairie, Idaho (from Wyman, 1994). periods of lake-emptying cycles occurred from 12,000 to These new data target coverage to specific profiles and 17,000 years ago (Waitt and Atwater, 1989). Proglacial validate the previous work. The combined data set was and flood processes deposited clasts with lithologies reduced and modeled with techniques not applied in the derived from local terranes that include the Precambrian earlier studies. Belt Series, the Cretaceous and Tertiary plutons and The thickening of the continental crust beneath the associated rocks, the lower Paleozoic sediments, and the northern Rocky Mountains (Winston and others, 1989; Miocene basalt (Breckenridge, 1997). Some evidence Harrison and others, 1972) east of the Rathdrum Prairie suggests the aquifer overlies the Latah Formation, which imposes a regional trend on gravity data. The regional is characterized by thick units of shale and clay with gradient is steep and obscures near-surface contributions some sands and gravels (Newcomb and others, 1953; to the gravity field. The effects of this gradient were Pardee and Bryan, 1926). The Latah beds are thought not incorporated in the most extensive previous gravity to lie unconformably over pre-Tertiary sediments, survey of the prairie (Purves, 1969). The models metasediments, and granitic rocks. Intercalated with the developed in our investigation account for the regional Latah Formation, and near the margins of the aquifer, trend and, as a result, more accurately define the basin lies Miocene basalt of the Columbia River Basalt Group geometry. (Breckenridge and Othberg, 1998a and 1998b). GEOLOGIC SETTING Our study employs gravity techniques to determine the subsurface geology. Previous gravity surveys (Purves, BASEMENT ROCKS 1969; Hammond, 1974; Cady and Meyer, 1976a) and seismic refraction and reflection studies (Newcomb and The country rocks surrounding, and presumably others, 1973; Gerstel and Palmer, 1994) have focused on underlying, the Rathdrum Prairie are mostly relatively small sections of the aquifer and do not produce Precambrian, but intrusions as young as Eocene are also an overall subsurface view of the Rathdrum Prairie. Our present. These pre-Miocene igneous and metamorphic study measured the gravity at 146 new locations on the units are described in Lewis and others (2002). The prairie to complement the same number of existing ones. units include low-grade metasedimentary rocks of the

Belt Supergroup and high-grade (amphibolite facies) Clark Fork Valley, and perhaps into Montana (Conners, metamorphic rocks whose protolith is either the Belt 1976). Supergroup or the basement rocks that predate the Belt metasedimentary rocks. The high-grade rocks are Younger Miocene basalt flows eventually overrode exposed in the Priest River metamorphic core complex the entire Rathdrum Prairie region. The separate west of the Rathdrum Prairie. Deformed granitic rock events and long interludes created alternating layers of (orthogneiss) of probable Cretaceous age is included basalt and Latah Formation interbeds, as observed by in the core complex. Plutonic rocks of Cretaceous age Hammond (1974) in well sections. The Latah sediments are also present as intrusions within the low-grade Belt were deposited in lakes formed where the basalt flows Supergroup. Relatively undeformed Eocene igneous impounded westward-flowing drainage systems. Kiver rocks are exposed as plutons northwest of the Rathdrum and Stradling (1989) suggest that basalt and Latah Prairie. A few Eocene rhyolite and dacite dikes are also sediments filled the valley to a present-day elevation present. of 730 m. Breckenridge and Othberg (1998a, 1998b) delineate where these younger basalt flows filled the The pre-Miocene igneous and metamorphic units embayments of the region. The increasing elevation of generally act as an aquiclude and constitute the basalt flows in the Miocene caused lake levels to the east impervious basement to the Rathdrum Prairie aquifer. to rise accordingly. Eventually, levels were high enough Because of their complexity and local variation, the to force the drainage pattern to alter from a westerly pre-Miocene rocks are treated as one unit for gravity to a northerly direction along the margin of the basalt modeling purposes. Differentiating the various host (Connors, 1976; Savage, 1965). The new drainage flowed rocks and intrusions and assigning unique physical north along the Purcell Trench through what is now Lake properties to each require more comprehensive data than Pend Oreille and then westward to the Columbia River. are available. The Latah sediments of the Rathdrum-Spokane valley may have been completely covered by one or more TERTIARY GEOLOGY basalt flows during the final extrusions of the Columbia River basalt. The rocks of Tertiary age include the Latah Formation and the Columbia River Basalt Group. These rocks were The downcutting from the late Miocene to the deposited on a mature erosional surface that developed early Pleistocene removed as much as 180 m of Latah during the Late Cretaceous and Early Tertiary. The sediments (Anderson, 1927). Developing drainages geography included ridge crests and mountain tops with probably eroded much of the exposed Latah beds and gently rounded forms (Molenaar, 1988) and rugged some of the marginal basalt. The Coeur d’Alene drainage canyons with depths exceeding 600 m (Connors, 1976). developed along the basalt marginal valley now occupied The west-flowing, Early Tertiary, ancestral Rathdrum by Coeur d’Alene Lake. Those sediments, protected River occupied a valley under the present-day Spokane by overlying basalt flows, were largely unaffected by Valley and Rathdrum Prairie (Savage, 1967). this erosional cycle, but a substantial amount of basalt may have been eroded during this time. The thickness During the Miocene, flows of the Columbia River of remaining Latah sediments is difficult to estimate Basalt Group spread northeastward from the Columbia owing to the cover of Pleistocene drift and the scarcity Plateau and filled these deep canyons. The basalt of evidence from boreholes. So far, researchers have not dammed drainageways, including the Rathdrum River, determined the extent of the Latah sediments. Anderson and these dams allowed lakes to form in which the (1940) discovered a 300-m-thick bed of Latah Formation sand, silt, and clay beds of the Latah Formation were below an exposed basalt flow in a well west of Hayden deposited. The Latah sediments consist predominantly Lake. Similarly, seismic velocities, intermediate to those of white, yellow, orange, and brown lacustrine silt and of bedrock and gravels, were interpreted by Newcomb clay, along with some fluvially deposited sand and gravel (1953) to represent Latah Formation near the state line. units (McKiness, 1988). The older basalt flows did not He estimated the thickness of these sediments to be extend to the eastern and northern Rathdrum Prairie, and 300 m. a relatively thick section of Latah Formation is believed to have formed at these locations, as evidenced in three The Missoula Floods eroded much of the Columbia water-well logs of Hammond (1974). Pardee and Bryan River Basalt Group and Latah Formation, significantly (1926) suggest that over 500 m of Latah sediments altering the geologic features that had developed accumulated in the Spokane area. Such deposits probably during the Miocene. Many Tertiary geologic and extended throughout the Lake Pend Oreille basin, up the geomorphic features were either destroyed or obscured.

QUATERNARY GEOLOGY exposure, and removed large amounts of rock material from the valleys extending north of the main Rathdrum The glacial and interglacial periods began between Prairie (Conners, 1976; Savage, 1964). 2 Ma and 3 Ma in this region. The Cordilleran glacier complex was the dominant geomorphic force during the Determining the southernmost extents to the various Pleistocene in North America. The epoch began with lobes of the Cordilleran ice sheet is difficult owing to high alpine glaciers in the Cascades, Coast Range, and the lack of exposed glacial and proglacial features. northern Rocky Mountains. As these glaciers coalesced Richmond (1986) has compiled the stratigraphy and in major valleys, the thickening ice eventually formed chronology of well-documented glacial deposits. The a massive sheet at least 2,200 m thick, over 3,400 km current consensus on the southernmost margin of long, and over 800 km wide (Conners, 1976). A series the ice sheet is shown on Figure 2, as summarized in of massive glacial lobes occupied the major north-south Richmond (1986) and modified by Breckenridge (1989). trending valleys in northern Idaho and produced the The locations shown are revisions of previous estimates primary landscaping event of the period. Erosion by the taken from numerous studies of Quaternary deposits advancing ice lobes deepened valleys, smoothed bedrock throughout the northwest.

Figure 2. Maximum extent of the Cordilleran ice sheet (Breckenridge, 1989). BRL = Bull River lobe; FL = Flathead lobe; PTL = Purcell Trench lobe; PRL = Priest River lobe; TRL = Thompson River lobe. Crosshatched area = Glacial Lake Missoula. Arrows show routes of major flood outbursts.

Of interest to our study is the Lake Pend Oreille lobe, Oscillations of the Lake Pend Oreille lobe significantly sometimes referred to as the Rathdrum lobe. This tongue affected the formation of the Rathdrum Prairie-Spokane of the Cordilleran ice sheet extended southward between Valley aquifer. Most sediments in the valley are thought the Selkirk Range and Cabinet Mountains. Flint (1937) to have been deposited catastrophically. With each and Bretz and others (1956) originally mapped the Lake major advance of the lobe in late Wisconsin time, an Pend Oreille lobe well onto the Columbia Plateau. Weis ice dam formed at Clark Fork and impounded the Clark and Richmond (1965) extended it through the Rathdrum Fork River to create another glacial Lake Missoula. Prairie-Spokane Valley to the present location of The ice dams eventually became unstable, producing Spokane. A more refined estimate of the southern extent periodic failures and recurrent, sudden flooding of the of the lobe is shown in Figure 3, taken from Breckenridge downstream reaches. With each failure, as much as 2.1 (1989) who mapped it as extending only to the present x 107 m3sec-1 of water was released (Baker, 1973). These southern shore of Lake Pend Oreille. events are now referred to as the Missoula Floods. The

Figure 3. Maximum terminal extent of the Lake Pend Oreille lobe of the Cordilleran ice sheet (from Breckenridge, 1989).

Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Adema, Breckenridge, and Sprenke Adema, Breckenridge, and Sprenke water released from Lake Missoula flowed forcefully 1998c). The most recent flood events are thought to have across the Rathdrum Prairie, scoured the region now occurred between 17,200 and 11,000 years ago (Waitt, known as the Channeled Scabland, and continued down 1985). The number of flooding episodes is unknown, the Columbia River to the Willamette Valley and the but at least fifty have been associated with depositional Pacific Ocean. rythmites (Waitt, 1980). Other researchers have found evidence for numerous episodes in other localities Evidence for the Missoula Floods was introduced by (Sieja, 1959; Chambers, 1971, 1984; Breckenridge and J Harlan Bretz in the 1920s (Bretz: 1923, 1925, 1928a, Othberg, 1998c). 1928b, 1928c, 1930a, 1930b, 1932). Bretz identified erosional and depositional features in the Channeled Several theories compete to explain the catastrophic Scabland. For him, the only explanation for these unique ice dam failures and have been summarized by landforms was a relatively brief, but enormous flood, Breckenridge (1989). Those theories include jökulhlaup which he called the Spokane Flood. Bretz investigated releases (Waitt, 1985), flotation of the ice (Thorarinsson, such examples as erratic boulders, deeply notched cliffs, 1939), the enlargement of subglacial tunnels by water streamlined loess hills, gravel deposits, and the Portland (Liestol, 1956), the deformation of ice by water pressure delta. (Glenn, 1954), and the enlargement of tunnels by icebergs (Aitkenhead, 1960). Whatever the mechanism of ice- Scientists were slow to accept his thesis. For years, dam failure, the geomorphic expression of the Missoula most denounced the idea as outrageous and physically Floods has been well documented and serves as the most impossible. Prominent among the old guard attacking recent significant event affecting the composition of the this perceived heresy was R.F. Flint (1935, 1936, 1938) Rathdrum Prairie-Spokane Valley aquifer. who, in rebuffing Bretz, decreed the scablands merely the result of normal proglacial discharge. The controversy The widely accepted geologic model of the Rathdrum finally prodded J.T. Pardee to show Bretz his own Prairie has the ancestral Rathdrum River valley first work on a large Pleistocene lake in western Montana. being filled with Miocene basalt and Latah sediments Pardee (1910) had been studying this evidence of a flood and then with Missoula Flood deposits. The Missoula source, which he named Lake Missoula, before Bretz Floods eroded much of the basalt and Latah sediments presented the idea of massive floods. Finally, Pardee and also obscured their exposures with extensive flood (1942) published his work on glacial Lake Missoula. deposits. These flood deposits are principally composed The paper strongly complemented Bretz’s great flood of fine to coarse gravels of glaciofluvial origin derived hypothesis and represents a much-cited example of how from glacial outwash of the Purcell Trench lobe. The the collaboration of evidence advances ideas in science. provenance of the gravels includes Precambrian Belt Both men agreed that the Lake Pend Oreille lobe of the Supergroup rocks, Tertiary and Cretaceous plutons, lower Wisconsin ice sheet had impounded a huge lake and that Paleozoic sediments, and Miocene basalt (Breckenridge the lake had drained catastrophically. Later, this idea and others, 1997). would be enlarged to include several episodes of ice dams and floods. Coarser gravels are found centrally in the valley, and finer sands and gravels are near the margins. Some of the Evidence described by Pardee (1942) included sands and gravels are classified as eddy and pendant bar severely scoured constrictions in the lake basin, huge bars deposits (Breckenridge and Othberg, 1998a and 1998b). of current-transported debris, and giant ripple marks with The high-energy depositional environment resulted in heights of 16 m and spacings of 160 m. Bretz and others cross-bedded gravel deposits with intercalated layers (1956) and Bretz (1959) provided revisions to his original of finer sands and clays. Discriminating these sand work that suggested several flood episodes. Chambers and gravel layers with gravity methods is impractical, (1971, 1984) followed with detailed descriptions of the but their existence is visible in local gravel pits. The sedimentation cycles of the rhythmically bedded floor gravel structure is further complicated by the occasional sediments in the Clark Fork valley. He interpreted them occurrence of clast cementation (Breckenridge and as evidence of multiple filling and flooding sequences. others, 1997). A primarily calcium carbonate cement varies in development, from minimal clast rinds to Present work is looking at whether sedimentary near complete matrix filling. The cement was found variations between rythmite beds represent distinct to have fine amounts of angular silica, perhaps carried flood events or are merely products of different energy downward by water infiltrating through surface ash. The levels from a single event, as well as at the timing of coatings, which must have originally formed in the zone each depositional feature (Breckenridge and Othberg, of a fluctuating water table, are now found in the vadose

zone. The cemented gravels and intercalated fine beds from Cady (1976) and Purves (1969) that complemented provide complicated hydrologic characteristics in the ours. Results from other geophysical investigations also aquifer that may adversely affect hydrologic modeling. helped in clarifying the subsurface conditions and in developing the model. The gravels are mantled with thin volcanic ash and loess deposits. The distribution of these eolian sediments SEISMIC SURVEYS varies greatly and is difficult to quantify. Deposits seldom reach a meter in thickness and in most places only a few Newcomb and others (1953) completed two seismic centimeters (R.M. Breckenridge and K.L. Othberg, oral refraction surveys near Spokane, Washington, in May commun., 1998). Some of the material is thought to and June of 1951 to locate the base of the glacial outwash percolate into the matrix of the underlying gravels. The aquifer. This geophysical investigation of the Rathdrum surficial geologic mapping of Breckenridge and Othberg Prairie aquifer obtained stratigraphic and hydrologic (1998a and 1998b) did not include ash and loess. information about the Spokane Valley. The study also sought to determine the type of material underlying the PREVIOUS GEOPHYSICAL aquifer and to locate the bedrock of the ancestral valley. INVESTIGATIONS One survey trended north-south across the Spokane Valley east of the Idaho-Washington border; the other Geophysical data from two previous studies have trended east-west across the Hillyard trough, north of been included in our research to create a more complete Spokane. The survey near the Washington-Idaho border model of the subsurface geology of the Rathdrum (Figure 4) provided the primary bedrock “ties” for the Prairie. We incorporated 484 gravity measurements gravity modeling in our report.

Figure 4. Locations of geophysical work performed on the Rathdrum Prairie.

Newcomb and others (1953) based their interpretation their time in the amount of data that could be obtained on limited reverse seismic refraction data. Their few and interpreted. The geologic interfaces shown in their refractions indicated a V-shaped valley with a maximum model were based on only a few data points. aquifer thickness of about 480 m where glacial outwash contacts the Latah Formation. The contact corresponds to Seismic reflections on the Rathdrum Prairie by Gerstel an elevation of about 530 m and occurs with the inference and Palmer (1994) followed the lines of Newcomb and of Pardee and Bryan (1926) about bedrock depth in this others (1953) on Idaho Road (Washington) so that results region. Newcomb and others (1953) interpreted five of the two studies could be compared (Figure 4). (Because subsurface units from their refractions: (1) soil and subsoil Idaho Road lies in both Washington and Idaho, its of the glacial outwash, (2) unsaturated glacial outwash, location will be identified with the state name following (3) saturated glacial outwash, (4) Latah Formation with in parentheses.) Their unconventional technique used intercalated igneous rocks, and (5) granitic rock. The a single point, zero-offset shot-receiver procedure and velocity obtained for the Latah Formation was so high a pneumatic acoustic source. They claimed to find that Newcomb and others (1953) reasonably presumed an undulating bedrock surface 160 m deep, with an that basalt sills and dikes were present. These sills and average bedrock-sediment interface at about 475 m dikes have also been found in the Latah Creek vicinity above sea level. Their interpretation revealed a much (Pardee and Bryan, 1925) and in the highway cuts more U-shaped valley floor than previously thought southeast of Coeur d’Alene (Conners, 1976). Newcomb (Figure 5). The velocities used for reflection processing and others (1953) were limited by the technology of were obtained from cross-hole seismic techniques and

Figure 5. Seismic reflection profile (top) and interpretation (bottom) of Gerstel and Palmer (1994). An unconventional single point receiver technique, with a pneumatic acoustic source, was used.

were unavailable for us to compare with the refraction of the aquifer, he claimed that the basement configuration velocities measured by Newcomb and others (1953). appeared to be a fluvially dissected erosional valley with erosional terraces composed of either a complex GRAVITY SURVEYS basement or basalt.

Bonini (1963) conducted the first regional gravity Purves (1969) never numerically modeled his data study of Idaho and produced a reconnaissance Bouguer because of the difficulty in running computations before anomaly map of the state using over 1,200 observations. the common use of computers. Instead, he relied on the The study was part of one on gravity anomalies, isostatic simple supposition that, in a relatively simple geologic equilibrium, and tectonic features of the northwestern system, the Bouguer gravity field will generally mimic United States. Bonini correlated the Idaho batholith with the underlying bedrock shape. The data from his study negative Bouguer anomalies (as low as -236 mGals), were used extensively in our study and provide the the Snake River Plain with a broad gravity high, and effective starting point for our investigation. the Columbia River Plateau as a relative gravity high. In northern Idaho and western Montana, he identified a In 1969, Hammond (1974) began a gravity survey modest variation in the Bouguer anomaly that generally and subsequent remodeling of the northern Rathdrum followed a north-south trend. Bonini concluded that the Prairie. This U.S. Geological Survey study, undertaken pattern reflected the broad structural fabric and variations in cooperation with the then Idaho Department of Water in density values of different members of the Belt Administration, evaluated previous estimates of (1) the Supergroup rocks. The survey was not detailed enough quantity of underflow moving toward the Rathdrum to identify individual features or units that the gravity Prairie from the Athol area across a line extending about variations were attributed to. Though no subsurface 10 km northwestward from Chilco and (2) the quantity geological modeling was performed, Bonini (1963) laid of water being recharged to the aquifer by Lake Pend the groundwork for future gravity work in Idaho. Oreille. A detailed gravity survey was conducted to define the configuration of the bedrock surface andto Purves (1969) performed an extensive gravity survey calculate the thickness of fill from the southern end of of the Spokane Valley-Rathdrum Prairie to identify Lake Pend Oreille south to the three Chilco channels, subsurface stratigraphic units, possible areas of underflow which were the focus of the study. Unfortunately, details impedance, and glacial conditions. Over 743 gravity about the gravity data reduction and modeling were not measurements were taken on 16 profiles in Idaho and included in the report. Depth and Bouguer anomaly Washington. All standard corrections, including terrain, contours produced by Hammond (1974) only overlap were made, and measurements were tied to the extended the very northern section of the region encompassed by gravity control network of North America established our study, but show a trend that continues to Lake Pend by Wollard and Behrendt (1961). The survey by Purves Oreille. (1969) provides the starting point for our study, and selected data from that work are modeled with ours. GRAVITY DATA COLLECTION

A significant finding of Purves (1969) was the Our investigation combines gravity data from three existence of a probable west-northwest trending main sources: Purves (1969); Cady and Meyer (1976), subsurface drainage divide within the Rathdrum Prairie which includes the data of Bonini (1963) and Hammond basin about 3.2 km west of the Washington-Idaho state (1974); and our own measurements taken in 1997. Data line. He suggests that this drainage divide was produced collected for our study was carefully planned to fill by the terminal position of the maximum glacial advance gaps in the coverage of previous studies. The combined of a proposed Hayden Lake lobe and the recognized Pend result is an extensive data set that allows for more Oreille lobe. He claimed that the subsurface configuration comprehensive modeling and interpretation. The three east of the divide, in the Rathdrum Prairie, appeared to data sets are assimilated as a final numerical adjustment. be dominantly influenced by glacial erosive processes, Principal facts for all gravity data are given in Table 1. with the U-shaped trough filled almost exclusively with glacial material and minimal basalt. He cites irregular PURVES DATA geohydrological evidence near the proposed flow divide that suggests the ultimate glacial terminus existed west In 1966, Purves (1969) collected over 743 gravity of this divide. His survey was inconclusive about the measurements on transects of the Rathdrum Prairie and existence of any Latah Formation in the Rathdrum Prairie Spokane Valley. Of these, 276 stations on 5 profiles were section. West of this divide, in the Spokane Valley section used in our study (Figure 4). Though Purves did the

Table 1. Principal Gravity Station Data.

Field descriptions:

Count Arbitrarily assigned. Point_ID Descriptive identification for each point. Alpha prefix describes data source. A = Adema, P = Purves (1969), D = Cady and Meyer (1976a) DD_lat Degree-decimal latitude, WGS84 geiod. DD_long Degree-decimal longitude, WGS84 geiod. Easting UTM zone 11 easting value, presented in meters. Northing UTM zone 11 northing value, presented in meters. Elevation Elevation above MSL, presented in meters. Obs_Grav Observed station gravity, presented in mGal. Corrected only for drift. C.B.A._1 Bouguer Anomaly, ρ=2.67 kg/m3. C.B.A._2 Residual Bouguer anomaly, ρ=2.67 kg/m3, regional trend correction of 0.11 mGal per km east.

Count Point_ID DD_lat DD_long Easting Northing Elevation Obs Grav C.B.A. 1 C.B.A. 2 1 AB285 47.8022 -116.9156 506,323.36 5,294,103.18 665.1 980,649.10 -92.38 -4.42 2 AG601 47.6770 -116.7979 515,179.26 5,280,195.86 651.7 980,632.59 -100.83 -3.13 3 AG602 47.6773 -116.7914 515,658.67 5,280,227.97 651.0 980,631.27 -102.33 -4.10 4 AG603 47.6773 -116.7881 515,908.83 5,280,228.65 659.1 980,629.50 -102.51 -4.01 5 AG604 47.6773 -116.7805 516,471.92 5,280,230.43 667.8 980,627.90 -102.42 -3.30 6 AG605 47.6772 -116.7762 516,805.57 5,280,231.24 668.7 980,627.50 -102.66 -3.18 7 AG606 47.6772 -116.7723 517,097.47 5,280,232.29 669.0 980,627.70 -102.38 -2.57 8 AG607 47.6772 -116.7673 517,472.87 5,280,233.17 665.5 980,628.60 -102.13 -1.90 9 AG608 47.6772 -116.7618 517,889.71 5,280,234.41 665.3 980,629.14 -101.59 -0.91 10 AG609 47.6769 -116.7567 518,265.29 5,280,204.81 662.4 980,630.56 -100.65 0.45 11 AH101 47.7593 -116.9584 503,122.67 5,289,315.06 659.9 980,643.52 -95.81 -11.38 12 AH102 47.7593 -116.9533 503,497.46 5,289,315.33 661.7 980,642.02 -96.97 -12.12 13 AH103 47.7594 -116.9479 503,913.80 5,289,346.43 662.0 980,641.51 -97.44 -12.13 14 AH104 47.7593 -116.9428 504,288.48 5,289,315.92 662.4 980,641.16 -97.72 -12.00 15 AH105 47.7593 -116.9370 504,725.73 5,289,316.28 661.1 980,641.81 -97.33 -11.13 16 AH106 47.7593 -116.9316 505,121.23 5,289,316.57 667.5 980,640.33 -97.56 -10.93 17 AH107 47.7592 -116.9264 505,516.74 5,289,316.87 675.3 980,637.82 -98.55 -11.48 18 AH108 47.7592 -116.9211 505,912.25 5,289,317.46 679.4 980,636.44 -99.15 -11.64 19 AH109 47.7592 -116.9159 506,307.76 5,289,317.76 684.4 980,635.13 -99.47 -11.53 20 AH110 47.7592 -116.9128 506,536.89 5,289,318.11 682.2 980,635.39 -99.65 -11.46 21 AH111 47.7592 -116.9075 506,932.40 5,289,318.40 679.7 980,635.15 -100.40 -11.78 22 AH112 47.7591 -116.9038 507,202.98 5,289,318.81 680.9 980,634.33 -100.98 -12.06 23 AH113 47.7590 -116.9007 507,432.00 5,289,288.39 683.0 980,633.45 -101.45 -12.28 24 AH114 47.7590 -116.8930 508,014.91 5,289,288.96 685.9 980,632.25 -102.08 -12.26 25 AH115 47.7590 -116.8885 508,347.95 5,289,289.47 688.5 980,631.63 -102.16 -11.98 26 AH116 47.7590 -116.8839 508,702.01 5,289,290.01 689.6 980,631.19 -102.41 -11.84 27 AH117 47.7589 -116.8788 509,076.50 5,289,290.57 688.4 980,630.92 -102.93 -11.94 28 AH118 47.7589 -116.8722 509,576.21 5,289,291.33 687.3 980,630.57 -103.49 -11.95 29 AH119 47.7589 -116.8669 509,971.72 5,289,291.93 686.6 980,630.34 -103.86 -11.89 30 AH120 47.7590 -116.8621 510,325.78 5,289,292.78 687.5 980,629.51 -104.53 -12.17 31 AH121 47.7590 -116.8564 510,763.04 5,289,293.44 689.5 980,628.31 -105.33 -12.49 32 AH122 47.7590 -116.8506 511,199.98 5,289,294.40 692.2 980,627.49 -105.64 -12.32 33 AH123 47.7590 -116.8441 511,678.98 5,289,295.13 692.9 980,627.21 -105.77 -11.92 34 AH124 47.7590 -116.8390 512,074.48 5,289,296.04 697.5 980,626.04 -106.04 -11.76 35 AH125 47.7589 -116.8334 512,490.71 5,289,296.97 697.6 980,625.15 -106.90 -12.16 36 AH126 47.7589 -116.8295 512,782.31 5,289,297.41 699.2 980,624.51 -107.22 -12.16 37 AH127 47.7589 -116.8257 513,052.89 5,289,298.13 701.0 980,623.76 -107.62 -12.26 38 AH128 47.7588 -116.8213 513,385.93 5,289,298.94 702.0 980,623.47 -107.70 -11.98 39 AH129 47.7588 -116.8172 513,698.25 5,289,299.72 702.0 980,623.40 -107.77 -11.70 40 AH130 47.7587 -116.8133 513,989.74 5,289,269.38 701.0 980,623.48 -107.88 -11.49 41 AH131 47.7587 -116.8085 514,343.81 5,289,270.23 700.4 980,623.20 -108.26 -11.48

10 11 Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Adema, Breckenridge, and Sprenke Adema, Breckenridge, and Sprenke

Count Point_ID DD_lat DD_long Easting Northing Elevation Obs Grav C.B.A. 1 C.B.A. 2 42 AH132 47.7587 -116.8050 514,614.39 5,289,270.94 700.7 980,622.81 -108.60 -11.53 43 AH133 47.7587 -116.7996 515,009.89 5,289,272.15 698.5 980,623.14 -108.68 -11.17 44 AH134 47.7587 -116.7937 515,467.86 5,289,273.15 699.3 980,622.91 -108.74 -10.72 45 AH135 47.7586 -116.7913 515,634.53 5,289,273.71 699.2 980,623.02 -108.63 -10.43 46 AH136 47.7587 -116.7884 515,863.36 5,289,274.36 697.2 980,623.54 -108.51 -10.06 47 AH137 47.7587 -116.7856 516,071.47 5,289,274.98 694.8 980,624.05 -108.46 -9.78 48 AH138 47.7587 -116.7825 516,300.61 5,289,275.63 693.2 980,624.16 -108.65 -9.72 49 AH139 47.7587 -116.7786 516,591.91 5,289,276.37 695.0 980,623.77 -108.65 -9.40 50 AH140 47.7587 -116.7754 516,841.77 5,289,277.06 693.7 980,624.17 -108.50 -8.98 51 AH141 47.7586 -116.7724 517,049.88 5,289,277.68 689.0 980,624.96 -108.58 -8.82 52 AH142 47.7587 -116.7681 517,383.22 5,289,278.80 690.3 980,624.54 -108.73 -8.61 53 AH143 47.7585 -116.7654 517,591.34 5,289,279.42 698.2 980,622.89 -108.79 -8.44 54 AH144 47.7585 -116.7630 517,757.71 5,289,279.67 699.4 980,622.93 -108.50 -7.97 55 AH145 47.7585 -116.7607 517,924.38 5,289,280.22 699.4 980,623.00 -108.42 -7.70 56 AH146 47.7586 -116.7577 518,153.52 5,289,280.88 699.3 980,623.21 -108.19 -7.23 57 AH147 47.7586 -116.7566 518,236.70 5,289,281.31 699.3 980,623.15 -108.22 -7.16 58 AM301 47.7767 -116.7781 516,628.05 5,291,283.22 700.8 980,622.32 -110.62 -11.33 59 AM302 47.7729 -116.7782 516,608.22 5,290,850.81 698.3 980,622.57 -110.51 -11.24 60 AM303 47.7681 -116.7783 516,609.69 5,290,326.11 700.2 980,622.20 -110.09 -10.82 61 AM304 47.7638 -116.7783 516,611.08 5,289,862.96 697.8 980,622.97 -109.39 -10.12 62 AM305 47.7604 -116.7784 516,612.38 5,289,461.67 698.1 980,623.26 -108.71 -9.44 63 AM306 47.7569 -116.7784 516,613.49 5,289,091.15 691.2 980,624.60 -108.39 -9.12 64 AM307 47.7544 -116.7783 516,614.02 5,288,813.26 689.5 980,624.93 -108.16 -8.88 65 AM308 47.7508 -116.7783 516,615.32 5,288,411.96 687.3 980,626.23 -106.94 -7.66 66 AN501 47.6860 -116.7937 515,489.46 5,281,215.57 669.5 980,627.95 -102.80 -4.76 67 AN502 47.6860 -116.7913 515,656.13 5,281,216.13 672.2 980,626.59 -103.68 -5.46 68 AN503 47.6860 -116.7883 515,885.27 5,281,216.78 675.4 980,625.29 -104.37 -5.90 69 AN504 47.6859 -116.7855 516,093.88 5,281,186.32 677.3 980,624.80 -104.48 -5.78 70 AN505 47.6859 -116.7811 516,427.54 5,281,187.13 676.9 980,625.58 -103.78 -4.71 71 AN506 47.6858 -116.7778 516,677.69 5,281,188.12 675.5 980,626.37 -103.26 -3.91 72 AN507 47.6859 -116.7738 516,969.60 5,281,188.87 674.8 980,627.58 -102.14 -2.48 73 AN508 47.6858 -116.7714 517,157.30 5,281,189.46 669.4 980,629.11 -101.64 -1.77 74 AN509 47.6858 -116.7686 517,365.71 5,281,190.08 666.1 980,630.07 -101.28 -1.18 75 AN510 47.6858 -116.7661 517,553.41 5,281,190.67 666.3 980,630.46 -100.81 -0.50 76 AN511 47.6859 -116.7631 517,782.55 5,281,191.32 664.8 980,630.48 -101.00 -0.44 77 AP001 47.7021 -116.9498 503,772.11 5,282,986.70 649.4 980,641.74 -93.81 -8.66 78 AP002 47.7845 -116.9369 504,723.30 5,292,125.66 660.0 980,648.02 -93.37 -7.18 79 AP004 47.7813 -116.9370 504,723.60 5,291,786.21 662.6 980,646.32 -94.35 -8.15 80 AP005 47.7784 -116.9370 504,723.91 5,291,446.47 664.1 980,644.99 -95.15 -8.96 81 AP006 47.7755 -116.9371 504,703.31 5,291,137.77 662.7 980,644.34 -95.85 -9.67 82 AP007 47.7720 -116.9371 504,703.70 5,290,736.47 668.7 980,641.75 -96.99 -10.81 83 AP008 47.7693 -116.9372 504,703.81 5,290,427.81 668.8 980,640.96 -97.53 -11.35 84 AP009 47.7655 -116.9373 504,704.20 5,290,026.21 666.5 980,640.79 -97.84 -11.67 85 AP010 47.7634 -116.9373 504,704.53 5,289,779.40 666.3 980,640.75 -97.72 -11.55 86 AP011 47.7606 -116.9373 504,704.64 5,289,470.73 661.5 980,642.05 -97.12 -10.95 87 AP012 47.7585 -116.9373 504,704.79 5,289,254.39 661.1 980,641.78 -97.28 -11.11 88 AP013 47.7557 -116.9373 504,705.20 5,288,945.72 660.0 980,641.24 -97.79 -11.62 89 AP014 47.7530 -116.9373 504,705.31 5,288,637.05 660.9 980,640.01 -98.60 -12.42 90 AP015 47.7500 -116.9373 504,705.62 5,288,297.61 668.1 980,637.45 -99.49 -13.32 91 AP016 47.7471 -116.9373 504,706.04 5,287,988.64 670.6 980,636.76 -99.45 -13.27 92 AP017 47.7447 -116.9373 504,706.26 5,287,710.75 674.1 980,636.48 -98.80 -12.62 93 AP018 47.7416 -116.9372 504,706.56 5,287,371.31 684.3 980,634.42 -98.62 -12.44 94 AP019 47.7386 -116.9372 504,706.56 5,287,031.56 681.2 980,634.89 -98.49 -12.31 95 AP020 47.7359 -116.9372 504,706.97 5,286,722.90 679.7 980,635.15 -98.27 -12.09 96 AP021 47.7331 -116.9372 504,707.08 5,286,414.23 679.4 980,635.02 -98.21 -12.03 97 AP022 47.7301 -116.9372 504,707.38 5,286,074.79 678.3 980,634.92 -98.24 -12.06 98 AP023 47.7269 -116.9371 504,707.69 5,285,735.04 676.0 980,634.99 -98.32 -12.14 99 AP024 47.7242 -116.9371 504,708.10 5,285,426.38 674.6 980,635.12 -98.20 -12.02 100 AP025 47.7214 -116.9372 504,708.21 5,285,117.71 672.4 980,635.47 -98.02 -11.84

Count Point_ID DD_lat DD_long Easting Northing Elevation Obs Grav C.B.A. 1 C.B.A. 2 101 AP026 47.7184 -116.9371 504,708.52 5,284,777.96 671.1 980,635.55 -97.89 -11.71 102 AP027 47.7157 -116.9371 504,708.74 5,284,500.07 669.1 980,635.84 -97.72 -11.54 103 AP028 47.7132 -116.9371 504,708.96 5,284,222.18 667.5 980,636.30 -97.30 -11.12 104 AP029 47.7108 -116.9372 504,709.17 5,283,944.60 663.5 980,637.45 -96.67 -10.49 105 AP030 47.7080 -116.9371 504,709.59 5,283,635.63 659.9 980,638.80 -95.69 -9.51 106 AP031 47.7047 -116.9369 504,730.49 5,283,265.44 664.2 980,638.12 -95.15 -8.95 107 AP032 47.7021 -116.9371 504,709.99 5,282,987.52 657.0 980,639.98 -94.31 -8.13 108 AP033 47.6982 -116.9375 504,689.55 5,282,555.11 663.5 980,638.78 -93.64 -7.48 109 AP034 47.6958 -116.9382 504,627.30 5,282,277.43 670.4 980,637.16 -93.41 -7.32 110 AP035 47.7885 -116.9369 504,723.13 5,292,588.81 656.6 980,649.20 -93.11 -6.91 111 AP036 47.7909 -116.9369 504,722.80 5,292,835.62 658.9 980,649.48 -92.51 -6.31 112 AP037 47.7939 -116.9369 504,722.48 5,293,175.37 665.0 980,649.47 -91.40 -5.21 113 AP038 47.7956 -116.9369 504,722.23 5,293,360.62 687.2 980,645.22 -91.50 -5.30 114 AP039 47.7969 -116.9368 504,742.90 5,293,514.84 722.7 980,637.10 -92.50 -6.28 115 AP040 47.7994 -116.9368 504,742.67 5,293,792.73 724.5 980,637.52 -92.34 -6.13 116 AP041 47.8025 -116.9367 504,742.66 5,294,132.48 723.5 980,639.20 -91.07 -4.86 117 AR401 47.7480 -116.7973 515,200.39 5,288,099.33 694.7 980,624.71 -106.87 -9.15 118 AR402 47.7534 -116.7973 515,198.84 5,288,685.88 696.9 980,623.83 -107.81 -10.09 119 AR403 47.7586 -116.7972 515,197.28 5,289,272.43 699.2 980,622.92 -108.75 -11.03 120 AR404 47.7661 -116.7971 515,195.09 5,290,106.10 702.5 980,622.71 -109.00 -11.29 121 AS201 47.7824 -116.7648 517,624.93 5,291,934.36 736.6 980,617.25 -109.09 -8.70 122 AS202 47.7800 -116.7648 517,625.76 5,291,656.47 734.9 980,617.23 -109.27 -8.88 123 AS203 47.7779 -116.7648 517,626.40 5,291,409.66 729.3 980,618.25 -109.17 -8.78 124 AS204 47.7758 -116.7648 517,627.14 5,291,193.62 729.3 980,617.78 -109.44 -9.05 125 AS205 47.7735 -116.7647 517,627.78 5,290,946.51 722.4 980,618.45 -109.88 -9.49 126 AS206 47.7707 -116.7648 517,628.81 5,290,637.85 705.8 980,621.43 -109.92 -9.53 127 AS207 47.7685 -116.7648 517,629.45 5,290,390.73 702.0 980,622.09 -109.82 -9.42 128 AS208 47.7670 -116.7648 517,630.01 5,290,205.47 699.8 980,622.57 -109.62 -9.23 129 AS209 47.7654 -116.7649 517,609.84 5,290,020.18 698.3 980,622.85 -109.49 -9.12 130 AS210 47.7633 -116.7650 517,610.59 5,289,804.15 698.9 980,622.60 -109.41 -9.03 131 AS211 47.7611 -116.7650 517,611.22 5,289,557.34 699.0 980,622.55 -109.21 -8.84 132 AS212 47.7587 -116.7651 517,612.06 5,289,279.45 699.2 980,622.72 -108.78 -8.40 133 AS213 47.7568 -116.7652 517,592.09 5,289,063.08 697.7 980,623.14 -108.48 -8.12 134 AS214 47.7542 -116.7654 517,592.92 5,288,785.19 696.9 980,623.50 -108.02 -7.67 135 AS215 47.7518 -116.7653 517,593.75 5,288,507.60 686.0 980,625.78 -107.54 -7.19 136 AS216 47.7490 -116.7652 517,594.47 5,288,198.63 685.0 980,626.25 -106.41 -6.06 137 AS217 47.7478 -116.7652 517,594.94 5,288,075.23 724.7 980,618.42 -106.90 -6.54 138 AV701 47.6734 -116.7861 516,055.96 5,279,797.11 652.0 980,632.79 -100.26 -1.60 139 AV702 47.6765 -116.7857 516,076.08 5,280,136.59 661.3 980,629.53 -101.99 -3.31 140 AV703 47.6837 -116.7858 516,073.80 5,280,939.17 670.7 980,626.49 -103.88 -5.20 141 AV704 47.6914 -116.7856 516,092.15 5,281,803.65 677.2 980,624.40 -105.41 -6.71 142 AV705 47.6968 -116.7857 516,069.89 5,282,390.17 680.2 980,624.03 -105.68 -7.01 143 AV706 47.7030 -116.7856 516,088.59 5,283,100.47 684.0 980,623.86 -105.68 -6.98 144 AV707 47.7098 -116.7862 516,044.95 5,283,841.14 680.5 980,624.55 -106.30 -7.65 145 AV708 47.7159 -116.7866 516,001.38 5,284,520.26 683.3 980,624.66 -106.19 -7.59 146 AV709 47.7225 -116.7863 516,020.19 5,285,261.33 681.0 980,625.28 -106.61 -7.98 147 D7928 47.6267 -116.7667 517,531.48 5,274,614.81 680.2 980,624.38 -98.44 1.85 148 D7942 47.6392 -116.9137 506,489.17 5,275,981.08 723.7 980,625.19 -88.97 -0.83 149 D7961 47.6467 -116.8883 508,386.83 5,276,817.03 677.7 980,633.69 -90.30 -0.07 150 D7962 47.6468 -116.8877 508,428.58 5,276,817.09 677.8 980,633.88 -90.14 0.14 151 D7972 47.6573 -117.0398 497,017.34 5,277,985.06 681.1 980,637.50 -86.96 -9.24 152 D7977 47.6583 -116.9138 506,466.09 5,278,111.24 810.9 980,607.50 -91.62 -3.50 153 D7984 47.6663 -116.7702 517,247.00 5,279,028.63 648.2 980,635.19 -97.51 2.46 154 D7985 47.6663 -116.8633 510,260.46 5,279,011.92 748.4 980,618.75 -94.56 -2.27 155 D7987 47.6668 -116.8298 512,783.82 5,279,048.05 651.2 980,638.32 -93.55 1.52 156 D7992 47.6698 -117.0917 493,118.46 5,279,377.43 634.2 980,646.50 -89.56 -16.13 157 D8006 47.6763 -116.8107 514,199.41 5,280,131.60 675.9 980,628.88 -99.27 -2.65 158 D8010 47.6787 -116.8168 513,760.92 5,280,377.75 707.6 980,625.00 -97.15 -1.02 159 D8012 47.6798 -116.8048 514,657.19 5,280,503.43 654.6 980,633.32 -99.53 -2.41

12 13 Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Adema, Breckenridge, and Sprenke Adema, Breckenridge, and Sprenke

Count Point_ID DD_lat DD_long Easting Northing Elevation Obs Grav C.B.A. 1 C.B.A. 2 160 D8013 47.6803 -116.8248 513,155.84 5,280,561.48 735.9 980,621.00 -95.92 -0.45 161 D8015 47.6818 -116.8052 514,615.01 5,280,719.40 647.9 980,634.32 -99.81 -2.74 162 D8021 47.6837 -116.8067 514,510.06 5,280,935.27 655.8 980,633.44 -99.00 -2.04 163 D8026 47.6848 -116.7798 516,532.21 5,281,064.19 663.1 980,629.00 -102.81 -3.62 164 D8027 47.6863 -116.8117 514,134.25 5,281,243.07 647.9 980,634.13 -96.62 -0.08 165 D8032 47.6868 -116.7948 515,405.89 5,281,276.99 661.6 980,629.50 -102.89 -4.94 166 D8037 47.6878 -116.7913 515,655.58 5,281,401.39 673.8 980,625.19 -104.96 -6.74 167 D8038 47.6887 -116.7848 516,155.63 5,281,495.38 673.8 980,624.38 -105.85 -7.08 168 D8044 47.6908 -116.7748 516,905.49 5,281,744.25 671.9 980,625.94 -104.79 -5.19 169 D8045 47.6908 -116.7798 516,530.09 5,281,743.37 672.2 980,625.19 -105.51 -6.32 170 D8047 47.6917 -116.9138 506,461.78 5,281,815.84 648.8 980,639.50 -95.08 -6.97 171 D8050 47.6928 -116.7637 517,738.40 5,281,963.07 658.5 980,630.75 -101.86 -1.35 172 D8051 47.6933 -116.7682 517,383.96 5,282,023.47 661.6 980,629.00 -103.70 -3.57 173 D8052 47.6933 -116.7717 517,133.79 5,282,022.79 672.2 980,626.63 -104.22 -4.37 174 D8053 47.6943 -116.8718 509,629.88 5,282,098.24 648.8 980,638.88 -95.79 -4.20 175 D8058 47.6963 -116.8923 508,087.14 5,282,342.71 648.5 980,639.69 -95.65 -5.75 176 D8059 47.6963 -116.9783 501,625.87 5,282,337.47 749.0 980,623.44 -91.64 -8.85 177 D8068 47.7017 -116.8103 514,234.22 5,282,941.34 664.6 980,631.32 -101.89 -5.23 178 D8070 47.7018 -116.7867 516,005.77 5,282,945.86 682.9 980,623.63 -106.03 -7.42 179 D8073 47.7033 -116.7848 516,151.17 5,283,131.34 656.1 980,633.00 -101.23 -2.46 180 D8074 47.7033 -116.9483 503,876.16 5,283,110.26 648.5 980,641.38 -94.59 -9.32 181 D8075 47.7053 -116.8698 509,773.58 5,283,333.43 656.4 980,633.88 -101.12 -9.37 182 D8076 47.7053 -116.8498 511,274.25 5,283,336.01 660.7 980,630.69 -103.51 -10.11 183 D8086 47.7098 -116.8917 508,126.60 5,283,824.85 673.2 980,631.38 -100.58 -10.64 184 D8087 47.7098 -116.9598 503,021.58 5,283,819.85 648.8 980,642.69 -93.97 -9.64 185 D8089 47.7108 -116.9167 506,251.28 5,283,946.02 665.8 980,634.07 -99.68 -11.80 186 D8091 47.7117 -116.8933 508,001.23 5,284,040.70 669.2 980,632.00 -100.63 -10.83 187 D8094 47.7133 -116.9798 501,521.12 5,284,220.70 656.4 980,641.69 -94.01 -11.34 188 D8095 47.7148 -116.9567 503,250.28 5,284,375.67 661.3 980,639.13 -95.75 -11.18 189 D8096 47.7148 -117.0000 499,999.98 5,284,374.70 647.6 980,643.00 -94.62 -13.62 190 D8097 47.7153 -116.8498 511,271.85 5,284,447.57 688.1 980,623.44 -106.50 -13.10 191 D8098 47.7153 -116.9418 504,375.47 5,284,438.02 662.2 980,637.57 -97.27 -11.45 192 D8100 47.7158 -116.8067 514,501.32 5,284,516.46 671.9 980,625.19 -107.90 -10.95 193 D8102 47.7163 -117.0223 498,333.30 5,284,560.17 639.3 980,646.25 -93.17 -14.00 194 D8104 47.7167 -116.9783 501,625.15 5,284,591.07 651.8 980,642.13 -94.84 -12.06 195 D8108 47.7187 -116.9578 503,166.82 5,284,807.92 662.9 980,638.88 -96.14 -11.66 196 D8109 47.7187 -116.9583 503,125.07 5,284,807.86 664.0 980,638.44 -96.36 -11.92 197 D8111 47.7193 -116.8288 512,833.49 5,284,882.92 687.2 980,623.57 -106.93 -11.81 198 D8116 47.7228 -116.9577 503,166.65 5,285,271.08 673.2 980,636.44 -96.95 -12.46 199 D8117 47.7232 -117.0223 498,333.33 5,285,331.99 642.4 980,645.32 -94.17 -15.00 200 D8124 47.7298 -116.9733 501,999.66 5,286,042.34 651.8 980,640.25 -98.01 -14.81 201 D8125 47.7302 -116.7647 517,642.50 5,286,130.37 683.8 980,624.07 -107.57 -7.16 202 D8126 47.7302 -116.8923 508,081.80 5,286,109.16 676.5 980,630.82 -102.74 -12.85 203 D8127 47.7303 -116.8718 509,623.30 5,286,111.50 678.6 980,628.69 -104.48 -12.89 204 D8128 47.7303 -116.8068 514,497.35 5,286,121.64 687.8 980,625.75 -105.59 -8.64 205 D8129 47.7303 -116.8288 512,830.92 5,286,117.89 693.3 980,625.38 -104.93 -9.82 206 D8130 47.7303 -116.9583 503,124.56 5,286,104.38 664.6 980,638.50 -97.32 -12.88 207 D8131 47.7308 -116.8498 511,268.62 5,286,176.16 693.9 980,625.32 -104.93 -11.53 208 D8132 47.7308 -116.9133 506,498.78 5,286,168.92 674.4 980,634.44 -99.58 -11.43 209 D8133 47.7308 -117.0223 498,333.58 5,286,165.67 643.9 980,647.07 -92.82 -13.65 210 D8142 47.7353 -117.0223 498,333.83 5,286,659.60 643.9 980,648.44 -91.83 -12.66 211 D8143 47.7358 -116.9578 503,165.78 5,286,721.78 659.4 980,639.32 -98.02 -13.54 212 D8147 47.7383 -116.9478 503,915.43 5,287,000.20 678.3 980,635.57 -98.31 -13.01 213 D8150 47.7398 -116.9903 500,728.77 5,287,153.20 649.0 980,643.25 -96.45 -14.65 214 D8153 47.7417 -116.9578 503,165.36 5,287,370.19 655.5 980,640.69 -97.96 -13.48 215 D8156 47.7442 -117.0677 494,918.88 5,287,649.58 651.2 980,649.75 -89.83 -14.42 216 D8157 47.7442 -117.0677 494,918.88 5,287,649.58 651.2 980,649.75 -89.83 -14.42 217 D8158 47.7443 -116.7942 515,430.41 5,287,667.60 684.1 980,626.07 -107.13 -9.16 218 D8159 47.7447 -116.8717 509,620.53 5,287,716.99 683.5 980,628.13 -105.41 -13.82

Count Point_ID DD_lat DD_long Easting Northing Elevation Obs Grav C.B.A. 1 C.B.A. 2 219 D8162 47.7448 -116.8072 514,451.62 5,287,727.06 705.8 980,623.82 -105.20 -8.31 220 D8163 47.7448 -116.8287 512,848.27 5,287,723.10 706.4 980,624.88 -104.04 -8.91 221 D8164 47.7448 -116.8498 511,265.63 5,287,719.79 689.6 980,626.57 -105.78 -12.39 222 D8165 47.7448 -116.8923 508,079.64 5,287,714.65 681.4 980,631.00 -102.95 -13.06 223 D8166 47.7448 -116.9138 506,455.26 5,287,712.49 680.5 980,633.07 -101.04 -12.94 224 D8167 47.7448 -116.9367 504,747.70 5,287,710.81 671.0 980,636.13 -99.74 -13.51 225 D8170 47.7452 -116.9578 503,165.28 5,287,771.49 658.2 980,640.38 -98.06 -13.57 226 D8181 47.7503 -117.0078 499,416.95 5,288,326.46 647.5 980,647.75 -93.06 -12.70 227 D8182 47.7503 -117.0078 499,416.95 5,288,326.46 647.5 980,647.75 -93.06 -12.70 228 D8185 47.7517 -117.0683 494,877.99 5,288,483.20 649.7 980,650.38 -90.09 -14.72 229 D8186 47.7517 -117.0683 494,877.99 5,288,483.20 649.7 980,650.38 -90.09 -14.72 230 D8187 47.7522 -116.9583 503,122.95 5,288,543.24 658.8 980,641.32 -97.63 -13.19 231 D8191 47.7563 -116.9583 503,122.78 5,289,006.39 659.4 980,641.75 -97.44 -13.00 232 D8193 47.7577 -117.0937 492,984.24 5,289,164.08 654.6 980,653.57 -86.03 -12.75 233 D8194 47.7583 -116.7898 515,759.35 5,289,243.12 693.3 980,623.57 -109.19 -10.85 234 D8196 47.7588 -117.0617 495,378.46 5,289,285.33 650.6 980,652.57 -88.00 -12.08 235 D8197 47.7588 -117.0617 495,378.46 5,289,285.33 650.6 980,652.57 -88.00 -12.08 236 D8199 47.7593 -116.8718 509,617.77 5,289,322.17 686.3 980,630.50 -103.80 -12.22 237 D8200 47.7597 -116.9367 504,746.36 5,289,378.16 660.3 980,641.63 -97.70 -11.47 238 D8201 47.7598 -116.9583 503,122.59 5,289,376.92 658.8 980,643.50 -96.10 -11.67 239 D8203 47.7633 -116.9358 504,808.44 5,289,779.55 665.2 980,640.50 -98.19 -11.90 240 D8205 47.7652 -117.0118 499,125.84 5,289,993.67 670.4 980,646.63 -90.88 -10.84 241 D8206 47.7652 -117.0118 499,125.84 5,289,993.67 670.4 980,646.63 -90.88 -10.84 242 D8207 47.7652 -117.0118 499,125.84 5,289,993.67 670.5 980,646.82 -90.67 -10.63 243 D8215 47.7697 -116.9348 504,891.12 5,290,489.64 668.0 980,640.75 -97.95 -11.57 244 D8216 47.7698 -116.7638 517,691.75 5,290,514.54 702.1 980,621.75 -110.25 -9.79 245 D8217 47.7718 -117.0618 495,379.58 5,290,705.57 651.2 980,654.00 -87.49 -11.58 246 D8218 47.7718 -117.0618 495,379.58 5,290,705.57 651.2 980,654.00 -87.49 -11.58 247 D8223 47.7737 -116.8498 511,259.47 5,290,930.76 694.8 980,628.25 -105.68 -12.29 248 D8224 47.7738 -116.8717 509,615.10 5,290,958.43 687.5 980,631.82 -103.53 -11.95 249 D8225 47.7738 -116.8933 507,991.94 5,290,955.97 686.3 980,633.94 -101.61 -11.82 250 D8227 47.7742 -116.9358 504,807.57 5,290,983.44 664.0 980,642.75 -97.09 -10.81 251 D8228 47.7743 -116.9148 506,389.30 5,290,984.93 676.8 980,637.32 -100.09 -12.07 252 D8229 47.7743 -116.9363 504,765.83 5,290,983.38 664.0 980,642.88 -96.97 -10.73 253 D8233 47.7757 -116.8092 514,297.35 5,291,184.31 704.5 980,624.07 -108.13 -11.40 254 D8236 47.7767 -116.9658 502,559.84 5,291,259.76 652.4 980,649.88 -91.95 -8.13 255 D8237 47.7767 -116.9658 502,559.84 5,291,259.76 652.4 980,649.88 -91.95 -8.13 256 D8243 47.7803 -117.0383 497,128.37 5,291,661.35 668.6 980,650.13 -88.65 -10.81 257 D8244 47.7803 -117.0383 497,128.37 5,291,661.35 668.6 980,650.13 -88.65 -10.81 258 D8247 47.7817 -116.9363 504,765.16 5,291,817.05 663.4 980,646.07 -94.46 -8.22 259 D8249 47.7828 -116.9797 501,518.96 5,291,938.58 732.6 980,635.50 -91.53 -8.86 260 D8250 47.7828 -116.9797 501,518.96 5,291,938.58 732.6 980,635.50 -91.53 -8.86 261 D8256 47.7858 -116.9417 504,369.48 5,292,279.61 657.6 980,648.13 -93.77 -7.96 262 D8257 47.7858 -116.9417 504,369.48 5,292,279.61 657.6 980,648.13 -93.77 -7.96 263 D8265 47.7883 -116.7638 517,685.44 5,292,582.86 741.1 980,615.63 -110.46 -10.01 264 D8266 47.7883 -116.7853 516,083.31 5,292,578.30 704.2 980,624.25 -109.09 -10.39 265 D8267 47.7883 -116.8068 514,481.18 5,292,574.04 703.0 980,625.82 -107.80 -10.87 266 D8268 47.7883 -116.8283 512,858.33 5,292,570.37 697.5 980,629.63 -105.07 -9.93 267 D8269 47.7883 -116.8502 511,214.45 5,292,566.96 693.3 980,631.44 -104.07 -10.73 268 D8270 47.7883 -116.8713 509,633.34 5,292,563.95 696.9 980,631.50 -103.27 -11.68 269 D8271 47.7883 -116.8933 507,989.46 5,292,561.46 678.9 980,638.57 -99.64 -9.85 270 D8272 47.7883 -116.9363 504,764.76 5,292,557.79 657.3 980,649.32 -92.85 -6.61 271 D8275 47.7887 -116.9148 506,387.43 5,292,590.42 664.3 980,643.94 -97.04 -9.02 272 D8283 47.7937 -116.9268 505,492.36 5,293,145.15 662.8 980,648.75 -92.75 -5.70 273 D8284 47.7937 -116.9268 505,492.36 5,293,145.15 662.8 980,648.75 -92.75 -5.70 274 D8285 47.7947 -117.0617 495,381.45 5,293,267.85 732.0 980,637.63 -89.27 -13.35 275 D8286 47.7947 -117.0617 495,381.45 5,293,267.85 732.0 980,637.63 -89.27 -13.35 276 D8287 47.7947 -117.0617 495,381.45 5,293,267.85 732.0 980,637.63 -89.27 -13.35 277 D8291 47.7957 -117.1223 490,846.63 5,293,396.87 718.0 980,642.88 -89.08 -18.15

14 15 Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Adema, Breckenridge, and Sprenke Adema, Breckenridge, and Sprenke

Count Point_ID DD_lat DD_long Easting Northing Elevation Obs Grav C.B.A. 1 C.B.A. 2 278 D8292 47.7958 -116.9598 503,016.60 5,293,390.34 718.0 980,638.82 -89.08 -4.76 279 D8293 47.7958 -116.9598 503,016.60 5,293,390.34 772.2 980,638.82 -89.90 -5.58 280 D8294 47.7958 -116.9797 501,518.67 5,293,389.59 772.2 980,630.19 -89.90 -7.23 281 D8296 47.7958 -116.9797 501,518.67 5,293,389.59 703.3 980,630.19 -93.38 -10.71 282 D8298 47.7963 -116.9383 504,618.36 5,293,453.10 648.2 980,640.25 -83.29 2.79 283 D8299 47.7968 -116.9798 501,518.70 5,293,482.22 648.2 980,650.63 -83.29 -0.62 284 D8304 47.7968 -116.9798 501,518.70 5,293,482.22 664.0 980,650.63 -92.59 -9.92 285 D8305 47.8023 -116.9148 506,385.83 5,294,102.97 664.0 980,649.13 -92.59 -4.57 286 D8309 47.8023 -116.9148 506,385.83 5,294,102.97 738.1 980,649.13 -109.55 -21.53 287 D8310 47.8028 -116.7637 517,701.24 5,294,188.38 701.5 980,618.44 -106.57 -6.10 288 D8311 47.8028 -116.8068 514,477.17 5,294,179.53 701.5 980,628.63 -106.57 -9.64 289 D8312 47.8028 -116.8068 514,477.17 5,294,179.53 704.5 980,628.63 -104.62 -7.70 290 D8313 47.8028 -116.8283 512,854.62 5,294,175.55 704.5 980,630.00 -104.62 -9.48 291 D8314 47.8028 -116.8283 512,854.62 5,294,175.55 689.9 980,630.00 -103.72 -8.58 292 D8315 47.8028 -116.8503 511,211.36 5,294,172.14 689.9 980,633.69 -103.72 -10.39 293 D8316 47.8028 -116.8503 511,211.36 5,294,172.14 680.8 980,633.69 -99.37 -6.04 294 D8317 47.8028 -116.8717 509,609.83 5,294,169.41 680.8 980,639.75 -99.37 -7.80 295 D8318 47.8028 -116.8717 509,609.83 5,294,169.41 723.1 980,639.75 -91.05 0.52 296 D8322 47.8028 -116.9352 504,846.38 5,294,163.41 723.1 980,639.19 -91.19 -4.86 297 D8323 47.8048 -116.9363 504,763.05 5,294,379.32 723.1 980,639.00 -91.19 -4.95 298 D8330 47.8048 -116.9363 504,763.05 5,294,379.32 669.2 980,639.00 -92.93 -6.69 299 D8331 47.8103 -116.8993 507,549.34 5,294,999.96 669.2 980,648.38 -92.93 -3.62 300 D8335 47.8103 -116.8993 507,549.34 5,294,999.96 674.1 980,648.38 -93.35 -4.05 301 D8338 47.8117 -116.8967 507,736.38 5,295,154.43 679.9 980,647.19 -89.52 -0.01 302 D8344 47.8128 -117.0088 499,334.57 5,295,272.72 735.9 980,648.88 -90.01 -9.74 303 D8345 47.8133 -116.9468 503,992.82 5,295,335.84 735.9 980,637.88 -90.01 -4.61 304 D8347 47.8133 -116.9468 503,992.82 5,295,335.84 703.9 980,637.88 -105.95 -20.55 305 D8348 47.8173 -116.8068 514,473.16 5,295,784.71 703.9 980,630.07 -105.95 -9.02 306 D8349 47.8173 -116.8068 514,473.16 5,295,784.71 686.9 980,630.07 -103.32 -6.40 307 D8350 47.8173 -116.8282 512,851.22 5,295,781.03 686.9 980,636.00 -103.32 -8.18 308 D8351 47.8173 -116.8282 512,851.22 5,295,781.03 683.2 980,636.00 -99.91 -4.77 309 D8352 47.8173 -116.8498 511,250.00 5,295,777.69 683.2 980,640.07 -99.91 -6.53 310 D8353 47.8173 -116.8498 511,250.00 5,295,777.69 676.2 980,640.07 -96.13 -2.75 311 D8354 47.8173 -116.8713 509,628.06 5,295,774.63 676.2 980,645.00 -96.13 -4.54 312 D8355 47.8173 -116.8713 509,628.06 5,295,774.63 707.9 980,645.00 -108.06 -16.47 313 D8356 47.8178 -116.7867 515,970.09 5,295,850.35 707.9 980,627.07 -108.06 -9.49 314 D8360 47.8178 -116.7867 515,970.09 5,295,850.35 684.1 980,627.07 -93.58 4.98 315 D8361 47.8192 -116.8848 508,629.40 5,295,989.15 684.1 980,645.82 -93.58 -3.09 316 D8364 47.8192 -116.8848 508,629.40 5,295,989.15 750.9 980,645.82 -109.17 -18.68 317 D8374 47.8247 -116.7747 516,861.97 5,296,624.74 705.2 980,617.94 -108.21 -8.67 318 D8380 47.8283 -116.7698 517,235.14 5,297,027.21 687.2 980,627.50 -104.25 -4.29 319 D8381 47.8318 -116.8067 514,469.14 5,297,390.19 693.6 980,636.32 -93.32 3.60 320 D8382 47.8318 -116.8713 509,625.26 5,297,380.11 693.6 980,645.38 -93.32 -1.73 321 D8391 47.8318 -116.8713 509,625.26 5,297,380.11 732.6 980,645.38 -86.52 5.07 322 D8398 47.8373 -117.0908 493,202.59 5,297,993.59 693.9 980,640.82 -101.98 -28.46 323 D8420 47.8462 -116.8068 514,465.11 5,298,995.68 695.4 980,638.57 -99.61 -2.70 324 D8421 47.8593 -116.7792 516,518.50 5,300,452.23 694.5 980,641.69 -100.08 -0.91 325 D8424 47.8597 -116.7797 516,476.67 5,300,513.72 694.5 980,641.44 -101.63 -2.51 326 D8426 47.8607 -116.7638 517,660.58 5,300,640.74 699.4 980,640.00 -104.69 -4.26 327 D8427 47.8608 -116.7502 518,678.90 5,300,643.81 694.5 980,635.88 -101.51 0.04 328 D8428 47.8608 -116.7637 517,681.60 5,300,640.78 695.4 980,640.13 -101.10 -0.65 329 D8429 47.8608 -116.7708 517,141.37 5,300,639.04 706.4 980,640.38 -99.60 0.26 330 D8430 47.8608 -116.8068 514,460.89 5,300,631.94 703.9 980,639.82 -99.75 -2.84 331 D8431 47.8612 -116.8018 514,834.87 5,300,663.58 694.5 980,640.19 -99.50 -2.18 332 D8432 47.8613 -116.7863 515,977.62 5,300,697.61 701.8 980,642.25 -100.28 -1.71 333 D8433 47.8613 -116.7918 515,582.72 5,300,696.40 703.0 980,640.07 -100.19 -2.04 334 D8434 47.8613 -116.7963 515,229.57 5,300,695.57 703.0 980,639.94 -100.44 -2.68 335 D8437 47.8613 -116.7963 515,229.57 5,300,695.57 710.9 980,639.69 -98.40 -0.65 336 D8440 47.8643 -116.8067 514,460.07 5,301,002.46 742.0 980,640.44 -91.13 5.78

Count Point_ID DD_lat DD_long Easting Northing Elevation Obs Grav C.B.A. 1 C.B.A. 2 337 D8445 47.8663 -116.8572 510,678.35 5,301,241.11 724.1 980,641.19 -95.55 -2.81 338 D8446 47.8678 -116.8487 511,322.14 5,301,396.57 715.8 980,640.75 -97.70 -4.24 339 D8447 47.8678 -116.8333 512,464.78 5,301,398.91 719.5 980,640.38 -97.57 -2.86 340 D8448 47.8678 -116.8387 512,069.88 5,301,398.31 719.5 980,639.75 -97.38 -3.10 341 D8449 47.8678 -116.8387 512,069.88 5,301,398.31 724.1 980,639.94 -95.73 -1.45 342 D8450 47.8678 -116.8483 511,342.86 5,301,396.60 726.8 980,640.57 -93.16 0.32 343 D8452 47.8678 -116.8533 510,968.98 5,301,396.03 706.4 980,642.57 -97.55 -4.48 344 D8453 47.8682 -116.8068 514,458.98 5,301,465.30 703.6 980,642.50 -96.83 0.08 345 D8454 47.8682 -116.8222 513,295.62 5,301,462.63 703.6 980,643.69 -96.82 -1.19 346 D8455 47.8682 -116.8237 513,191.71 5,301,462.47 707.3 980,643.69 -97.74 -2.23 347 D8456 47.8682 -116.8282 512,838.56 5,301,461.63 714.3 980,641.94 -96.94 -1.82 348 D8460 47.8682 -116.8433 511,716.64 5,301,459.32 703.9 980,641.25 -95.94 -2.05 349 D8462 47.8683 -116.8152 513,814.84 5,301,463.72 705.8 980,644.57 -100.09 -3.90 350 D8463 47.8688 -116.7998 514,978.11 5,301,528.55 706.4 980,640.13 -97.74 -0.26 351 D8467 47.8698 -116.8068 514,458.61 5,301,619.79 704.8 980,642.44 -99.43 -2.53 352 D8468 47.8728 -116.7697 517,220.44 5,301,967.07 708.2 980,641.32 -98.99 0.95 353 D8470 47.8728 -116.7923 515,537.87 5,301,962.08 707.3 980,641.13 -96.22 1.88 354 PCR01 47.7578 -116.9899 514,457.21 5,302,082.94 642.7 980,626.61 -91.74 5.17 355 PCR02 47.7533 -116.9902 500,749.38 5,289,160.02 640.9 980,627.87 -93.09 -11.27 356 PCR03 47.7519 -116.9902 500,728.71 5,288,666.06 645.4 980,629.48 -93.57 -11.77 357 PCR04 47.7506 -116.9902 500,728.77 5,288,511.58 646.3 980,630.51 -94.29 -12.48 358 PCR05 47.7492 -116.9899 500,728.82 5,288,357.40 645.7 980,631.19 -94.99 -13.19 359 PCR06 47.7475 -116.9899 500,749.60 5,288,202.94 647.0 980,632.18 -95.58 -13.76 360 PCR07 47.7461 -116.9899 500,749.55 5,288,017.68 647.0 980,632.86 -96.14 -14.31 361 PCR08 47.7447 -116.9902 500,749.60 5,287,863.50 647.6 980,633.24 -96.25 -14.43 362 PCR09 47.7433 -116.9902 500,728.94 5,287,708.98 648.2 980,633.55 -96.30 -14.50 363 PCR10 47.7419 -116.9899 500,728.99 5,287,554.80 647.6 980,633.23 -95.98 -14.18 364 PCR11 47.7403 -116.9899 500,749.76 5,287,400.35 647.6 980,633.74 -96.35 -14.53 365 PCR12 47.7389 -116.9899 500,749.71 5,287,215.09 648.8 980,634.45 -96.66 -14.84 366 PCR13 47.7375 -116.9899 500,749.77 5,287,060.60 648.2 980,634.85 -97.07 -15.24 367 PCR14 47.7361 -116.9899 500,749.82 5,286,906.42 644.5 980,634.60 -97.50 -15.68 368 PCR15 47.7344 -116.9899 500,749.88 5,286,751.93 644.5 980,635.08 -97.85 -16.03 369 PCR16 47.7331 -116.9899 500,749.82 5,286,566.67 645.7 980,635.48 -97.86 -16.04 370 PCR17 47.7317 -116.9899 500,749.87 5,286,412.49 648.2 980,695.76 -98.26 -16.43 371 PCR18 47.7303 -116.9899 500,749.93 5,286,258.01 650.0 980,637.15 -98.32 -16.50 372 PCR19 47.7289 -116.9899 500,749.99 5,286,103.52 650.9 980,637.61 -98.45 -16.62 373 PCR20 47.7272 -116.9899 500,749.73 5,285,949.34 652.4 980,638.03 -98.40 -16.57 374 PCR21 47.7258 -116.9899 500,749.98 5,285,764.08 652.4 980,638.17 -98.41 -16.59 375 PCR22 47.7244 -116.9899 500,750.04 5,285,609.59 650.9 980,637.77 -98.22 -16.39 376 PCR23 47.7231 -116.9899 500,750.09 5,285,455.41 648.8 980,637.21 -98.00 -16.17 377 PCR24 47.7217 -116.9899 500,749.84 5,285,300.92 646.3 980,636.53 -97.71 -15.89 378 PCR25 47.7203 -116.9899 500,749.89 5,285,146.74 645.4 980,636.01 -97.25 -15.42 379 PCR26 47.7186 -116.9899 500,749.95 5,284,992.26 645.4 980,635.63 -96.73 -14.90 380 PCR27 47.7172 -116.9896 500,749.89 5,284,806.99 645.7 980,634.86 -95.76 -13.93 381 PCR28 47.7161 -116.9899 500,770.97 5,284,652.54 643.3 980,634.25 -95.56 -13.71 382 PCR29 47.7144 -116.9899 500,750.11 5,284,529.11 645.7 980,633.93 -94.56 -12.74 383 PCR30 47.7131 -116.9899 500,750.06 5,284,343.84 646.7 980,633.73 -94.04 -12.21 384 PCR31 47.7114 -116.9899 500,750.11 5,284,189.66 645.4 980,633.60 -94.04 -12.22 385 PCR32 47.7100 -116.9899 500,750.05 5,284,004.40 644.5 980,633.41 -93.93 -12.10 386 PCR33 47.7086 -116.9896 500,750.11 5,283,849.92 644.2 980,633.07 -93.51 -11.68 387 PCR34 47.7072 -116.9896 500,770.88 5,283,695.76 644.8 980,633.40 -93.55 -11.70 388 PCR35 47.7061 -116.9899 500,770.93 5,283,541.28 642.4 980,633.29 -93.85 -12.00 389 PCR36 47.7061 -116.9904 500,750.07 5,283,417.84 639.3 980,632.59 -93.83 -12.00 390 PCR37 47.7042 -116.9902 500,729.35 5,283,417.81 637.2 980,631.61 -93.18 -11.38 391 PCR38 47.7031 -116.9899 500,729.49 5,283,201.78 629.9 980,630.48 -93.56 -11.76 392 PHL01 47.7508 -117.0063 500,750.38 5,283,078.10 648.8 980,630.02 -93.13 -11.31 393 PHL02 47.7522 -117.0063 499,521.08 5,288,388.17 649.7 980,629.95 -92.99 -12.51 394 PHL03 47.7536 -117.0063 499,521.02 5,288,542.65 651.8 980,630.13 -93.10 -12.62 395 PHL04 47.7550 -117.0063 499,521.28 5,288,696.84 662.2 980,632.26 -92.75 -12.27

16 17 Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Adema, Breckenridge, and Sprenke Adema, Breckenridge, and Sprenke

Count Point_ID DD_lat DD_long Easting Northing Elevation Obs Grav C.B.A. 1 C.B.A. 2 396 PHL05 47.7564 -117.0066 499,521.22 5,288,851.32 664.9 980,632.44 -92.44 -11.96 397 PHL06 47.7581 -117.0066 499,500.44 5,289,005.78 670.7 980,633.81 -92.63 -12.18 398 PHL07 47.7594 -117.0066 499,500.50 5,289,190.73 674.7 980,634.32 -92.31 -11.86 399 PHL08 47.7608 -117.0066 499,500.44 5,289,345.22 674.7 980,633.86 -91.93 -11.48 400 PHL09 47.7619 -117.0068 499,500.38 5,289,499.70 671.3 980,632.15 -91.08 -10.63 401 PHL10 47.7628 -117.0087 499,500.52 5,289,623.11 670.7 980,631.64 -90.90 -10.45 402 PHL11 47.7636 -117.0104 499,354.60 5,289,715.82 671.3 980,631.31 -90.55 -10.26 403 PHL12 47.7644 -117.0120 499,229.70 5,289,808.26 671.3 980,631.26 -90.58 -10.43 404 PHL13 47.7650 -117.0139 499,104.79 5,289,901.01 671.7 980,631.83 -91.15 -11.14 405 PHL14 47.7658 -117.0158 498,959.37 5,289,962.65 668.6 980,631.41 -91.50 -11.64 406 PHL15 47.7669 -117.0175 498,813.44 5,290,055.36 668.6 980,631.36 -91.54 -11.85 407 PHL16 47.7672 -117.0191 498,688.65 5,290,178.88 669.2 980,631.08 -91.14 -11.59 408 PHL17 47.7672 -117.0224 498,563.83 5,290,209.77 675.6 980,632.18 -90.82 -11.40 409 PHL18 47.7658 -117.0224 498,313.97 5,290,210.00 681.7 980,632.94 -90.02 -10.87 410 PHL19 47.7644 -117.0224 498,314.02 5,290,055.52 684.8 980,633.05 -89.33 -10.19 411 PHL20 47.7636 -117.0224 498,313.78 5,289,901.03 687.2 980,633.74 -89.40 -10.25 412 PHL21 47.7636 -117.0202 498,313.75 5,289,808.70 676.8 980,632.37 -90.35 -11.20 413 PHL22 47.7636 -117.0180 498,480.42 5,289,808.65 671.7 980,632.21 -91.34 -12.01 414 PHL23 47.7636 -117.0150 498,647.09 5,289,808.60 676.2 980,633.71 -91.81 -12.30 415 PHL24 47.7631 -117.0137 498,875.93 5,289,808.34 677.7 980,633.97 -91.61 -11.85 416 PHL25 47.7619 -117.0120 498,979.92 5,289,746.64 682.3 980,635.39 -91.86 -11.98 417 PHL26 47.7603 -117.0120 499,105.02 5,289,623.12 685.1 980,636.58 -92.31 -12.29 418 PHL27 47.7589 -117.0120 499,104.96 5,289,437.86 685.1 980,637.00 -92.62 -12.61 419 PHL28 47.7575 -117.0117 499,104.71 5,289,283.67 680.2 980,636.22 -92.84 -12.83 420 PHL29 47.7558 -117.0117 499,125.49 5,289,129.22 666.5 980,632.83 -92.34 -12.30 421 PHL30 47.7539 -117.0117 499,125.74 5,288,943.96 662.8 980,631.78 -91.86 -11.82 422 PHL31 47.7522 -117.0117 499,125.57 5,288,727.92 653.0 980,629.59 -91.71 -11.67 423 PHL32 47.7508 -117.0117 499,125.52 5,288,542.66 650.6 980,629.14 -91.66 -11.63 424 PHR01 47.7106 -117.1107 499,125.57 5,288,388.18 636.9 980,624.02 -85.98 -5.94 425 PHR02 47.7092 -117.1107 491,685.93 5,283,917.51 635.7 980,623.57 -85.68 -13.83 426 PHR03 47.7075 -117.1107 491,685.67 5,283,763.32 634.2 980,623.23 -85.55 -13.69 427 PHR04 47.7058 -117.1107 491,685.31 5,283,578.06 633.5 980,623.62 -85.98 -14.13 428 PHR05 47.7042 -117.1107 491,685.26 5,283,392.80 632.9 980,624.00 -86.37 -14.52 429 PHR06 47.7022 -117.1107 491,684.89 5,283,207.54 632.3 980,624.54 -86.93 -15.08 430 PHR07 47.7006 -117.1107 491,684.73 5,282,991.50 631.7 980,624.98 -87.39 -15.54 431 PHR08 47.6989 -117.1107 491,684.36 5,282,806.24 631.1 980,625.62 -88.04 -16.19 432 PHR09 47.6975 -117.1107 491,684.00 5,282,620.98 630.2 980,626.00 -88.51 -16.66 433 PHR10 47.6961 -117.1107 491,683.75 5,282,466.80 629.3 980,626.45 -89.09 -17.23 434 PHR11 47.6947 -117.1107 491,683.50 5,282,312.31 629.0 980,626.86 -89.45 -17.60 435 PHR12 47.6933 -117.1107 491,683.55 5,282,157.83 629.6 980,627.61 -89.95 -18.10 436 PHR13 47.6919 -117.1104 491,683.30 5,282,003.64 628.4 980,627.97 -90.43 -18.58 437 PHR14 47.6906 -117.1104 491,724.79 5,281,849.22 627.7 980,628.30 -90.83 -18.93 438 PHR15 47.6892 -117.1104 491,724.54 5,281,694.73 627.4 980,628.49 -90.87 -18.97 439 PHR16 47.6878 -117.1104 491,724.29 5,281,540.55 626.5 980,628.55 -90.99 -19.09 440 PHR17 47.6864 -117.1104 491,724.04 5,281,386.07 625.3 980,628.36 -90.92 -19.02 441 PHR18 47.6850 -117.1104 491,723.79 5,281,231.58 620.4 980,627.46 -90.99 -19.09 442 PHR19 47.6822 -117.1101 491,723.54 5,281,077.40 619.5 980,626.60 -90.05 -18.15 443 PHR20 47.6806 -117.1101 491,744.06 5,280,768.46 618.3 980,626.48 -90.08 -18.16 444 PHR21 47.6792 -117.1101 491,743.69 5,280,583.50 621.0 980,626.93 -89.79 -17.87 445 PHR22 47.6778 -117.1101 491,743.44 5,280,429.01 622.9 980,627.37 -89.69 -17.77 446 PHR23 47.6764 -117.1101 491,743.19 5,280,274.52 621.7 980,627.41 -89.88 -17.96 447 PHR24 47.6750 -117.1101 491,742.94 5,280,120.34 623.5 980,627.79 -89.72 -17.81 448 PHR25 47.6736 -117.1098 491,742.69 5,279,965.86 629.0 980,628.74 -89.34 -17.42 449 PHR26 47.6722 -117.1098 491,763.46 5,279,811.40 630.5 980,629.17 -89.31 -17.37 450 PHR27 47.6714 -117.1098 491,763.21 5,279,657.22 631.4 980,629.18 -88.98 -17.04 451 PHR28 47.6697 -117.1098 491,763.18 5,279,564.59 633.2 980,629.17 -88.44 -16.50 452 PIH01 47.8103 -116.8926 491,762.82 5,279,379.33 671.7 980,630.15 -93.72 -21.78 453 PIH02 47.8086 -116.8926 508,048.76 5,295,000.42 666.5 980,629.77 -94.36 -4.51 454 PIH03 47.8072 -116.8926 508,049.01 5,294,815.16 666.8 980,630.73 -95.13 -5.28

Count Point_ID DD_lat DD_long Easting Northing Elevation Obs Grav C.B.A. 1 C.B.A. 2 455 PIH04 47.8058 -116.8926 508,049.07 5,294,660.97 668.0 980,631.91 -95.90 -6.05 456 PIH05 47.8044 -116.8926 508,049.43 5,294,506.49 669.8 980,633.07 -96.53 -6.67 457 PIH06 47.8031 -116.8926 508,049.49 5,294,352.31 671.3 980,633.60 -96.59 -6.74 458 PIH07 47.8017 -116.8926 508,049.85 5,294,197.82 673.5 980,634.49 -96.88 -7.02 459 PIH08 47.8000 -116.8926 508,049.91 5,294,043.34 672.6 980,635.19 -97.65 -7.79 460 PIH09 47.7986 -116.8926 508,050.16 5,293,858.07 673.8 980,635.65 -97.70 -7.85 461 PIH10 47.7972 -116.8926 508,050.52 5,293,703.90 674.7 980,636.09 -97.81 -7.96 462 PIH11 47.7958 -116.8926 508,050.58 5,293,549.41 669.5 980,636.39 -99.14 -9.28 463 PIH12 47.7944 -116.8926 508,050.94 5,293,395.23 672.0 980,636.61 -98.68 -8.82 464 PIH13 47.7928 -116.8926 508,051.00 5,293,240.74 672.0 980,636.97 -98.91 -9.06 465 PIH14 47.7914 -116.8926 508,051.25 5,293,055.48 672.3 980,637.35 -99.12 -9.26 466 PIH15 47.7900 -116.8926 508,051.61 5,292,901.30 674.4 980,638.35 -99.52 -9.67 467 PIH16 47.7883 -116.8926 508,051.67 5,292,746.82 678.1 980,639.46 -99.71 -9.86 468 PIH17 47.7869 -116.8926 508,051.92 5,292,561.56 681.4 980,640.43 -99.83 -9.97 469 PIH18 47.7856 -116.8929 508,052.28 5,292,407.07 684.8 980,641.18 -99.72 -9.86 470 PIH19 47.7842 -116.8929 508,031.62 5,292,252.86 686.3 980,641.78 -99.86 -10.03 471 PIH20 47.7828 -116.8929 508,031.98 5,292,098.38 687.8 980,642.39 -100.00 -10.17 472 PIH21 47.7814 -116.8929 508,032.04 5,291,944.19 688.1 980,642.85 -100.27 -10.44 473 PIH22 47.7797 -116.8929 508,032.40 5,291,789.71 688.1 980,643.32 -100.61 -10.77 474 PIH23 47.7783 -116.8929 508,032.65 5,291,604.45 688.1 980,643.65 -100.81 -10.97 475 PIH24 47.7769 -116.8929 508,032.70 5,291,450.27 686.9 980,643.73 -101.02 -11.19 476 PIH25 47.7756 -116.8929 508,033.07 5,291,295.78 686.0 980,643.84 -101.22 -11.38 477 PIH26 47.7739 -116.8929 508,033.13 5,291,141.30 685.1 980,643.84 -101.30 -11.46 478 PIH27 47.7725 -116.8929 508,033.38 5,290,956.04 683.5 980,643.67 -101.33 -11.50 479 PIH28 47.7711 -116.8929 508,033.73 5,290,801.86 681.7 980,643.74 -101.68 -11.85 480 PIH29 47.7697 -116.8929 508,033.79 5,290,647.37 682.6 980,644.10 -101.71 -11.88 481 PIH30 47.7683 -116.8929 508,034.15 5,290,493.19 682.6 980,644.40 -101.75 -11.91 482 PIH31 47.7667 -116.8929 508,034.21 5,290,338.70 683.2 980,644.73 -101.95 -12.11 483 PIH32 47.7653 -116.8929 508,034.46 5,290,153.44 683.2 980,645.12 -102.21 -12.37 484 PIH33 47.7639 -116.8929 508,034.82 5,289,998.96 683.8 980,645.32 -102.15 -12.31 485 PIH34 47.7625 -116.8929 508,034.88 5,289,844.78 682.9 980,645.24 -102.15 -12.31 486 PIH35 47.7611 -116.8929 508,035.24 5,289,690.29 680.8 980,645.14 -102.39 -12.55 487 PIH36 47.7594 -116.8929 508,035.29 5,289,536.11 681.1 980,645.26 -102.51 -12.67 488 PIH37 47.7581 -116.8929 508,035.54 5,289,350.85 683.8 980,645.75 -102.05 -12.21 489 PIH38 47.7567 -116.8929 508,035.90 5,289,196.37 685.1 980,645.91 -101.81 -11.97 490 PIH39 47.7553 -116.8929 508,035.96 5,289,042.18 684.8 980,646.06 -101.90 -12.06 491 PIH40 47.7539 -116.8929 508,036.32 5,288,887.70 683.8 980,646.00 -101.91 -12.07 492 PIH41 47.7522 -116.8929 508,036.38 5,288,733.21 682.0 980,645.78 -101.98 -12.14 493 PIH42 47.7508 -116.8929 508,036.63 5,288,547.95 681.1 980,645.78 -102.05 -12.21 494 PIH43 47.7494 -116.8932 508,036.98 5,288,393.77 680.5 980,645.93 -102.20 -12.36 495 PIH44 47.7481 -116.8932 507,995.60 5,288,239.23 679.0 980,646.36 -102.84 -13.05 496 PIH45 47.7467 -116.8932 507,995.65 5,288,085.04 679.0 980,646.43 -102.79 -12.99 497 PIH46 47.7453 -116.8932 507,996.02 5,287,930.56 679.0 980,646.66 -102.88 -13.09 498 PIH47 47.7436 -116.8932 507,996.07 5,287,776.38 679.0 980,646.73 -102.82 -13.02 499 PIH48 47.7422 -116.8932 507,996.32 5,287,591.12 679.0 980,646.99 -102.95 -13.15 500 PIH49 47.7408 -116.8932 507,996.68 5,287,436.63 679.0 980,647.15 -102.97 -13.18 501 PIH50 47.7394 -116.8932 507,996.74 5,287,282.15 678.7 980,647.37 -103.13 -13.33 502 PIH51 47.7381 -116.8932 507,997.09 5,287,127.97 678.1 980,647.39 -103.40 -13.60 503 PIH52 47.7364 -116.8932 507,997.15 5,286,973.48 677.4 980,647.50 -103.27 -13.47 504 PIH53 47.7350 -116.8932 507,997.71 5,286,788.22 676.5 980,647.50 -104.04 -14.24 505 PIH54 47.7336 -116.8932 507,997.76 5,286,634.04 675.0 980,647.32 -103.37 -13.57 506 PIH55 47.7322 -116.8932 507,998.12 5,286,479.56 674.7 980,647.26 -103.25 -13.45 507 PIH56 47.7306 -116.8932 507,998.17 5,286,325.38 673.8 980,647.12 -103.18 -13.38 508 PIH57 47.7292 -116.8932 507,998.42 5,286,140.11 673.2 980,646.80 -102.87 -13.07 509 PIH58 47.7278 -116.8932 507,998.78 5,285,985.63 672.9 980,646.68 -102.68 -12.89 510 PIH59 47.7264 -116.8932 507,998.84 5,285,831.14 672.9 980,646.60 -102.48 -12.68 511 PIH60 47.7250 -116.8932 507,999.19 5,285,676.97 672.9 980,646.42 -102.16 -12.36 512 PIH61 47.7233 -116.8932 507,999.25 5,285,522.48 673.2 980,646.36 -101.86 -12.06 513 PIH62 47.7219 -116.8932 507,999.50 5,285,337.22 672.6 980,646.24 -101.73 -11.93

18 19 Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Adema, Breckenridge, and Sprenke Adema, Breckenridge, and Sprenke

Count Point_ID DD_lat DD_long Easting Northing Elevation Obs Grav C.B.A. 1 C.B.A. 2 514 PIH63 47.7206 -116.8932 507,999.85 5,285,183.04 672.0 980,646.05 -101.52 -11.72 515 PIH64 47.7192 -116.8932 507,999.91 5,285,028.55 670.7 980,645.56 -101.14 -11.34 516 PIH65 47.7178 -116.8932 508,000.27 5,284,874.37 668.9 980,645.12 -100.95 -11.15 517 PIH66 47.7161 -116.8932 508,000.32 5,284,719.89 668.0 980,644.94 -100.83 -11.03 518 PIH67 47.7147 -116.8932 508,000.57 5,284,534.63 666.5 980,645.05 -101.14 -11.34 519 PIH68 47.7133 -116.8932 508,000.93 5,284,380.14 666.5 980,645.36 -101.32 -11.52 520 PIH69 47.7122 -116.8934 508,000.98 5,284,225.96 670.7 980,646.17 -101.05 -11.25 521 PIH70 47.7114 -116.8937 508,001.15 5,284,102.55 672.3 980,646.38 -100.78 -10.98 522 PIH71 47.7103 -116.8940 507,980.40 5,284,009.89 670.7 980,646.03 -100.65 -10.87 523 PIH72 47.7094 -116.8943 507,959.85 5,283,886.15 669.2 980,645.25 -100.07 -10.32 524 PIH73 47.7086 -116.8934 507,939.10 5,283,793.49 669.8 980,645.28 -99.78 -10.05 525 PIH74 47.7072 -116.8929 508,001.84 5,283,700.96 667.7 980,644.44 -99.29 -9.48 526 PIH75 47.7061 -116.8929 508,043.64 5,283,546.84 664.0 980,643.01 -98.49 -8.64 527 PIH76 47.7044 -116.8929 508,043.80 5,283,423.43 661.6 980,641.71 -97.57 -7.72 528 PIH77 47.7031 -116.8929 508,044.05 5,283,238.17 663.7 980,641.46 -96.67 -6.82 529 PIH78 47.7017 -116.8929 508,044.41 5,283,083.69 662.8 980,641.47 -96.73 -6.88 530 PIH79 47.6997 -116.8926 508,044.46 5,282,929.20 662.2 980,641.36 -96.60 -6.75 531 PIH80 47.6983 -116.8929 508,065.62 5,282,713.20 658.5 980,640.51 -96.43 -6.56 532 PIH81 47.6967 -116.8929 508,044.95 5,282,558.98 649.4 980,638.25 -96.08 -6.23 533 PIR01 47.7439 -117.0470 508,045.20 5,282,373.72 665.5 980,631.25 -89.71 0.14 534 PIR02 47.7422 -117.0464 496,480.69 5,287,617.52 665.5 980,630.76 -89.12 -11.99 535 PIR03 47.7408 -117.0464 496,522.38 5,287,432.33 665.5 980,630.91 -89.15 -11.97 536 PIR04 47.7394 -117.0464 496,522.13 5,287,277.84 657.3 980,629.98 -89.96 -12.78 537 PIR05 47.7381 -117.0464 496,522.19 5,287,123.35 649.4 980,628.22 -89.87 -12.69 538 PIR06 47.7364 -117.0464 496,521.94 5,286,969.17 647.0 980,627.99 -90.03 -12.85 539 PIR07 47.7353 -117.0464 496,521.88 5,286,783.91 643.6 980,627.97 -90.63 -13.46 540 PIR08 47.7336 -117.0464 496,521.74 5,286,660.50 643.6 980,628.48 -91.03 -13.85 541 PIR09 47.7322 -117.0464 496,521.69 5,286,475.24 642.4 980,628.64 -91.30 -14.13 542 PIR10 47.7308 -117.0464 496,521.75 5,286,320.76 642.4 980,629.07 -91.61 -14.43 543 PIR11 47.7294 -117.0464 496,521.50 5,286,166.27 642.4 980,629.53 -91.94 -14.76 544 PIR12 47.7278 -117.0462 496,521.55 5,286,012.09 642.1 980,629.71 -92.06 -14.88 545 PIR13 47.7261 -117.0464 496,542.21 5,285,826.86 641.8 980,629.84 -92.12 -14.93 546 PIR14 47.7247 -117.0462 496,521.13 5,285,641.56 640.5 980,629.72 -92.15 -14.97 547 PIR15 47.7233 -117.0462 496,541.91 5,285,487.11 640.5 980,630.00 -92.31 -15.11 548 PIR16 47.7217 -117.0462 496,541.96 5,285,332.93 638.7 980,629.73 -92.33 -15.13 549 PIR17 47.7203 -117.0462 496,541.60 5,285,147.66 635.4 980,629.49 -92.70 -15.50 550 PIR18 47.7186 -117.0462 496,541.66 5,284,993.18 636.0 980,629.66 -92.60 -15.41 551 PIR19 47.7172 -117.0462 496,541.60 5,284,807.92 634.8 980,629.84 -92.92 -15.73 552 PIR20 47.7156 -117.0462 496,541.35 5,284,653.74 634.8 980,630.13 -93.07 -15.88 553 PIR21 47.7142 -117.0462 496,541.29 5,284,468.48 636.3 980,630.27 -92.74 -15.54 554 PIR22 47.7125 -117.0462 496,541.35 5,284,313.99 637.5 980,630.34 -92.40 -15.21 555 PIR23 47.7111 -117.0462 496,541.29 5,284,128.73 637.5 980,630.13 -92.24 -15.05 556 PIR24 47.7094 -117.0462 496,541.04 5,283,974.55 638.4 980,630.02 -91.52 -14.32 557 PIR25 47.7081 -117.0462 496,540.98 5,283,789.29 639.3 980,629.88 -91.03 -13.84 558 PIR26 47.7067 -117.0462 496,540.73 5,283,634.80 639.3 980,629.78 -90.77 -13.58 559 PIR27 47.7050 -117.0462 496,540.78 5,283,480.62 639.6 980,629.51 -90.29 -13.10 560 PIR28 47.7036 -117.0462 496,540.73 5,283,295.36 639.6 980,629.07 -89.82 -12.63 561 PIR29 47.7019 -117.0462 496,540.48 5,283,140.87 639.3 980,628.83 -89.49 -12.30 562 PIR30 47.7003 -117.0462 496,540.42 5,282,955.61 638.4 980,628.34 -89.05 -11.86 563 PIR31 47.6967 -117.0470 496,540.36 5,282,770.35 628.7 980,626.10 -88.64 -11.45 564 PIR32 47.6953 -117.0467 496,477.67 5,282,369.26 630.2 980,626.61 -88.67 -11.54 565 PIR33 47.6936 -117.0467 496,498.44 5,282,214.81 629.6 980,626.50 -88.56 -11.42 566 PIR34 47.6925 -117.0464 496,498.08 5,282,029.54 630.2 980,626.37 -88.18 -11.03 567 PMA01 47.6697 -117.1098 496,518.96 5,281,906.17 631.4 980,629.14 -88.81 -11.64 568 PMA02 47.6706 -117.1074 491,762.82 5,279,379.33 631.4 980,629.60 -89.33 -17.39 569 PMA03 47.6708 -117.1052 491,929.83 5,279,471.60 630.8 980,629.27 -89.20 -17.07 570 PMA04 47.6714 -117.1033 492,096.61 5,279,502.33 630.8 980,629.04 -89.01 -16.71 571 PMA05 47.6714 -117.1011 492,242.79 5,279,563.80 631.7 980,629.94 -89.71 -17.25 572 PMA06 47.6714 -117.0986 492,409.46 5,279,563.74 632.3 980,630.62 -90.25 -17.60

Count Point_ID DD_lat DD_long Easting Northing Elevation Obs Grav C.B.A. 1 C.B.A. 2 573 PMA07 47.6714 -117.0964 492,597.16 5,279,563.42 633.8 980,631.41 -90.70 -17.85 574 PMA08 47.6714 -117.0943 492,764.14 5,279,563.37 636.0 980,632.19 -90.97 -17.93 575 PMA09 47.6714 -117.0921 492,930.82 5,279,563.01 639.0 980,633.00 -91.10 -17.88 576 PMA10 47.6714 -117.0902 493,076.77 5,279,562.93 640.2 980,633.70 -91.53 -18.14 577 PMA11 47.6714 -117.0883 493,222.73 5,279,562.54 641.8 980,633.78 -91.27 -17.73 578 PMA12 47.6714 -117.0861 493,368.68 5,279,562.46 643.9 980,633.89 -90.91 -17.20 579 PMA13 47.6714 -117.0842 493,535.66 5,279,562.41 644.2 980,633.55 -90.49 -16.60 580 PMA14 47.6714 -117.0817 493,681.61 5,279,562.02 647.6 980,633.59 -89.76 -15.71 581 PMA15 47.6714 -117.0795 493,869.31 5,279,562.00 648.5 980,633.30 -89.27 -15.01 582 PMA16 47.6714 -117.0773 494,035.99 5,279,561.64 648.5 980,632.97 -88.92 -14.48 583 PMA17 47.6714 -117.0754 494,202.97 5,279,561.60 648.2 980,632.50 -88.52 -13.90 584 PMA18 47.6714 -117.0732 494,348.92 5,279,561.51 648.8 980,631.66 -87.52 -12.74 585 PMA19 47.6714 -117.0710 494,494.87 5,279,561.43 647.9 980,630.85 -86.77 -11.82 586 PMA20 47.6714 -117.0689 494,661.55 5,279,561.07 649.4 980,631.16 -86.44 -11.31 587 PMA21 47.6714 -117.0667 494,828.53 5,279,561.02 646.0 980,630.49 -86.51 -11.20 588 PMA22 47.6714 -117.0653 494,995.20 5,279,560.97 649.4 980,630.97 -86.24 -10.75 589 PMR01 47.6700 -117.0885 495,099.72 5,279,560.83 644.5 980,634.42 -91.19 -15.58 590 PMR02 47.6683 -117.0885 493,347.70 5,279,408.25 648.8 980,635.64 -91.31 -17.63 591 PMR03 47.6669 -117.0885 493,347.65 5,279,222.99 653.0 980,636.66 -91.25 -17.57 592 PMR04 47.6656 -117.0885 493,347.40 5,279,068.50 653.3 980,637.08 -91.46 -17.78 593 PMR05 47.6642 -117.0885 493,347.14 5,278,914.32 651.8 980,637.23 -91.81 -18.13 594 PMR06 47.6625 -117.0885 493,346.89 5,278,759.83 645.1 980,635.89 -91.82 -18.14 595 PMR07 47.6611 -117.0885 493,346.83 5,278,574.57 641.5 980,635.11 -91.72 -18.03 596 PMR08 47.6597 -117.0885 493,346.58 5,278,420.39 641.8 980,635.42 -91.83 -18.15 597 PMR09 47.6583 -117.0885 493,346.33 5,278,265.90 642.1 980,635.39 -91.58 -17.90 598 PMR10 47.6567 -117.0885 493,346.38 5,278,111.42 636.0 980,634.18 -91.57 -17.88 599 PMR11 47.6556 -117.0885 493,346.01 5,277,926.46 632.0 980,632.97 -91.11 -17.43 600 PNL01 47.7439 -117.0470 493,345.87 5,277,802.75 665.5 980,631.24 -89.91 -16.23 601 PNL02 47.7439 -117.0489 496,480.69 5,287,617.52 665.5 980,631.38 -90.09 -12.96 602 PNL03 47.7439 -117.0511 496,335.04 5,287,617.61 666.2 980,631.51 -90.19 -13.22 603 PNL04 47.7439 -117.0533 496,168.37 5,287,617.66 666.2 980,631.41 -90.10 -13.32 604 PNL05 47.7439 -117.0555 496,001.70 5,287,617.71 971.0 980,631.68 -90.38 -13.78 605 PNL06 47.7439 -117.0574 495,835.33 5,287,617.76 665.8 980,631.81 -90.57 -14.15 606 PNL07 47.7439 -117.0593 495,689.37 5,287,617.84 664.3 980,632.06 -91.14 -14.89 607 PNL08 47.7439 -117.0615 495,564.44 5,287,617.96 662.2 980,631.50 -91.05 -14.93 608 PNL09 47.7439 -117.0637 495,397.77 5,287,618.31 651.8 980,628.89 -90.74 -14.80 609 PNL10 47.7439 -117.0656 495,231.40 5,287,618.37 650.6 980,628.29 -90.41 -14.66 610 PNL11 47.7439 -117.0678 495,085.44 5,287,618.45 651.5 980,628.21 -90.13 -14.53 611 PNL12 47.7456 -117.0678 494,919.08 5,287,618.50 651.5 980,628.23 -90.15 -14.74 612 PNL13 47.7469 -117.0678 494,919.13 5,287,803.76 650.9 980,627.95 -90.12 -14.71 613 PNL14 47.7483 -117.0678 494,919.38 5,287,958.25 650.6 980,627.71 -90.08 -14.66 614 PNL15 47.7497 -117.0678 494,919.33 5,288,112.43 650.6 980,627.63 -90.13 -14.72 615 PNL16 47.7511 -117.0678 494,919.57 5,288,266.92 650.3 980,627.46 -90.16 -14.74 616 PNL17 47.7525 -117.0678 494,919.52 5,288,421.40 650.0 980,627.37 -90.25 -14.84 617 PNL18 47.7539 -117.0680 494,919.77 5,288,575.59 649.4 980,627.06 -90.20 -14.78 618 PNL19 47.7553 -117.0680 494,898.99 5,288,730.04 649.1 980,626.78 -90.11 -14.72 619 PNL20 47.7569 -117.0680 494,899.24 5,288,884.53 648.8 980,626.62 -90.15 -14.76 620 PNL21 47.7583 -117.0680 494,899.30 5,289,069.48 649.4 980,626.46 -89.98 -14.59 621 PNL22 47.7597 -117.0689 494,899.54 5,289,223.97 648.5 980,626.13 -89.97 -14.58 622 PNL23 47.7608 -117.0699 494,837.33 5,289,378.36 647.6 980,625.99 -90.14 -14.82 623 PNL24 47.7622 -117.0710 494,753.98 5,289,501.95 648.2 980,625.89 -90.01 -14.78 624 PNL25 47.7633 -117.0721 494,671.04 5,289,656.31 648.5 980,625.79 -89.96 -14.82 625 PNL26 47.7644 -117.0732 494,587.69 5,289,779.89 648.5 980,625.34 -89.64 -14.60 626 PNL27 47.7656 -117.0743 494,504.64 5,289,903.48 648.8 980,625.15 -89.51 -14.56 627 PNL28 47.7669 -117.0754 494,442.31 5,290,027.09 648.8 980,625.08 -89.56 -14.67 628 PNL29 47.7683 -117.0757 494,359.07 5,290,181.45 648.8 980,624.52 -88.99 -14.19 629 PNL30 47.7700 -117.0762 494,317.58 5,290,335.87 650.0 980,624.17 -88.41 -13.66 630 PNL31 47.7711 -117.0765 494,297.22 5,290,521.11 650.6 980,623.34 -87.21 -12.48

20 21 Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Adema, Breckenridge, and Sprenke Adema, Breckenridge, and Sprenke

standard gravity reductions to his data, we chose to work Bouguer, and terrain were applied to standard gravity directly from his original data to ensure that identical measurements but could not be checked. corrections were applied to all observations. Purves’ original data and accompanying notes were provided NEW OBSERVATIONS to us by the Washington State Department of Natural Resources (Stephen Palmer, written commun., 1997). In 1997, we undertook 146 gravity measurements along three main transects in the Rathdrum Prairie as This information included the following data: gravity well as at the mouths of Hayden Lake and Coeur d’Alene readings, elevations, times, locations, temperatures, base Lake (Figure 4). The primary transects were along Idaho readings, and meter constants. It also contained Purves’ Road (Idaho) and Hayden Avenue with five smaller notes on how he collected and recorded his findings. transects near the mouths of Hayden Lake and Coeur He logged horizontal positions using an automobile’s d’Alene Lake. odometer and marked in tenths of a mile. He noted vertical positions with an altimeter, and these were Measurements were read with a Lacoste and Romberg verified daily against local benchmarks at numerous model G gravity meter (no. 1069). A standard base plate places. He monitored elevation drift due to changes in was used. At least two measurements were taken for each barometric pressure by allowing no more than a 0.03 station, with the requirement that they agree to within percent disparity in elevations between local benchmarks. 0.01 mGal. These were read at the base station at least His correction correlated to a vertical accuracy of ± 8 cm three times a day to monitor instrument drift. The base (approx.). The horizontal locations were plotted by us station was a U.S. Geological Survey first-order, second- on a topographic map from the details of starting points class benchmark—designation P285 on the Rathdrum for each line. We digitally sampled the plotted points to 7.5-minute quadrangle, near the corner of Idaho Highway obtain coordinates for each gravity station. Therefore, 53 and Greensferry Road, about 0.6 km from Rathdrum. the accuracy of the station positions can be no better than Principal facts for the benchmark are given in Table 2. 1/10 of a mile according to the odometer. Because of the All gravity measurements are thought to be precise to at Rathdrum Prairie’s relatively consistent trends and the least 0.01 mGal before correction. substantial amount of additional data being considered, we are confident that Purves’ locations are sufficient for Coordinates and elevations for each gravity station our study. We have no reason to suspect inconsistent were obtained using the differential Global Positioning levels of precision or accuracy in the data. System (GPS) technique. Leica SR 399 receivers were used, with the previously described benchmark CADY AND MEYER DATA as a coordinate tie to the geodetic system. With these techniques, horizontal and vertical accuracy is at least In their geologic study of uranium deposits in the ± 2 cm (J.S. Oldow, oral commun., 1997). Normally, a region, Cady and Meyer (1976a) compiled the principal 2-cm-vertical variation should result in no more than a facts for 2,077 gravity stations around Spokane, 0.01 mGal gravity variation. Measurements were Washington. From these data, they constructed a Bouguer recorded on level road surfaces, with the GPS receiver gravity anomaly map of the Okanagan, Sandpoint, and gravity meter at the same elevation. Ritzville, and Spokane 1° x 2° quadrangles (Cady and Meyer, 1976b). The principal facts for the stations were The field survey was designed with two goals: to cover obtained from various sources, including USGS open- previously uncovered sections of the Rathdrum Prairie, file reports and the U.S. Department of Defense’s gravity and to enable a meaningful comparison and check of library. Data from investigations by Bonini (1963) and Purves’ (1969) data. The Idaho Road (Idaho) profile fits Hammond (1975) were included; data from Purves between two profiles of Purves (1969), Corbin Road and (1969) were not. The Cady and Meyer (1976a) principal Idaho Highway 41. These three profiles, on the western facts were obtained in digital form from Hittelman and side of the aquifer, are constrained to the north and south others (1994), a CD-ROM collection of significant by bedrock exposure, which reduces the uncertainty gravity data sets. of the profile’s geometry at these locations. The Idaho Road (Idaho) profile provides a meaningful comparison We used 206 stations from this data set (Figure 4). with the data of Purves (1969), indirectly verifying their They are spaced generally one per square mile over validity. The Hayden Avenue profile is oriented east-west the study area to complement the profiles measured by and designed to tie the north-south profiles and improve Purves (1969) and us. Unfortunately, raw gravity data the model of the east-west trend. The difficulty, to be were unavailable. Corrections for drift, latitude, free-air, discussed later, is its orientation with the regional trend

Table 2. Principal Facts for Primary Benchmark.

The NGS Data Sheet DATABASE = Sybase ,PROGRAM = datasheet, VERSION = 5.70 Starting Datasheet Retrieval. 1 National Geodetic Survey, Retrieval Date = NOVEMBER 29, 1998 SV0371 ***************************************************************** SV0371 DESIGNATION - P 285 SV0371 PID - SV0371 SV0371 STATE/COUNTY- ID/KOOTENAI SV0371 USGS QUAD - RATHDRUM (1986) SV0371 SV0371 *CURRENT SURVEY CONTROL SV0371 ______SV0371* NAD 83(1986)- 47 48 08. (N) 116 54 56. (W) SCALED SV0371* NAVD 88 - 665.103 (meters) 2182.09 (feet) ADJUSTED SV0371 ______SV0371 GEOID HEIGHT- -17.41 (meters) GEOID96 SV0371 DYNAMIC HT - 665.142 (meters) 2182.22 (feet) COMP SV0371 MODELED GRAV- 980,649.1 (mgal) NAVD 88 SV0371 SV0371 VERT ORDER - FIRST CLASS II SV0371 SV0371.The horizontal coordinates were scaled from a topographic map and have SV0371.an estimated accuracy of +/- 6 seconds. SV0371 SV0371.The orthometric height was determined by differential leveling SV0371.and adjusted by the National Geodetic Survey in June 1991. SV0371 SV0371.The geoid height was determined by GEOID96. SV0371 SV0371.The dynamic height is computed by dividing the NAVD 88 SV0371.geopotential number by the normal gravity value computed on the SV0371.Geodetic Reference System of 1980 (GRS 80) ellipsoid at 45 SV0371.degrees latitude (G = 980.6199 gals.). SV0371 SV0371.The modeled gravity was interpolated from observed gravity values. SV0371 SV0371; North East Units Estimated Accuracy SV0371;SPC ID W - 682,440. 712,690. MT (+/- 180 meters Scaled) SV0371 SV0371 SUPERSEDED SURVEY CONTROL SV0371 SV0371 NGVD 29 - 663.931 (m) 2178.25 (f) ADJ UNCH 1 2 SV0371 SV0371.Superseded values are not recommended for survey control. SV0371.NGS no longer adjusts projects to the NAD 27 or NGVD 29 datums. SV0371.See file format.dat to determine how the superseded data were derived. SV0371 SV0371_MARKER: DD = SURVEY DISK SV0371_SETTING: 7 = SET IN TOP OF CONCRETE MONUMENT (ROUND) SV0371_STAMPING: P 285 1944 P.C. 395+90.8 40.00 SV0371_STABILITY: C = MAY HOLD, BUT OF TYPE COMMONLY SUBJECT TO SV0371+STABILITY: SURFACE MOTION SV0371

SV0371 HISTORY - Date Condition Recov. By SV0371 HISTORY - 1944 MONUMENTED IDDT SV0371 HISTORY - 1944 GOOD NGS SV0371 HISTORY - 1980 GOOD SV0371 SV0371 STATION DESCRIPTION SV0371 SV0371’DESCRIBED BY NATIONAL GEODETIC SURVEY 1944 SV0371’1.0 MI SW FROM RATHDRUM. SV0371’1.0 MILE SOUTHWEST ALONG STATE HIGHWAY NO. 53 FROM THE NORTHERN SV0371’PACIFIC RAILROAD STATION AT RATHDRUM, ABOUT 0.1 MILE NORTHWEST OF THE SV0371’TRACK, 89 FEET WEST OF CENTER-LINE OF A SIDE ROAD LEADING SOUTH AND SV0371’ACROSS THE TRACK, 40 FEET SOUTHEAST OF CENTER-LINE OF HIGHWAY, 2 FEET SV0371’NORTHEAST OF REFERENCE POST. A IDAHO STATE HIGHWAY BRONZE SV0371’RIGHT-OF-WAY DISK SET IN TOP OF A CONCRETE POST PROJECTING ABOUT 0.4 SV0371’FOOT ABOVE THE GROUND. SV0371 SV0371 STATION RECOVERY (1980) SV0371 SV0371’RECOVERED 1980 SV0371’RECOVERED IN GOOD CONDITION.

of the gravity field and its location over a rather distinct a relative sense and should not be confused with an non-two-dimensional basin. Typical spacing was about absolute gravity measurement. For our study, the tie to 300 m between gravity stations. This spacing was based the local standard was a final step of the data reduction. on an analysis of the profiles presented in Purves (1969). The meter readings were converted to mGal with Small-scale variations identified in Bouguer gravity a calibration table provided by Lacoste & Romberg profiles of Purves (1969) are equally prominent when specific to our gravity meter G-1069 for a given gravity sampled every 300 m, as opposed to the approximate meter temperature. Purves (1969) used a standard 150-m spacing that he typically used. Predictive forward Worden gravity meter and had to multiply readings by the modeling also confirmed the efficacy of a 300-m spacing meter constant of 0.09869-mGal/scale division. Original in identifying the bedrock-sediment interface that has information on meter readings was not available for the been presumed to exist at depths by previous authors data from Cady and Meyer (1976a). (Newcomb, 1953; Hammond, 1974; Gerstel and Palmer, 1994). TIDAL AND INSTRUMENT DRIFT CORRECTIONS GRAVITY DATA REDUCTION All instrumental readings in the field require a Because our survey is using gravity measurements correction to compensate for the effects of instrument from the two previous studies, various reductions to all drift and earth tides. Instrument drift may occur with these field measurements are necessary to properly model the Lacoste & Romberg gravity meter even though and interpret the combined gravity data. Reductions its integral parts are housed in a vacuum at a fixed must be consistent for each data set in the final product temperature. Readings are corrected for the tidal drift to ensure that artifacts from inconsistent reduction do not caused by the variable forces applied to the earth by the appear as anomalies on the final gravity map. sun and the moon.

INSTRUMENT CALIBRATION After the tidal drift was removed, instrument drift CORRECTION was removed by calibrating the measurements to a fixed value at the previously introduced benchmark P285. Every gravity meter has a unique calibration function This instrument drift was never greater than 0.10 mGal that is determined shortly after its production. This over one day. Data from Cady and Meyer (1976a) were calibration allows readings to be converted to a milliGal received already corrected for drift, but with no way to (mGal) scale. This scale is accurate, however, only in verify the accuracy of those calculations. Purves (1969)

22 23 Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Adema, Breckenridge, and Sprenke Adema, Breckenridge, and Sprenke properly corrected for combined tidal and instrument could not be obtained. The DEM-based, automated drift values as much as 0.10 mGal/day. procedure is composed of three software routines: one that combines and reorganizes local 1:24,000 DEMs, a TERRAIN CORRECTIONS second that uses the reduced DEMs and any additional data provided by the user (GPS points, etc.) to calculate The Rathdrum Prairie is a relatively flat plain with “inner-zone” terrain corrections, and a third that applies elevation variations generally less than 50 m. These local the “outer-zone” terrain corrections from a specially variations in topography, as well as the surrounding hills, produced DEM that provides regional coverage. have an impact on the gravity and must be considered with a terrain correction procedure. The nearby hills The digital elevation models supplied by the USGS are give an upward component of gravitational attraction preprocessed for use by the terrain correction software. that counteracts a part of the downward pull exerted A routine called DEMREAD, written by Alan H. Cogbill by the rest of the earth. Conversely, the surrounding at the Los Alamos National Laboratory, performs the valleys below station elevations, which can be modeled required algorithms that combine numerous DEMs into as holes in the theoretically continuous slab that extends one large, project specific DEM. The preprocessing to the datum, produce a smaller downward pull than is enhances the execution speed of the terrain correction calculated with the Bouguer correction. program and reduces the data storage space of the DEM file for that program to about 30 percent of the ASCII In the past, these corrections followed the method data size provided by the USGS. Table 3 lists the USGS developed by Hammer (1939) that calculated the 7.5-minute DEMs (30-m x 30-m data spacing cast on gravitational effect of flat-topped, cylindrical sections a Universal Transverse Mercator projection) that were or sloping planes, both based on elevations manually input to DEMREAD for combination and reduction to read from topographic maps. Other methods developed binary format. Surface coverage was required to a radius from Hammer’s original work include one for improved of 2,000 m around each gravity station. This radius for sloping planes (Sandberg, 1958) and another for inner-zone corrections is suggested by Cogbill (1997) for improved conical prisms (Olivier and Simard, 1981). regions of nonextreme topographic relief. All of the DEMs The accuracy of these methods depends on how closely used are classified as level 1 or 2, correlating to a vertical a particular geometrical model conforms to the actual RMS error of 7-15 m or 3 m, respectively. As a check of terrain near a gravity station. Today, data manipulation the DEMs’ vertical accuracy, twenty GPS measurements by computers, combined with the availability of low- were taken within 2.5 m of DEM data points. Assuming price 30-m digital elevation models (DEMs) produced that the GPS measurements were accurate in the vertical by the U.S. Geological Survey’s (USGS) National direction within 0.05 m, the average vertical error of the Cartographic Information Center, allows for a more DEM points that were near the GPS measurements was accurate and less arduous method of terrain correction. 3.4 m. Although this variation appears large, it agrees Methods developed by Plouff (1966) and Cogbill (1990) with the 3-m RMS error reported by the USGS for level that use the exhaustive surface coverage provided by 1 DEMs and is well below the 7- to 15-m RMS error DEMs have an accuracy that depends mostly on how well reported for level 2 DEMs. the DEM represents the terrain near the gravity station. Corrections calculated using the DEMs are potentially The terrain corrections are decomposed into two more accurate than hand procedures because of the more parts: inner-zone and outer-zone corrections. Inner-zone subjective manual methods. corrections account for topographic variations very close to the gravity stations (5 m) out to a specified distance Because data from the three sources (ours; Purves, (2,000 m), whereas outer-zone corrections are based on 1969; Cady and Meyer, 1976a) are to be compared with coarser terrain data, and broader surface fitting methods each other and presented as one product, the same terrain are calculated. The inner-zone corrections use a procedure correction technique would ideally be applied uniformly. explained in Cogbill (1990) that takes advantage of the This would avoid any inconsistencies between manual USGS 7.5-minute DEMs. The inner zone is further estimation methods and automated DEM-based divided into an inner zone (radius of 250 m) and medium techniques. Unfortunately, nonterrain corrected data zone (radius 250-2,000 m). A continuously differentiable from Cady and Meyer (1976a) were unavailable and surface is fit to the elevation data within the smaller inner therefore not re-corrected in relation to the other data zone. This surface is integrated numerically to obtain that sets. Furthermore, parameters of the terrain corrections portion of the overall terrain correction. Mathematically, performed on the data from Cady and Meyer (1976a), the algorithm effectively integrates a line element between including Hammer zones and correction densities, the station and the medium zone, repeated every 3

Table 3. Digital Elevation Models. more than 18 km from each gravity station. Outer terrain corrections ranged from approximately 0.17 mGal for USGS DEM Level points in the central part of the prairie to 0.78 mGal for Athol, ID 1 points in the surrounding hills. Fernan Lake, ID 2 Greenacres, WA 2 LATITUDE AND ELEVATION Hayden Lake, ID 1 Hayden, ID 2 CORRECTIONS Liberty Lake, WA-ID 2 Mica Bay, ID 1 Because the earth is not a perfect, nonrotating sphere Mica Peak, ID 1 but has an equatorial bulge and significant rotation, the Coeur d’Alene, ID 1 Mt. Coeur d’Alene, ID 2 effects of latitude must be considered so that gravity Bayview, ID 1 adjustments can be compared. Centrifugal acceleration Newman Lake, ID 1 due to rotation, maximum at the equator and minimum Post Falls, ID 2 at the poles, acts to oppose gravitational acceleration. Rathdrum, ID 2 Rockford Bay, ID 1 Conversely, polar flattening acts to increase gravity Spirit Lake East, ID 1 at the poles by making the geoid closer to the earth’s center of mass. The latitude adjustment is calculated by differentiating the Geodetic Reference System (GRS degrees of azimuth. In the medium zone, the terrain effect 1967) formula. is calculated by numerically integrating the elevation data using a rectangular integration rule. Cogbill (1990) Because gravity varies inversely with distance from estimates that integration errors are always less than the center of the earth, it is necessary to apply the free-air 0.001 mGal, but notes that the calculated corrections correction, which reduces all readings to a datum surface. can be no better than the elevation data used to represent The correction is 0.3086 mGal/m (Dobrin, 1976). the terrain about each gravity station. The accuracy of the calculated corrections is also limited by the inherent BOUGUER CORRECTIONS uncertainty of the estimated mean terrain density. The Bouguer correction removes the effect of a The inner-zone corrections from all three data sets were presumed infinite slab of material between the horizontal conducted using the method of Cogbill (1990), through plane of each station and a datum plane. The correction the software package, INNERTC, which he developed. factor is -0.112 mGal/m above the datum, assuming an The terrain corrections ranged from approximately 0.02 average density for crustal rocks of 2,670 kg-m-3 (Dobrin, mGal in the proximal regions of the prairie to ± 1.20 1976). mGal in the surrounding hills. A correction density (r) of 2.68 g-cm-3 was used. By applying the reductions listed above to the field measured data, the Bouguer anomaly is produced. Outer-zone corrections were calculated using a The Bouguer anomaly is the observed value of gravity method developed by Plouff (1966) and modified by minus the theoretical value at the latitude and elevation Alan Cogbill in his software package, OUTERTC. of the observation point. This allows for variations in The correction procedure fits a multiquadric surface the Bouguer anomaly to be interpreted and modeled in to elevation data from USGS 3-arcsec DEMs, and it relation to variations in subsurface geologic features. calculates the effect of this surface on each gravity measurement. A compilation of the entire set of USGS The Bouguer anomaly map produced by our study 3-arcsec DEMs was provided by Cogbill, with the is shown in Figure 6. The Bouguer anomaly has a accompanying program MAPFILE that selectively fairly straightforward basinal appearance, except for a removes the nonrequired DEMs and formats the significant overriding, east-west regional trend. remaining DEMs for input to OUTERTC. Outer-zone corrections cover the distance from the outermost radius CORRELATION OF used in INNERTC (4,000 m for our study), to a maximum DATA SETS radius of 111 km. The standard maximum radius of 167 km would have been preferred, but significant The data from all three sources must be properly complications arise with the integration of Canadian correlated. The data from Cady and Meyer (1976a) are DEMs. A correction density of r=2670 kg-m-3 was used. referenced to GRS 1967. The data from Purves (1969) The earth’s curvature is incorporated for elevation data were referenced to the control network of North America

24 25 Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Adema, Breckenridge, and Sprenke Adema, Breckenridge, and Sprenke established by Wollard and Behrendt (1961). No control REMOVAL OF point was available for our study, so this correlation REGIONAL TREND is used to reference the data to GRS 1967. To insure consistency between the three data sets, eight locations Any pattern seen on the Bouguer anomaly map is the were identified where gravity measurements were taken sum of the attractions of local sources and broader, more for all three stidies. The correlations of Cady and Meyer distant regional sources. A regional trend that affects (1976a) to our data and to Purves (1969) are shown in Bouguer anomaly because of changes in crustal thickness Figure 6. The data sets, as represented by these points, are is apparent in Figure 7. This poses a difficult problem evidently well correlated and only require the addition to gravity modeling of the Spokane Valley-Rathdrum of a constant to reference our data and those of Purves Prairie aquifer. The thickening of the continental crust (1969) to the GRS 1967 system. beneath the northern Rocky Mountains (Winston and

Figure 6. Locations of eight gravity measurements overlap between the three data sets. By correlating these points, it is possible to indirectly reference the data of Purves (1969) and ours to GRS 1967, to which the data of Cady and Meyer (1976a) are referenced.

26 27 Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Adema, Breckenridge, and Sprenke Adema, Breckenridge, and Sprenke

others, 1989; Harrison and others, 1972), east of the A Bouguer gravity map that is adjusted to compensate study area, causes a regional decrease in the Bouguer for the regional trend is shown in Figure 9. Compared anomaly of 0.8 to 1.9 mGal per km eastward. This trend with the Bouguer anomaly map in Figure 7, the trend- can be clearly observed on a portion of the Bouguer adjusted gravity apparently has a basinlike appearance. anomaly map of Idaho shown in Figure 8. Bankey and The method used respects the general geologic model others (1985) is a complete Bouguer anomaly map of of the subsurface and maintains the simplicity and Idaho produced through a compilation of existing data reproducibility of the corrections. sources. The collection of data from Cady and Meyer (1976a) and Hammond (1974) is that included on the part covering the Rathdrum Prairie, but data sets peripheral to GRAVITY DATA MODELING the study area allow for an interpolation of the regional trend. Ten measurements were averaged to estimate The overall geologic model of the Rathdrum Prairie a regional trend of 1.1 mGal per km east. Though this has been described as an ancestral valley that was correction is subjective, it appears to reasonably estimate filled with various sediments during the late Tertiary the effect of crustal thickening. The trend was simplified and Quaternary periods. The basin-fill geology has to an E-W orientation. Variations in this correction will generally been inferred from Bouguer anomaly (Figure 7). significantly alter models of east-west oriented profiles, Unfortunately, this general impression cannot predict such as the Hayden Avenue profile. the specific depths and morphologies of buried surfaces.

Figure 7. Bouguer anomaly map of the Rathdrum Prairie based on our data and those from Purves (1969) and Cady and Meyer (1976a).

Floods. Measured densities of the basement rocks range from 2.64 to 2.80 g-cm-3, with an average of 2.67 g-cm-3 (Purves, 1969; Birch, 1942; Harrison and others, 1972). The densities of the Columbia River basalt fall into a very broad range from 2.78 g-cm-3 for vesicular samples to 3.21 g-cm-3 for massive samples. Latah sediments have a measured dry density of 1.09-1.62 g-cm-3 and a saturated density of 2.13-2.42 (Hosterman, 1960; Purves, 1969). The gravels of the Missoula Floods vary widely in their densities because of the influence of different particle sizes on porosity, as well as the presence of any cementation. By using a source rock density of 2.60- 2.80 g-cm-3 and porosity ranging from 20 percent for cemented gravels to 40 percent for coarse uncemented gravels, a dry bulk density range of 1.56 to 2.24 g-cm-3 can be inferred. Given this dry density and porosity range, the saturated gravel density range is calculated at 1.96 to 2.44 g-cm-3. Measuring the densities of surface samples is quite simple, but extending these densities to subsurface gravels should be done with caution.

The density information introduced above is difficult to model. The contrast in density between bedrock and basalt is very small, as is also that between Latah sediments and flood gravels. Adding to this problem is the uncertainty about the locations of basalt dikes and remaining basalt deposits, and the location and quantity of Latah sediments. A complex geologic environment probably exists beneath the prairie, considering the numerous reworking episodes that have occurred. Differentiating the basalt deposits from bedrock or the Latah sediments from gravels is nearly impossible through a gravity model because an infinite number of combinations may create the same signature. Because of Figure 8. Bouguer gravity map of the Rathdrum Prairie showing a regional trend of +1.1 mGal/km east. the locally complex setting and small density contrasts, the geologic models have been simplified to elements that will materially affect gravity signatures.

With a detailed gravity survey, however, small variations The rocks of the Belt Supergroup, Cretaceous in the local gravity field can be accurately detected. intrusions, other dikes and sills, and Miocene basalt have These anomalies can presumably be attributed to buried all been combined for the model and are referred to as geologic surfaces that can be readily modeled. The bedrock, with a density of 2.67 g-cm-3. Latah sediments limiting factor to the models is determining the densities and flood gravels are modeled as dry sediments with a of the geologic units. Because the densities of surface density of 1.7 g-cm-3, and the intermingled sands and rocks can be accurately measured and mapped, doing clays are modeled as saturated sediments with a density so provides a starting point for estimating subsurface of 2.1 g-cm-3. The justification for these densities is densities. twofold: They are well within the range of average measured and calculated densities, and they provide the Numerous studies have documented the surface most realistic model when the seismic refraction bedrock densities of rocks in and around the Rathdrum Prairie. tie of Newcomb and others (1953) is imposed. Obviously, The rocks can be generally categorized into four main this will oversimplify the actual geology in many places, units: (1) basement rocks of the Belt Supergroup but will allow a more realistic and probable test of the and Cretaceous igneous intrusions, (2) Tertiary basalt, proposed geologic model without the complication of (3) Latah sediments, and (4) gravels of the Missoula determining contacts between units of similar densities.

28 29 Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Adema, Breckenridge, and Sprenke Adema, Breckenridge, and Sprenke

Figure 9. Residual Bouguer gravity map of the Rathdrum Prairie.

The modeling densities used by Hammond (1974) could the Rathdrum Prairie, because it gives some control over not be procured for comparison. the 3-dimensional subsurface structure that ultimately defines the gravity field. The 2¾-dimensional modeling The theoretical gravity field of five proposed cross- technique normally overestimates basement depth, unless sections was modeled using the software GM-SYS of the basin segment of the model is chosen to be narrower Northwest Geophysical Associates in Corvallis, Oregon. in the direction normal to the profile than the actual It uses the methods of Talwani and others (1959) and basin, in which case the depth would be underestimated. Talwani and Heirtzler (1964) to calculate the gravity Adding additional uncertainty to non-3-dimensional model response. A powerful feature of the program models is the elevation gradient of the basement walls in is its capability to calculate 2¾-dimensional models the basin. In less than three dimensions, it is impossible from the routines of Rasmussen and Pedersen (1979). to exactly and correctly identify the perpendicular extent A 2¾-dimension model allows for the extension of of each modeled profile segment. each discrete model along its perpendicular axis to a user-defined interface where an infinite half-slab of the Five profiles were modeled on the basis of trend- same profile shape is modeled with user-defined density adjusted Bouguer gravity; their locations are shown on values. This is useful in a basin-type structure, such as Figure 4. Two of the models, Hayden Avenue and Idaho

Road (Idaho), were based on our data. Three models models have been interpolated from well-inventory data. are based on data collected by Purves (1969) and re- Profile element extensions in the third dimension, for corrected as part of our study: Idaho Road (Washington), 2¾-dimensional modeling, were estimated individually Corbin Road, and Idaho Highway 41. The profiles and for each profile. calculated and measured gravity values are shown in Figure 10(a-e). All four north-south profiles are shown at GRAVITY MODEL INTERPRETATION the same scale for reliable comparison. The profiles are based on surficial mapping by Breckenridge and Othberg The gravity profiles in the preceding section represent (1998a, 1998b) and other geologic and geophysical data the possible subsurface structure of the Rathdrum Prairie. previously mentioned. The depth tie for these models is Understanding this morphology will aid in interpreting the seismic refraction profile of Newcomb and others the region’s geologic history. No attempt has been made (1953). The seismic reflection profile of Gerstel and to identify specific hydrologic boundaries within the Palmer (1994) confirms the depths of Newcomb and sediments, as such divisions would be overwhelmingly others (1953) in the chosen location. The deepest point subjective because of the low density contrasts involved. on Newcomb and others’ profile has been fixed as the The models are based on the existing pool of geological deepest point on the Idaho Road (Washington) model. No hypotheses for the region. The reversed seismic refraction existing wells penetrate to bedrock in the proximal part profile of Newcomb and others (1953) was the only of the prairie, most only extend a short distance beneath bedrock tie used for these data; therefore, the modeled the water table. The ground-water levels shown in the depths to bedrock are not definitive. Additionally,

Figure 10a. The Idaho Road (Washington) profile is at the western end of the Rathdrum Prairie. At this point, the valley appears to be generally V-shaped, with a small bench on the southern edge of the profile. A deeper section in the middle may be the result of fluvial erosion. The valley is only half as wide at this location, as it is 8 km to the west at Idaho Road (Idaho). Based on the refraction profile of Newcomb and others (1953), the maximum depth to bedrock in this profile has been fixed to a depth of 152 m below the ground surface at the indicated position. The maximum aquifer thickness is 216 m, and the lowest elevation of the bedrock-sediment interface is 426 m.

30 31 Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Adema, Breckenridge, and Sprenke Adema, Breckenridge, and Sprenke

Figure 10b. The Corbin Road profile is 5 km east of the Idaho Road (Washington) profile. This profile is characterized by a generally smooth floor and an apparent embedded valley on the north edge, with shallow benches on both sides of the profile. The southern edge shows a small depression in the present location of the Spokane River. It is not possible to determine, through gravity modeling, how much of the subsurface marginal rocks is Miocene basalt rimrock. The maximum thickness of the aquifer is 263 m and the lowest elevation of the bedrock-sediment interface 384 m.

Figure 10c. The Idaho Road (Idaho) profile is significantly wider than the two profiles to its west. It is characterized by what appears to be an incised river channel in the center of the valley (Rathdrum River?) and a perched depression on the north side. The maximum sediment thickness is 332 m, and the lowest elevation of the bedrock-sediment interface is 337 m. This model does not conclusively prove or disprove that this location was overridden by the Cordilleran ice sheet.

Figure 10d. The Idaho Highway 41 profile is the widest of the north-south profiles. The incised valley evident on the Idaho Road (Idaho) profile is less apparent here. Instead, there is a significant feature on the southern side of the valley. The interface is characterized by a smooth, undulating surface. Because of the wide, smooth profile, Cordilleran glaciation possibly did override this location. The maximum aquifer thickness is 356 m, and lowest elevation of the bedrock-sediment interface is 319 m.

Figure 10e. The Hayden Avenue profile is the only east-west profile. This makes accurate modeling particularly difficult because of the east-west regional trend, causing the shape of this profile to be strongly affected by the trend correction factor. The general bedrock surface has a smooth, undulating character similar to the Idaho Highway 41 profile. Evidence of Pleistocene glaciation across this profile is inconclusive. The valley appears to be deeper on the eastern side, but that may be an influence of the regional trend. The maximum aquifer thickness is 283 m, and the lowest elevation of the bedrock-sediment interface is 395 m.

variations in sediment densities and anomalies within control provided by the seismic refraction of Newcomb the metamorphic and crystalline bedrock can cause and others (1953). Bedrock ties act to constrain the variations in the gravity field that would lead to grossly model. The valley at this point is relatively constricted misinterpreting the shown depths. Table 4 presents the and V-shaped. This suggests that the ancestral valley was modeled aquifer thickness and bedrock elevations. subject to fluvial erosion. It is not possible to determine whether a major glacial advance overrode this profile, Table 4. Modeled Aquifer Characteristics but it seems unlikely from the geomorphic evidence of Breckenridge (1989) and Waitt and Thorson (1983). Along Maximum Lowest this profile, Newcomb and others (1953) interpreted the Sediment Elevation of lower half of the aquifer as Latah sediments intercalated Thickness Bedrock with Miocene basalt. Profile (meters) (meters) Idaho Road (Washington) 216 426 Five kilometers to the east is the Corbin Road profile. Corbin Road 263 384 This profile has a smooth bedrock surface with what Idaho Road (Idaho) 332 337 appears to be an incised stream channel on the north Idaho Highway 41 356 319 side. This feature may be the remnant of a flow on the Hayden Avenue 283 411 north side of the valley during the Miocene when the region was impounded by the Columbia River Basalt The data contained in Table 2 reveal an interesting Group. Similarly, the apparent hump may be a large feature that was first presented by Purves (1969), basalt deposit that was not eroded during the Pleistocene. though he noticed it only in Bouguer anomaly profiles. Shallow benches exist on both sides of the valley. The elevation of the aquifer’s base rises significantly beneath Corbin Road, effectively thinning the sediments Five more kilometers to the east, the valley widens by about 80 m compared to the neighboring profiles. significantly where the Idaho Road (Idaho) profile is Hypotheses vary for explaining this feature. Purves located. The bedrock surface shows a large feature (1969) suggests that the local bedrock high is evidence channel in the center of the profile. Perhaps this is the that the advancement of the Pend Oreille lobe stopped location of the ancestral Rathdrum River. Modeling just east of the Idaho Road (Washington) profile. His such extreme variations in bedrock effectively with explanation is possible but not overwhelmingly evident gravity data is difficult because of the broad influence from these gravity data alone. A major basalt remnant that relatively deep features have on their surrounding may exist in the locality of the bedrock high. It would morphology. This may be the cause for the rise to the have had to survive Pleistocene erosion and would right of the incised channel. Aside from the channel, have acted as a shielding mechanism from the Missoula however, this profile has a consistent depth and a smooth Floods for the Latah sediments to the west that were bedrock-sediment interface. identified by Newcomb and others (1953). The Latah sediments may have been overridden by younger Two more kilometers to the east is the Idaho basalt flows that slowed their erosion. A localized dike Highway 41 profile. This incised stream channel is less or intrusion in the country rock could also explain the evident in the Idaho Highway 41 profile. The valley feature. If such an event occurred and the intrusion was is broad and smooth with few large variations in the a significantly higher density, the modeled feature could bedrock-sediment interface. Possibly the Pend Oreille be a misinterpretation. These very simplified hypotheses lobe did advance this far west, but evidence for or against are based on a feature that has only been identified with such an idea is not contained completely in this profile. gravity methods. Its existence should be studied further. The Hayden Avenue profile was quite difficult to Discussions of each profile are useful in attempting model. It is oriented perpendicularly to the regional trend to decipher the paleogeomorphology of the Rathdrum and therefore significantly affected by any adjustments Prairie. It should be reiterated that small-scale variations to the trend correction. The valley appears to be deepest in a modeled surface are a non-unique solution to a on the eastern side and shallower on the west. This is complex physical situation. Many of these variations likely an effect in the difficulty of modeling a basin in could quite easily appear somewhat different in another less than three dimensions. The profile forms an acute model based on the same data. angle with the north valley wall for about a kilometer on the western end. The shallow bedrock on the north The most reliable of the models is the Idaho Road side of the profile acts to increase the apparent gravity (Washington) profile. This profile has the limited bedrock on that end, which leads to a shallower modeled depth

32 33 Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Adema, Breckenridge, and Sprenke Adema, Breckenridge, and Sprenke to bedrock. This profile should be limited to the depth Two primary factors limit the reliability of these to bedrock near the center of the profile, where the edge results. First, the models were created using 2¾ effect is minimized. dimensional modeling techniques. Any model produced with 3-dimensional techniques will be more accurate. Analysis of the minimum elevations of the bedrock- A 3-dimensional inversion of the basin would more sediment interface suggests that a low point exists clearly define the aquifer boundaries. Second, only one between Idaho Road (Idaho) and Hayden Avenue, control point was used for our study. Future studies as has been previously suggested by Purves (1969). must include two kinds of data gathering: (1) seismic Unfortunately, the reliability of depths modeled becomes and gravity surveys to map the subsurface depths and less accurate when the profiles are further from the structure; and (2) well logs to distinguish geologic units location of the seismic refraction tie. Furthermore, the and confirm structure, rock densities, and areal and depth adjustment for the regional trend is a notable subjective measurements. action that would also affect this conclusion. ACKNOWLEDGMENTS CONCLUSION This study was funded by the Idaho Geological Understanding the thickness and subsurface geology Survey. The gravity meter and GPS equipment necessary of the Rathdrum Prairie aquifer is critical to interpreting to our research were provided by Dr. John Oldow of the the local and regional geologic history that will University of Idaho. Dr. Oldow provided insights on ultimately lead to ensuring better aquifer management. the details of GPS and gravity surveying and training To that end, we have incorporated new gravity data with in the use of the equipment. Dr. Stephen Palmer at the a sizable existing gravity data set to create a broader Washington Department of Natural Resources provided base from which to interpret the region’s subsurface the original data of Purves (1969). Loudon Stanford at structure. From this information, we have developed the the Idaho Geological Survey gave helpful advice on geometry for each of the five modeled profiles as well manipulating USGS DEM and DRG data. The late Dan as the plausible depth and thickness relationships. The Weisz was an excellent field assistant, and thanks should results provide a much clearer picture of the aquifer’s also go to his parents, who allowed a GPS station to be structure and extent. placed in their yard on the Rathdrum Prairie.

The sediments that compose the Rathdrum Prairie have a maximum thickness of about 356 m in the center REFERENCES of the valley on Idaho Road (Idaho) between Post Falls and Rathdrum. The aquifer thins to about 215 m only Aitkenhead, N., 1960, Observation on the drainage of a 10 km to the west at the Idaho-Washington state line. glacier-dammed lake in Norway: Journal of Glaciology, The bedrock-sediment interface is generally smooth east v. 3, p. 607-609. of Idaho Road (Idaho). From this point to the west, an Anderson, A.L., 1927, Some Miocene and Pleistocene incised channel becomes apparent in the ancestral valley drainage changes in northern Idaho: Idaho Bureau of floor. The lower boundary of the aquifer remains ata Mines and Geology Pamphlet 18, 29 p. generally consistent elevation across the study area, with ———, 1940, Geology and metalliferous deposits of a shallow slope to the east. A low point in the elevation Kootenai County, Idaho: Idaho Bureau of Mines and of the bedrock-sediment interface is presumed to exist Geology Pamphlet 53, 31 p. between Idaho Road (Idaho) and Hayden Avenue. Bankey, V., M. Webring, D.R. Mabey, M.D. Kleinkopf, This conclusion should be interpreted with caution, and E.H. Bennett, 1985, Complete Bouguer gravity as the regional trend correction ultimately determines anomaly map of Idaho: U.S. Geological Survey the magnitude of the interface’s slope in the east-west Miscellaneous Field Studies Map, MF-1773. direction. Baker, V.R., 1973, Paleohydrology and sedimentology of Lake Missoula flooding in eastern Washington: Gravity modeling has its limitations. With the existing Geological Society of America Special Paper 144, data, it cannot establish definitive depths to the bedrock- 79 p. sediment interface or differentiate their various geologic Birch, F., (ed.), 1942, Handbook of Physical Constants: units. The next step in studying the aquifer further should Geological Society of America Special Paper 36, be more seismic investigations, especially on the eastern 319 p. half of the prairie, and more well drilling that penetrates Bonini, W.E., 1963, Gravity anomalies in Idaho: Idaho bedrock. Bureau of Mines and Geology Pamphlet 132, 39 p.

34 35 Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Adema, Breckenridge, and Sprenke Adema, Breckenridge, and Sprenke

Breckenridge, R.M., 1989, Glacial Lake Missoula and gravity stations in the Okanogan, Sandpoint, Ritzville, the Channeled Scablands, chapter 3: Lower glacial and Spokane 1° x 2° quadrangles, northeastern Lake Missoula and Clark Fork ice dams, T310, Washington and northern Idaho: U.S. Geological in Glacial Geology and Geomorphology of North Survey Open-File Report 76-290, 57 p. America, v. 1, 28th International Geologic Congress ———, 1976b, Bouguer gravity anomaly map of the Field Trip Guidebook: American Geophysical Union, Okanogan, Sandpoint, Ritzville, and Spokane 1° Washington, D.C., 72 p. by 2° quadrangles, northeastern Washington and Breckenridge, R.M., and K.L. Othberg, 1998a, Surficial northern Idaho: U.S. Geological Survey Geophysical geologic map of the Post Falls and part of the Liberty Investigation Map GP-914. Lake quadrangles, Kootenai County, Idaho: Idaho Chambers, R.L., 1971, Sedimentation in glacial Lake Geological Survey Surficial Geologic Map 98-1, Missoula: University of Montana M.S. thesis, 100 p. 1:24,000. ———, 1984, Sedimentary evidence for multiple ———, 1998b, Surficial geologic map of the Rathdrum Glacial Lakes Missoula: Montana Geological Field and part of the Newman Lake quadrangles, Kootenai Conference on Northwestern Montana, Kalispell, County, Idaho: Idaho Geological Survey Surficial Montana, p. 189-199. Geologic Map 98-2, 1:24,000. Cogbill, A.H., 1990, Gravity terrain corrections calculated ———, 1998c, Chronology of Glacial Lake Missoula using digital elevation models: Geophysics, v. 55, Floods; paleomagnetic evidence from Pleistocene lake no. 1, p. 102-106. sediments near Clark Fork, Idaho: Geological Society Conners, J.A., 1976, Quaternary history of northern of America Abstracts with Programs, v. 30, no. 6, Idaho and adjacent areas: University of Idaho Ph.D. p. 30. thesis, 504 p. Breckenridge, R.M., K.L. Othberg, J.A. Welhan, and Costa, J.E., 1988, Floods from dam failures in V.R. C.R. Knowles, 1997, Geologic characteristics of the Baker, R.C. Kochel, and P.C. Patton, eds., Flood Rathdrum aquifer: Inland Northwest Water Resources Geomorphology: John Wiley and Sons, New York, Conference, p. 23. p. 439-463. Bretz, J H., 1923, The channeled scablands of the Columbia Dobrin, M.B., 1960, Introduction to geophysical Plateau: Journal of Geology, v. 31, p. 617-649. prospecting: McGraw-Hill Book Company, Inc., ———, 1925, The Spokane flood beyond the channeled New York, p. 169-262. scablands: Journal of Geology, v. 33, p. 97-115, 236- Flint, R.F., 1935, Glacial features of the southern 259. Okanogan region: Geological Society of America ———, 1928a, Bars of channeled scabland: Geological Bulletin, v. 46, p. 169-194. Society of America Bulletin, v. 39, p. 643-702. ———, 1936, Stratified drift and deglaciation in eastern ———, 1928b, The channeled scabland of eastern Washington: Geological Society of America Bulletin, Washington: Geographical Review, v. 18, p. 446- v. 47, p. 1849-1884. 477. ———, 1937, Pleistocene drift border of eastern ———, 1928c, Alternate hypothesis for channeled Washington: Geological Society of America Bulletin, scabland: Journal of Geology, v. 36, p. 193-223, 312- v. 48, p. 203-232. 341. ———, 1938, Origin of the Cheney-Palouse scabland ———, 1930a, Lake Missoula and the Spokane flood tract: Geological Society of America Bulletin, v. 49, (Abs.): Geological Society of America Bulletin, p. 461-524. v. 41, p. 92-93. Gerstel, W.J., and S.P. Palmer, 1994, Geologic and ———, 1930b, Valley deposits immediately west of geophysical mapping of the Spokane Aquifer— the Channeled Scabland: Journal of Geology, v. 38, Relevance to growth management: Washington p. 385-422. Geology, v. 22, no. 2, p. 18-24. ———, 1932, The Grand Coulee: American Geographical Glenn, J.W., 1954, The stability of ice-dammed lakes Society Special Publication 15, 89 p. and other water filled holes in glaciers: Journal of ———, 1959, Washington’s Channeled Scabland: Glaciology, v. 2, p. 316-318. Washington Department of Conservation, Division Hammer, S., 1939, Terrain corrections for gravimeter of Mines and Geology Bulletin 45, 57 p. stations: Geophysics, v. 4, p. 184-194. Bretz, J H., H.T.U. Smith, and G.E. Neff, 1956, Hammond, R.E., 1974, Ground-water occurrence and Channeled scabland of Washington: New data and movement in the Athol area and the northern Rathdrum interpretations: Geological Society of America Prairie, northern Idaho: Idaho Department of Water Bulletin, v. 67, p. 957-1049. Administration, Water Information Bulletin 35, 18 p. Cady, J.W., and R.R. Meyer, 1976a, Principal facts for Harrison, J.E., M.D. Kleinkopf, and J.D. Obradovich,

1972, Tectonic events at the intersection between the Rasmussen, R., and L.B. Pedersen, 1979, End corrections Hope fault and the Purcell trench, northern Idaho: in potential field modeling: Geophysical Prospecting, U.S. Geological Survey Professional Paper 719, 24 p. v. 27, p. 749-760. Hittelman, A., D. Dater, R. Buhman, and S. Racey, 1994, Richmond, G.M., 1986, Tentative correlation of deposits Gravity CD-ROM—1994 Editions and User’s Manual, of the Cordilleran Ice Sheet in the northern Rocky revised, 1996: U.S. Department of Commerce, National Mountains, in V. Sibrava, D.Q. Bowen, and G.M. Geophysical Data Center, Boulder, Colorado. Richmond, eds., Quaternary Glaciations in the Hosterman, J.W., 1960, Investigations of some clay Northern Hemisphere: A Report of the International deposits in parts of Washington and Idaho: U.S. Geological Correlation Programe, Project 24, Geological Survey Bulletin 1091, 146 p. International Union of Geological Sciences and Kiver, E.P., and D.F. Stradling, 1989, Glacial Lake UNESCO: Quaternary Science Reviews, v. 5, p. 129- 144. Missoula and the Channeled Scablands, chapter 4: The Sandberg, C.H., 1958, Terrain corrections for an inclined Spokane Valley and northern Columbia Plateau, T310, plane in gravity computations: Geophysics, v. 23, in Glacial Geology and Geomorphology of North p. 701-711. America, v. 1, 28th International Geologic Congress Savage, C.N., 1965, Geologic history of Pend Oreille Field Trip Guidebook: American Geophysical Union, Lake region in north Idaho: Idaho Bureau of Mines Washington, D.C., 72 p. and Geology Pamphlet 134, 18 p. Lewis, R.S., R.F. Burmester, R.M. Breckenridge, M.D. ———, 1967, Geology and mineral resources of Bonner McFaddan, and J.D. Kauffman, 2002, Geologic map County: Idaho Bureau of Mines and Geology County of the Coeur d’Alene 30 x 60 minute quadrangle, Report 6, p. 131. Idaho: Idaho Geological Survey Geologic Map 33. Sieja, D.M., 1959, Clay mineralogy of glacial Lake Liestol, O., 1955, Glacier-dammed lakes in Norway: Missoula varves, Missoula Co., Montana: University Norsk Geographisk Tidsskrift, v. 15, p. 122-149. of Montana M.S. thesis, 52 p. McKiness, J.P., 1988, The Quaternary system of the Talwani, Manik, and J.R. Heirtzler, 1964, Computation eastern Spokane Valley and the lower, northern slopes of magnetic anomalies caused by two-dimensional of the Mica Peak uplands, eastern Washington and bodies of arbitrary shape, in G.A. Parks, ed., northern Idaho: University of Idaho M.S. thesis, Computers in the Mineral Industries, Part 1: Stanford 100 p. University Publication, Geological Sciences, v. 9, Molenaar, D., 1988, The Spokane aquifer, Washington: p. 464-480. Its geologic origin and water-bearing quality Talwani, Manik, J.L. Worzel, and M. Landisman, 1959, characteristics: U.S. Geological Survey Water Supply Rapid gravity computations for two-dimensional Paper 2265, 74 p. bodies with application to the Mendocino submarine Newcomb, R.C., and others, 1953, Seismic cross sections fracture zone: Journal of Geophysical Research, across the Spokane River Valley and the Hillyard v. 64, p. 49-59. trough, Idaho and Washington: U.S. Geological Thorarinsson, S., 1939, The ice-dammed lakes of Iceland Survey Open-File Report, 16 p. with particular reference to their values as indicators Olivier, R.J., and R.G. Simard, 1981, Improvement of of glacier oscillations: Geografiska Annaler, v. 21, the conic prism model for terrain correction in rugged Ht. 3-4, p. 216-242. topography: Geophysics, v. 46, p. 1054-1056. Waitt, R.B., 1980, About forty last-Glacial Lake Missoula Pardee, J.T., 1910, The glacial Lake Missoula: Journal of jökulhlaups through southern Washington: Journal of Geology, v. 18, p. 376-386. Geology, v. 88, p. 653-679. ———, 1942, Unusual currents in glacial Lake ———, 1985, Case for periodic, colossal jökulhlaups Missoula: Geological Society of America Bulletin, from Pleistocene Glacial Lake Missoula: Geological v. 53, p. 1569-1600. Society of America Bulletin, v. 96, p. 1271-1286. Pardee, J.T., and Kirk Bryan, 1926, Geology of the Latah Waitt, R.B., and B.F. Atwater, 1989, Glacial Lake Formation in relation to the lavas of the Columbia Missoula and the Channeled Scablands, chapter Plateau near Spokane, Washington: U.S. Geological 5: Stratigraphic and geomorphic evidence for Survey Professional Paper 140-A, p. 1-16. dozens of last-glacial floods, T310, in Glacial Plouff, Donald, 1966, Digital terrain corrections based Geology and Geomorphology of North America, upon geographic coordinates: Geophysics, v. 31, v. 1, 28th International Geologic Congress Field p. 1208. Trip Guidebook: American Geophysical Union, Purves, W.J., 1969, The stratigraphic control of ground Washington, D.C., 72 p. water through the Spokane Valley: Washington State Waitt, R.B., and R.M. Thorson, 1983, The Cordilleran University M.S. thesis, 213 p. ice sheet in Washington, Idaho, and Montana, in

36 37 Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Adema, Breckenridge, and Sprenke Adema, Breckenridge, and Sprenke

Late-Quaternary Environments of the United States, v. 1: University of Minnesota Press, p. 53-70. Weis, P.L., and G.M. Richmond, 1965, Maximum extent of late Pleistocene Cordilleran glaciation in northeastern Washington and Idaho: U.S. Geological Survey Professional Paper 525-c, p. 128-132. Winston, D., R.J. Horodyski, and J.W. Whipple, 1989, Middle Proterozoic Belt Supergroup, western Montana, in Volcanism and Plutonism of Western North America, v. 2: 28th International Geologic Congress, American Geophysical Union, p. 1-103. Wollard, G.P., and J.C. Behrendt, 1961, An evaluation of the gravity control network in North America: Geophysics, v. 50, p. 866-881. Wyman, S.A., 1994, The potential for heavy metal migration from sediments of Coeur d’Alene Lake into the Rathdrum Prairie aquifer, Kootenai County, Idaho: University of Idaho M.S. thesis, 141 p.

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