DEPARTMENT OF THE INTERIOR UNITED STATES GEOLOGICAL SURVEY
PALEOVALLEY AND GEOLOGIC MAP OF NORTHEASTERN COLORADO
By Glenn R. Scott
N Vl 0 0 0 0 MISCELLANEOUS INVESTIGATIONS SERIES Published by the U.S. Geological Survey, 1982 DEPARTMENT OF THE INTERIOR TO ACCOMPANY MAP 1-1378 UNITED STATES GEOLOGICAL SURVEY
PALEOVALLEY AND GEOLOGIC MAP OF NORTHEASTERN COLORADO By Glenn R. Scott
INTRODUCTION County, Nebraska, apparently is the earliest post-Ogallala While mapping the geology of the Sterling 1 o x 2° deposit of the Central Great Plains. The formation contains quadrangle, northeastern Colorado (Scott, 1978), I found the proboscidean, Stegomastodon, the horse, that the courses of many former streams could be inferred Dolichohippus, the camel, Gigantocamelus, and other fossils from the preserved remnants of their gravel deposits. After the that demonstrate its equivalence with the Blancan (of geologic mapping was completed, the inferred courses of Pliocene and Pleistocene age) ofTexas ( Dalq uest, 197 5). The these ancient streams were plotted on a copy of the geologic Broadwater lies almost 490 ft ( 150 m) above the North Platte map of the Sterling quadrangle (Scott, 1978) either directly River in Nebraska, nearly the same as the elevation of the from stream deposits visible on aerial photographs or by Nussbaum Alluvium above the South Platte River in northeast connecting mapped outcrops of gravelly stream deposits. The Colorado. Thus, the Broadwater apparently is an age resulting map shows positions of paleovalleys of seven ages correlative of the Nussbaum Alluvium in the Sterling ranging from Oligocene to Quaternary (Illinoian?). In quadrangle. addition, a small scale map (fig. 1) was made that shows In 1964, Smith and others (p. 32-33) discussed a postulated ancient stream channels in a larger area of the westward shift in the course of the South Platte River from eastern half of Colorado and adjoining southwestern Beebe Draw (fig. 1) to the present valley of the South Platte Nebraska. River. The stream that captured the South Platte was a The Cenozoic time scale as used here is: The Paleocene tributary of the Cache Ia Poudre River. In addition, they Epoch ended 55 (54-56) m.y. ago, the Eocene 38 (34-38) described another capture that involved Box Elder Creek and m.y., the Oligocene, 24 (23-26) m.y., the Miocene, 5 an ancestral creek in Beebe Draw. (4.9-5.3) m.y., and the Pliocene, 1.8 (1. 7-2.2) m.y. ago. Gardner (1967) mapped the surficial geology of the Orchard, Weldona, and Fort Morgan quadrangles in ACKNOWLEDGMENTS northeast Colorado, and showed the distribution of I am indebted to G. Edward Lewis of the U.S. Geological Quaternary terrace gravels and a convergence of profiles Survey for identifying vertebrate fossils I collected in the drawn on the bases of the alluvial deposits as they are traced Sterling area and for supplying much information about the eastward (Gardner, 1967, fig. 9). Cenozoic stratigraphy and vertebrate fossils of northeastern Vondra and others ( 1969) traced a channel of Gering age Colorado. Robert W. O'Donnell of the U.S. Geological (lower part of the Arikaree Formation through the Wildcat Survey prepared the vertebrate fossils. I also have benefited . Ridge area of western Nebraska. Apparently, the channel was from discussions and field trips with Glen A. Izett and Norman traced by using a characteristic pumice bed in the Gering M. Denson of the U.S. Geological Survey. Cary T. Madden of (Twin Sisters Pumice Conglomerate Bed of Vondra, Schultz, the University of Colorado Museum and Department of and Thompson, 1969). Anthropology provided information about fossil Sharps ( 1969) showed that the Arkansas River migrated proboscidean teeth found west of Sterling and north of Brush. across its valley during the Pliocene and Pleistocene in a style The trends of paleovalleys in the mountains were derived very similar to the early migrations of the South Platte River. from work by William A. Braddock of the U.S. Geological The migration apparently always was the same direction as Survey and University of Colorado, Charles E. Chapin of the the transport direction of the tributaries carrying the greatest New Mexico Bureau of Mines and Mineral Resources, Rudy quantity of water and sediment. In one reach of the valley, the C. Epis of the U.S. Geological Survey and Colorado School of Arkansas migrated northward; in another reach it migrated Mines, Malcolm E. McCallum of the U.S. Geological Survey southward. and Colorado State University, and Richard B. Taylor of the Denson ( 1969) studied the detrital heavy mineral U.S. Geological Survey. assemblages in the Miocene and Pliocene (now all included in the Miocene) rocks of central Wyoming and parts of adjacent PREVIOUS WORK ON PALEOVALLEYS states. The Arikaree Formation of Oligocene(?) and Miocene AND ORIGIN OF SEDIMENTS age contains a volcanic assemblage characterized by Many vertebrate paleontologists have collected fossils in green-brown hornblende and oxyhornblende, augite, northeastern Colorado. Reports on the fossils were written by hypersthene, and red-brown biotite; the volcanic assemblage, Matthew (1901), Galbreath (1953), and Wilson (1960). Many which was largely reworked from ash falls, makes up 70-90 reports have been published on the geomorphology of percent of the nonopaque heavy minerals in the formation. Colorado, Nebraska, Wyoming, and Kansas; however, very According to Denson this suite suggests a time of active little has been written about the paleovalleys or the origin of volcanism and tectonism far to the west. The Ogallala the paleovalley sediments in the Sterling quadrangle. The Formation of Miocene age chiefly contains a metamorphic following reports are the principal ones on this subject: and plutonic assemblage of nonopaque heavy minerals In 1951, Schultz and others described the Quaternary characterized by blue-green hornblende, garnet, epidote, deposits of Nebraska and their associated fossils. Along the zircon, tourmaline, actinolite, sphene, and gray-green biotite; North Platte River, north of the Sterling quadrangle, the the plutonic and metamorphic assemblage makes up more Broadwater Formation of Schultz and Stout (1945), well than 46 percent of the nonopaque heavy minerals in the preserved in the NE% sec. 20, T. 19 N., R. 47 W., Morrill formation and suggests derivation largely from Precambrian rocks. erosion prior to the deposition of the Blanco Formation of Denson and Chisholm (1971) studied the detrital heavy Texas. He believed that the Arikaree Formation was thinned minerals of the older Tertiary rocks in the middle Rocky ''by onlap from a northern area along the paleovalley of the Mountains and the northern Great Plains. The Oligocene North Platte River to a southern area in northeast Colorado." rocks (White River Group) contain about equal amounts of This conclusion was reached because only the upper part volcanic and plutonic-metamorphic heavy minerals; the (equivalent to Marsland Formation) of the Arikaree crops out volcanically derived heavy-mineral assemblage generally in northeastern Colorado. Had the Arikaree been deposited in contains more than 40 percent green-brown hornblende and its entirety and then eroded in pre-Ogallala time only the less than 15 percent augite. More than 30 percent of the heavy lower part would have been preserved. From several minerals is blue-green hornblende, epidote, zircon, garnet, radiometric ages of volcanic ash beds, he assigned a Miocene sphene, and green-brown biotite derived from age( 16-17 m.y. to about 5 m.y.) to the Ogallala Formation. plutonic-metamorphic sources. Thus, the sediments came He inferred, from the abrupt increase in grain size in passing from distant volcanic sources and from erosion of from the sandy Arikaree upward into conglomeratic Ogallala, Precambrian rocks. Most of the Oligocene rocks are very fine that the Front Range was uplifted several hundred meters at grained and contain a low percentage of heavy minerals. The the beginning of Ogallala time. He considered it fairly wen· Brule Formation was inferred to be largely of eolian origin established that the South Platte River was in about its present whereas the Chadron Formation is almost entirely of fluviatile course for the past 10-12 m.y. origin. Denson and Chisholm also believe that the .major Clark ( 1975) presented evidence for a few stream streams and main drainage framework were in existence at channels in the White River Group. He described one set of the close of Eocene time and were only slightly modified since (Chadron) channels trending eastward through the middle of then. T. 11 N., R. 55 W. Clasts of Proterozoic Sherman Granite Pearl (1971) studied the late Cenozoic erosional history characterize this set, but the gravel also contains reddish of east central Colorado and northwestern Kansas. He mudstone and clasts of plutonic rocks of the Front Range from concluded that the Arikaree River was the main river crossing south of the Sherman Granite. A second river (of Chadron east-central Colorado at the end of Ogallala time; it left the age) about 25 mi (40 km) south of the first went eastward and mountains near Pikes Peak, followed the north-eastward turned southeastward north of Sterling. This channel contains trend of modern Big Sandy Creek, then assumed the trend of neither clasts of Sherman Granite nor red mudstone. He the present Arikaree River. He inferred that the upper end of found little volcanic ash in the lower part of the Chadron of the ancestral Arikaree River was captured in Pliocene (p. 1, northeastern Colorado. this report) and Pleistocene time presumably somewhere near Blackstone (1975, p. 269 and fig. 14) suggested that the Lake George and that afterward the new northeastward 28-27 m.y.-old Specimen Mountain volcanic rocks in Rocky course became part of the upper reach of the South Platte Mountain National Park are a probable source for rhyolitic River. He also inferred that another capture took place in volcanic clasts found at least 75 mi ( 120 km) east of the Pliocene (p. 1, this report) and Pleistocene time when Big present mountain front in the upper Tertiary (Ogallala) Sandy Creek cut headward from the Arkansas River and sedimentary rocks. captured the ancestral Arikaree near present River Bend, Scott (1975) and Epis and others (1976) inferred the Colorado. Presumably Pearl inferred that these two captures routes of streams of Paleocene and later age that flowed out of so depleted the Arikaree River that it could not cut down the mountains and onto the Great Plains. In many of the through the Ogallala as rapidly as the Arkansas and South preserved alluvial deposits, the clasts were traced back to their Platte Rivers; therefore, its course eastward from River Bend, points of origin; however, because of great later erosion the Colorado, was detached from its headward course. Pearl also location of few paleovalleys east of the mountains could be stated that removal of the Ogallaia Formation along the east determined with confidence. Paleovalleys of Pliocene and side of the mountains began in late Pliocene (p. 1, this report) later age in the mountains were easy to locate because of the time. canyons cut in Pliocene time; contemporaneous paleovalleys Stanley and Wayne ( 1972) studied the lower Pleistocene along major valleys in the plains were tentatively located from fluvial sediments of Nebraska. In particular, they sought the gravelly valley fills; however, along small valleys with little information about the provenance of stream gravel and or no gravel, paleovalleys could not be traced. The above movements of the major early Pleistocene streams through authors mapped an Oligocene stream that crossed the time. Based on the size of clasts in deposits of various ages Thirtynine Mile volcanic field, the present site of the Rampart they determined that the carrying power of early Pleistocene Range, and that deposited the Castle Rock Conglomerate on streams was greater than that of either Pliocene (equals(?) the plains. Quite likely this stream (the ancestral South Platte Miocene Ogallala) streams or the modern Platte River. They River?) also deposited the Chadron Formation in the Fremont discussed the possible Blancan age of the early post-Ogallala Butte area. The same stream later carried Miocene gravel stream deposits in Nebraska. They believed that the granitic (gravel at Divide, Colo.) eastward, possibly along a trend gravel in the lower Pleistocene alluvium was derived directly similar to that of the ancestral Arikaree valley suggested by from the Rocky Mountains, not reworked from underlying Pearl ( 1971). The above authors showed that in Pliocene time Tertiary deposits. Shifts in the courses of streams were this stream was interrupted by several faults and forced into its considered to be the result of spasmodic uplift of folds in the present course as the South Platte River from Lake George bedrock. northeastward to Denver. Post-Ogallala cyclic cutting of Izett ( 1975) discussed the late Cenozoic sedimentation narrow canyons in the mountains and broader valleys on the and stratigraphy of northeastern Colorado. He equated the plains was described for all large streams; depth of canyons Pliocene (5 to 2 m.y. old) approximately with part of the described is as much as 1,640 ft (500 m). Blancan land mammal age and inferred a period of uplift and Stanley ( 1976) inferred that based on paleocurrents,
2 depositional structures, and the geometry of sandstone Except for the trench cut by the South Platte River and its bodies, the Tertiary streams were similar to modern braided tributaries, the surface on the Ogallala has remained relatively streams on the Great Plains. He concluded that most of the unchanged since its establishment in Miocene time. As the Tertiary fine-grained sandstone· originated as alluvial plain scarp was being cut, all drainage continued to flow eastward deposits and that a smaller part was deposited as eolian sand on the original surface of the Ogallala. and loess. Based on the presence of distinctive volCanic clasts Many tributaries north and a few south of the river are in the Arikaree and Ogallala Formations, Stanley (1976, p. alined in a northwest direction. The northwest alined 303, 304) believed that for the first time streams headed in tributaries are almost exactly parallel with the direction of the areas west of the Front Range and the Laramie Mountains (in prevailing wind-southeastward at S. 30°-35° E. I infer that North Park, Colo.). He also reiterated that the Laramie and the wind actively assisted in the dissection of the stream Medicine Bow Mountains were almost completely buried channels. Perhaps the streams were further controlled by under Tertiary sediments by Arikaree time, thus helping to being forced to stay in channels between long linear dunes. In explain why the amount of plutonic material in the Arikaree addition, a case could be made that the stream alinement was Formation is so small compared to the amount of volcanic in part controlled by the southeast-trending conglomeratic material. From his work on petrofacies of the Tertiary rocks he channel deposits of the Chadron Formation (Oligocene); believed that during Oligocene and Miocene time the stream however, this seems unlikely because southeastward systems shifted progressively southward. trending valleys are also fairly abundant south of the river Sharps (1976) and Scott and others (1978) showed that on the Ogallala Formation where the Chadron Formation the greater part of the Nussbaum Alluvium north of the is not exposed. Arkansas River is a broad thick alluvial fan deposit, similar to A poorly developed tributary stream system along the the Ogallala, that grades southeastward into main-stem gravel south side of the river and east of Beaver Creek resulted from along the Arkansas River. deposition of a thick cover of wind-blown sand. The blanket of eolian sand has effectively smothered a northwestward PHYSIOGRAPHY sloping area 3-15 mi (5-24 km) wide where rainfall and The two major physiographic features of northeastern surface water are quickly absorbed and little or no runoff from Colorado are a high sloping tableland created by the Ogallala the south reaches the river. To the west, Beaver Creek, Formation (Miocene) and a large and intricately dissected Badger Creek, Sand Arroyo Creek, Bijou Creek, Kiowa stream-valley system created by the South Platte River and its Creek, Lost Creek, and Box Elder Creek have, over the years, tributaries. The high tableland is well preserved north and developed drainage areas extensive enough to permit them to south of the river in virtually its original form. The tableland keep flowing and not be impeded by the eolian sand. north of the river can be traced continuously from the Peetz Eolian deposits impose a strong southeastward-trending Table westward to the Gangplank where Tertiary clastic rocks . physiographic pattern on the area. Many of the contacts of the overlie Precambrian rocks just west of Cheyenne, Wyo. loess and eolian sand parallel the direction of the prevailing Well-defined scarps were created where tributaries of the windS. 30°-35° E. The axes of barchans and linear dunes, South Platte River cut headward into the tableland. The scarp some of which are more than 100ft (30m) high, also parallel of the tableland (Chalk Bluffs) north of the river is steeper than the wind direction. The large dune field in the southeastern the scarp south of the river because the tributaries that have part of the quadrangle is an excellent place to see dune cut headward into the northern bluffs are larger and more lineation and morphology. The dunes range in age from active than those south of the river. Excellent exposures of the Pinedale Glaciation to modern; some dunes are still moving. Ogallala and pre-Ogallala clastic deposits can be seen along This large dunefield ends at the North Fork of the Republican the two scarps. River; loess lies south of the river. Apparently the sand at the The valley of the south Platte River is broader and deeper south end of the dune field is carried away by the river as fast (below the tableland formed by the Ogallala Formation) in the as it is deposited on the floodplain. west than it is in the east. In the west, from near Pawnee Buttes Several small high landmarks, Pawnee Buttes, Fremont southeastward to near Akron, the valley is about 70 mi (112 Butte, Twin Buttes, Flattop Butte, and Kirchnavy Butte are km) wide and about 900 ft (275 m) deep. In the east near erosional remnants of the Tertiary tableland. Ovid, the valley is only 5 mi (8 km) wide and 400ft (120m) An unexplained undrained depression in the deep. Near Fremont Butte, the Ogallala escarpment lies about southwestern part ofT. 5 N., R. 51 W. is thought to be either 18 mi (29 km) southeast of the river, but near Ovid, it is only 5 an unusually large wind blowout or a meteor impact crater. miles (8 km) southeast of the river. From Fremont Butte, the The hollow occupies an area of about 4 mi2 (10.4 km2) and escarpment trends southward out of the quadrangle as far as has more than 20 smaller hollows downwind (southeast) from Lindon, where it turns southwestward to Cedar Point (fig. 1), it. The meteor origin seems unlikely, but I believe the the highest and westernmost point on the Ogallala Formation, possibility of finding iron-bearing meteorites should be tested between the South Platte and Arkansas Rivers. by making a few traverses across the hollow with a hand-held The South Platte River shifted southward through magnetometer. Unfortunately a magnetometer would not Quaternary time because many streams were bringing water detect stony meteorites which apparently are more common. and sediment from the northwest and few from the southeast. Remnants of stream terraces and pediments are The sediment clogged the South Platte River and forced it to numerous and well preserved north of the South Platte River, move against its southern valley wall. A lack of major tributary whereas few are preserved on the south side of the river. streams coming into the South Platte from the south, These surfaces range in age from Pliocene to Holocene, and downstream from Beaver Creek, resulted first from the south they lie between 450ft (135m) and less than 10ft (3m) above Platte River staying close to the scarp and later from eolian the present major streams. The terraces are used here to sand coverjng the slope between the scarp and the river. indicate how the position of the ancestral South Platte River
3 has changed through time. White River Group- Oligocene-about 34-30 m.y. old Brule and Chadron PALEO VALLEYS Formations The trends of paleovalleys of seven ages have been The older three of the above deposits lie one over reconstructed by use of high altitude Army Map Service aerial another with the youngest at the top. The younger four photographs and the geologic map of the Sterling 1o x 2o main-stem gravel deposits were laid down on the ancient quadrangle (Scott, 1978). A list of the deposits in the valley floors of the South Platte River as it continuously cut paleovalleys and their ages is as follows: downward; therefore they are not superposed, and the oldest Slocum Alluvium Pleistocene (Sangamon or Illinoian) is topographically the highest. The paleovalleys and their Verdos Alluvium Pleistocene (Yarmouth or Kansan) deposits will be discussed from oldest to youngest (table 1). Rocky Flats Alluvium Pleistocene (Blancan) Brule and Chadron Formations-The Brule and Nussbaum Alluvium Pliocene (Blancan) Chadron paleovalleys were recognized by their widespread Ogallala Formation- Miocene-about 18-? m.y. old conglomerate-capped ridges and more local broad sheetlike lower part sandstone and conglomerate deposits, suggesting that there Arikaree formation Miocene-about 20-18 m.y. old were valleys ranging greatly in width. Indeed, one broad area (Sterling area only) extending almost unbroken from North Sterling Reservoir to
Table I.-Stratigraphy of Oligocene and younger deposits in the Sterling 1 ox 2° quadrangle
Series Geologic Unit Approximate Vertebrate fossils of climate or radiometric this area faunal age unit Sangamon or Slocum Alluvium 160,0001· Illinoian Mammuthus sp. Pleistocene Yarmouth or Verdos Alluvium (older part Kansan Pearlette type-0 600,0002 only) ash 1.33 ______Rocky Flats Alluvium 1.4 Equus (Piesippus) (Dolichohippus) sp. ____1.8 Blancan in part Pliocene Nussbaum Alluvium 3.0 Stegomastodon rexroadensis ______5.0 ______Upper Aphelops, Te/eoceras, Ustatochoerus major part Pliohippus, Neohipparion Ogallala Formation Lower part Merychippus republican us 18.0 ______Miocene Arikaree Formation (Equivalent to Merycochoerus and Merychyus Marsland Formation only) 20.0 ______24.0 30.0 ______Whitney 4 Leptauchenia White Brule Mbr. Fm. Orella4 Oligocene River { Mbr. Paradjidaumo, Mesohippus eulophus Group Chadron Formation Merycoidodon, Mesohippus proeulophus, Trigonias osborni, brontothere ___ 38.0 ______
1. Barney J. Szabo, written commun., 1978. 2. Naeser and others, 1973. 3. This and following ages from Glen A. lzett, and John D. Obradovich, oral commun., 1979. 4. Of Schultz and Stout, 1955.
4 Willard suggests a probable broad gravel fan rather than the The following partial section of the Chadron describes gravel-covered floor of a well-defined stream valley. the material in the broad sheetlike deposits near Pawnee· Pass Conglomerate-filled channels probably are continuous across in the SE'4 sec. 25, T. 8 N., R. 55 W., Logan County, the quadrangle, but most of the Brule and Chadron are now Colorado. either buried or removed by erosion. The preserved and Feet exposed channels show two trends; one is a southeastward trend nearly parallel to modern streams, and as stated earlier, Sandstone, coarse- to medium-grained, pale- 4 might have controlled the locations of the modern streams. yellowish-brown, olive-gray, or yellowish-gray. The second trend is northeastward from near Pawnee Buttes Contains granules and clay balls. Highly cross to Chimney Canyons. Most of these channels are considered stratified. Contains many fragments of bone to be of Brule age. Channels of both trends contain many and teeth of brontotheres and rhinoceros topographically high, well-preserved deposits of Conglomerate composed of granules of quartz 2 conglomerate. The deposits are at several different levels, and and feldspar in a clayey matrix unless scour took place simultaneously at many depths I Claystone, light-olive-gray (damp), yellowish- 12+ suggest that the deposits are of several nearly gray (dry), blocky, sandy, silty contemporaneous but slightly different ages. In fact, the material in some of the channels mapped as Brule or Chadron Fossil bones of titanotheres and other animals could be as young as the Ogallala Formation. The clasts are characteristic of the Chadron were found in some of the chiefly of plutonic and volcanic igneous rocks, but some are of channel deposits. Fossil bones characteristic of the Brule were metamorphic and sedimentary rocks. Table 2 shows the found in Brule siltstone topographically below the sandstone composition of the gravel. Fragments of ash-flow tuff similar and conglomerate in the paleovalleys assigned to the Brule; to ash-flow tuff near Specimen Mountain in Rocky Mountain this evidence supports the assignment to the Brule of these National Park and fragments of tourmaline-bearing pegmatite channel deposits. suggest a provenance in the Big Thompson River-Rocky Mountain National Park area rather than from the Sherman Arikaree Formation .-Arikaree paleovalleys are Granite as suggested by Clark (1975, p. 104). In addition to characterized by abundant ovoid, biotite-rich, ashy, the sandstone and conglomerate, the Brule and Chadron calcareous siltstone nodules. The trends of three possible contain siltstone, limestone, and clay; however, the paleovalleys were derived from the distribution of Arikaree paleovalleys are recognized by the presence of the sandstone outcrops on the geologic map. On an early version of the and conglomerate. geologic map of Colorado (Burbank and others, 1935), a large area of Arikaree was shown in the area southeast of the South Platte River and east of Sterling. Later, Edward Lewis Table 2.-Composition of grauel in Chadron Formation of the U.S. Geological Survey checked this area and found no paleoualleys Arikaree, but only White River and Ogallala Formations. [Values represent the number of 1- to 2-in. (2.5- to S-cm) When I mapped the Sterling quadrangle (Scott, 1978), I pebbles identified by hand lens at three localities: 1) NE 14 found two small areas of rocks I assigned to the Arikaree along NE'4 sec. 19, T. 11 N., R. 55 W., Logan Co.; 2) SE'4NY2 the scarp southeast of Sterling. Although diagnostic fossils sec. 30, T. 8 N., R. 57 W., Weld Co., and 3) SW'4 SWlh could not be found, these outcrops were mapped as Arikaree sec. 16, T. 9 N., R. 59 W., Weld Co., Colorado] based on lithology similar to that of the Arikaree north of the river. Each of these areas represents a small segment of an Rock type Sample localities inferred paleovalley. Also, a large paleovalley is inferred that 1 2 3 trends eastward from Pawnee Buttes to the quadrangle Pegmatite ...... 44 61 58 border north of Sedgwick. Fossils diagnostic of Arikaree time Quartz ...... 23 14 19 were found at several places along this inferred paleovalley. Feldspar ...... 5 3 2 Gravel in the paleovalley is cross-bedded and composed Quartzite ...... 10 8 6 chiefly of gray rounded siltstone nodules, but also of clasts of Hornblende gneiss ...... 2 1 1 plutonic and volcanic igneous rocks; clasts are as large as 10 Hornblende granite ...... 1 0 0 in. (25 em). Sand beds are gray, loose to well cemented, Ashflow tuff-reddish-brown ...... 8 4 6 cross-stratified, and chiefly composed of quartz and feldspar. Porphyry-orange-weathering...... 1 1 2 According to Denson ( 1969), the microscopic fraction of fine Porphyry-gray ...... 3 1 0 sand in samples from the Arikaree at Martin Canyon, sec. 27, Rhyolite with bipyramidal quartz . . . . . 1 0 0 T. 11 N., R. 53 W., contains 0.8 percent heavy minerals Felsite-red ...... 1 0 0 composed of the following percentages of individual minerals: Chert...... 1 0 0 58 greenish-brown hornblende, 10 augite, 4 oxyhornblende, Granite ...... 0 1 0 1 hypersthene (totaling 73 percent volcanic minerals); 18 Felsic gneiss ...... 0 2 3 bluish-green hornblende, 6 epidote, 1 garnet, 1 sphene, and 1 Quartz diorite ...... 0 1 0 biotite (totaling 27 percent plutonic and metamorphic Andesite ...... 0 3 0 minerals.) Apparently most of the volcanic material came Aplite ...... 0 0 1 from vents west of the high plains. Ashflow tuff-gray ...... 0 0 1 A section of the Arikaree Formation measured in the Fossiliferous siltstone ...... 0 0 1 SW'4SE'4 sec. 18, T. 11 N., R. 50 W., Logan County, Colorado shows the characteristic fluvial lithology and the Total 100 100 100 placement of vertebrate fossils in th~ formation:
5 Feet Ogallala Formation, upp~r parl.-Paleovalleys were not Gravel, gray, loose to well-cemented, 8+ mapped in the upper part of the Ogallala Formation in the cross-stratified; composed of quartz and Sterling quadrangle. The formation is covered almost feldspar in a sandy to silty matrix. Larger everywhere by so thick a Quaternary mantle that channel de rounded clasts are chiefly siltstone, but include posits were not observed. Undoubtedly some channels exist; plutonic and volcanic rocks; largest size, 10 in. however, the upper part of the formation probably was deposited (25 em). Contains fossil bones at USGS locality as a broad fan and its great thickness, well beyond the possible D920. Merycochoerus cf. M. proprius, 3rd depth of scour to bedrock, suggests that channeling was not a upper molar, fragment of 3rd lower molar. Also dominant part of the mode of deposition. The great areal extent fragments of an artiodactyl foot bone of the Ogallala and the way it laps up onto the Laramie Moun Sand, yellowish-gray, fine, friable, contains 7 tains at the Gangplank indicate that it was deposited as a series of orange rusty streaks. Coarse in lower part; broad coalescing fans and that few if any streams were in well contains fossil bones formed discrete channels. Locally, north of Proctor in T. 11 N., R. Conglomerate, gray, clasts to 10 in. (25 em), 3-4 49, 50, and 51 W., well-sorted uncemented gravel in the upper mostly clasts of light-gray siltstone. Cross part of the Ogallala could possibly be the deposit of a through stratified flowing braided stream. Sand, yellowish-gray, loose, cross-stratified in 7 Nussbaum Alluvium.-The paleovalley in which the upper half, evenly stratified in lower half Nussbaum Alluvium was deposited was inferred from outcrops of Claystone and siltstone rubble, pale- 6 the alluvium on the geologic map. The belt of outcrops, which lies yellowish-brown or yellowish-gray, contains about 10-12 mi (1~19km) north ofthe river, extends only as far sand in irregular streaks throughout. Contains east as Willard. Because the South Platte River valley narrowed loose sand and siltstone pebbles in upper part to the east, the older stream deposits (Nussbaum and Rocky Rats and rarely lower down. Contains fossil bones Alluviums) east of their easternmost outcrops were removed by Sand, dark-yellowish-gray, loose, contains 1 + erosion in post-Nussbaum and in post-Rocky Rats time. In the pebbles of gray siltstone. Composed mostly of western part of the valley, there was room for the river to move quartz and feldspar laterally without destroying all of its earlier deposits, but to the Total 32-33 east, the valley was so narrow that all of the Pliocene and lower Ogallala Formation, lower part.-A sinuous paleovalley in Quaternary alluviums were eroded from the valley east of the lower part of the Ogallala Formation was extrapolated from Sedgwick. . contacts on the geologic map. The paleovalley extends from The Nussbaum paleovalley is preserved as a terrace that was Pawnee Buttes eastward to Martin Canyon where it apparently called the Dead Horse terrace by Gardner (1967, p. 141). The extends farther eastward under the upper part of the Ogallala Nussbaum paleovalley is about 450ft (137m) above the present Formation. The presence of deep intraformational channels floor of the South Platte River northwest of Fort Morgan. This suggests frequent severe erosional activity by the streams. De elevation above the river decreases eastward to only 400 ft ( 120 posits in the channel (Galbreath, 1953) include cobbly conglom m) northwest of Merino. Thus, the profile of the South Platte erate lenses cemented by calcium carbonate; brown fine-grained River in Nussbaum time converges downstream with the modern sandstone that locally weathers to a honeycomb appearance; profile of the river as also shown by Gardner (1967, fig. 9). This gray or brown soft or slightly indurated silty sand; reddish-brown, finding, if carried to its obvious conclusion, suggests that the early light-brown, or grayish-olive calcareous soft or indurated sandy Quaternary river profiles probably cross the modern river profile silt that commonly contains layers of gray calcareous siltstone and that somewhere in western Nebraska the Pliocene deposits nodules; and several layers of loose to cemented gray volcanic should be overlain by younger deposits in a superposed or ash as thick as 2ft (0. 6 m) that contain fossil bones. Some cobbles layercake sequence. Such a relationship also is implied by in the conglomerate apparently came from near Specimen Gardner (1967, fig. 9). A sequence like this also is shown by Mountain in Rocky Mountain National Park. Geologic sections 1, Schultz and others (1951, figs. la and 9 and p. 4) along the 2, 3, 4, 5, 8, 9, and 10 of Galbreath ( 1953) show the characteris North Platte River, but poor exposures and lack of detailed tics of the above deposits. mapping of surficial deposits along the South Platte River in The above deposits are equivalent to the upper part of southwestern Nebraska has prevented finding such a Galbreath's Pawnee Creek Formation (the lower part being sequence there. ·nt to my Arikaree). Deposits in the paleovalley con The Nussbaum paleovalley was wider than 4 mi ( 6. 4 km) and the alluvium originally was thicker than 70ft (21 m). The tain th~ -:>and Canyon local fauna of Wood and others (1941), and the Vim-Peetz, Kennesaw, and Eubanks local faunas of alluvium is composed of gravel, sand, and silt; clasts in the Galbreath ( 1953). These faunas are characteristic of the Val gravel are chiefly of Precambrian granitic rocks and are larger entine Member of the Ogallala of Condra and Reed ( 1959) than the average clast in the Ogallala Formation. The and of the lower Snake Creek Beds of McKenna (1965). The following measured section of the Nussbaum Alluvium shows deposits in the paleovalley apparently include two formations its typical lithology. as used by vertebrate paleontologists of the American Section near Dead Horse Springs in the SWY 6 Feet when Cary T. Madden (oral commun.) reported the discovery Silt, moderate-brown, sandy, clayey, pebbly, has 1.4 of a gomphothere tooth by Gordon E. Williams from the strong medium columnar structure, calcareous Rocky Flats Alluvium at the intersection of Antelope Draw and in lower part. The B horizon of a Bull Lake-age the Riverside Canal in the NWY 7 than 3 mi (5 km), however, the deposits are too severely lie along the north side of the river between Sedgwick and eroded on the south side to permit a good estimate of original Ovid, suggesting that the river course was still moving south width. ward, even after Louviers time. Fossils in the Verdos Alluvium include Equus {Equus) caballu$; the earlier horse, E. ( Dolichohippus) sp. is not GEOMORPHIC HISTORY characteristic of the Verdos. However, in the Van Gundy CRETACEOUS TO EOCENE gravel pit in the SEY 8 characterized the earlier paleovalleys was decreasing. major change in stream activity took place when alluviation, Nevertheless, the sandstone apparently was deposited in characteristic of Ogallala time, ceased and downcutting discrete mappable channels. (Unfortunately, time was not began, with occasional times of stability of base level, when available during this study to map these deposits.) The upper alluvium was deposited. The change probably resulted from part (Whitney Member) of the Brule, is composed of massive uplift of the mountains and increased runoff. Gradients of tan to gray silt, which, according to Harksen ( 1969) is a streams were steepened and deep canyons were cut in the combination of fluviatile and eolian deposits. flanks of the mountains. The dissection apparently either MIOCENE caused stoppage of deposition of the Ogallala or started soon In early Miocene time, the volcanism that started about after the latest sediments of the Miocene Ogallala Formation 36 million years ago in the mountains to the west continued to were deposited. The present pattern of streams just east of the mountains produce volcanic flows and crystal-rich ash that were incorporated as clasts in the Arikaree Formation (The suggests that dissection started with streams flowing Arikaree is Miocene and locally Oligocene(?) in age, but only semi-parallel to the mountain front and emptying into two the early Miocene upper part equivalent to the Marsland master streams, the South Platte and Arkansas Rivers. The Formation has been recognized in northeastern Colorado.) sudden change from eastward drainage that deposited fan Block faulting began in the mountains about 28 m.y. ago in alluvium in the Ogallala Formation to stream dissection the Oligocene and channels in the mountains were disrupted. semi-parallel to the mountain front apparently came about Channel deposits of the Arikaree on the plains apparently because· the two greatest systems of drainage from the were unaffected by the faulting and are spread rather evenly mountains at the end of Miocene time were in the areas west across the Oligocene rocks and are not channeled deeply into of Canon City and west of Greeley. When dissection began, them. Deposits of early Miocene age correlative to the the headward-cutting tributaries of these two drainage systems cut northward and southward near the mountain Marsland Formation of Nebraska have not been documented front and because they apparently had steeper profiles than in the Front Range, but are known from the lower parts of the slope on the Ogallala they were able to capture all of the basin fills in Middle Park (Troublesome Formation) and near smaller streams coming out of the mountains. Very soon after Howard, Colo. (Dry Union Formation). The fineness and downcutting started all streams flowing across the Ogallala good sorting of much of the Arikaree on the plains suggest a Formation would have been beheaded except the South gentle braided stream, and yet part of the formation is coarse Platte and Arkansas Rivers which would have become conglomerate and rubble beds that indicate periods of strong entrenched into the Ogallala. On the plains, erosion would currents and great carrying power. The scarcity of channels have been as fast as changing base level would have suggests that the Arikaree was either confined to a few small permitted as the master streams and their new tributaries cut channels or a great part of it was later removed by streams that down through nonresistant Tertiary deposits and into deposited the Ogallala Formation. even-less-resistant Cretaceous marine shale. In late Miocene (Ogallala) time, erosion and channeling The capture of most drainage from the mountains by the into the late Eocene surface on the Precambrian rocks of the two master streams would have left the Ogallala with no major mountains ranged from 100-800 ft (30-240 m) on various drains. I am unable to prove that in Miocene time any of these streams leaving the Front Range. Some erosion of older units streams was flowing in its present position and depositing the also took place on the plains by the streams that deposited the Ogallala. Therefore; I believe that all the streams now crossing Ogallala. Generally, however, the Ogallala was laid down by the Ogallala started after the beginning of dissection. At the broad shallow aggrading streams that did not cut the earlier beginning of dissection, the streams on the Ogallala were sedimentary rocks. flowing eastward down the depositional slope; even now the The possibility that these streams had well developed drainage on preserved Ogallala deposits is chiefly eastward. valleys at the end of Ogallala time seems slight, and I do not At the end of Ogallala time, the South Platte River probably believe that the present valleys of Frenchman Creek, was north of its position in Pliocene (Nussbaum) time. Republican River, Arikaree River, the Smoky Hill River, or Eastward from Greeley, the ancestral South Platte River, their tributaries existed as well-developed valleys in their while downcutting, apparently migrated southward in present positions at the close of Ogallala deposition. Pliocene time, the same as the proved movement in most of its Both the approximate sources and courses of streams reach in Quaternary time. The extent of migration is that deposited the Ogallala can be inferred from trends of unknown, but surely was more than 10 mi ( 16 km). It moved preserved gravel channels in the mountains (Scott, 1975; and southward because the principal drainage by tributaries was Epis and others, 1976) and from the lithology of clasts in the from the northwest. The southward movement forced the Ogallala. For instance, one of the channels providing scarp on the south side of the river to retreat southward, kept sediments to the Ogallala came from the mountains north of the reaches of the streams from the south very short, but Pikes Peak (Scott, 1975; Epis and others, 1976). I have seen lengthened the reaches of the streams from the north. The many fragments of amazonstone and smoky quartz from the valley of the south Platte was being constantly widened from Pikes Peak Granite in the Ogallala of east central Colorado; in Pliocene to the present time. addition, fragments of volcanic rocks similar to those in the The style of drainage in Pliocene and later time differs Thirtynine Mile volcanic field were found in the Ogallala at considerably from the style in Ogallala time. In Ogallala time Cedar Point. Other streams coming out of the mountains also most drainage flowed nearly due east from the mountains; in carried unique rocks and minerals that can be recognized in Pliocene and later time the drainage flowed semi parallel to the the Ogallala Formation.. mountains from three divides, one at the Wyoming border, PLIOCENE one at the South Platte-Arkansas divide, and one just north of At the beginning of Pliocene time, about 5 m.y. ago, a the New Mexico border, to two master streams, the South 9 Platte and Arkansas Rivers. Of the streams joining the South Creek were created after the Nussbaum Alluvium was Platte River from the north and south, those to the west deposited 3 m.y. ago. The Nussbaum Alluvium lies about 350 developed as major tributaries parallel to and close to the ft (107m) above the floor of Big Sandy Creek and earlier had mountains first and then streams to the east gradually were been transported across the present course of Big Sandy added. New tributaries were added continuously as the scarp Creek. For this reason, and based on the distribution of the on the Ogallala Formation retreated farther and farther east Rocky Flats Alluvium along Big Sandy Creek in the Limon 1ox from the mountains and the valleys of the South Platte and 2° quadrangle (Sharps, written commun., 1979) I believe that Arkansas Rivers were constantly widened. By now, the Big Sandy Creek could not have formed any earlier than Ogallala has been removed from more than one half of the Rocky Flats time about 1.4 m.y. ago. area of Colorado east of the mountains. In addition, capture of any ancestral Arikaree River also Enlargement of. the drainage area of the South Platte took place long before the Nussbaum Alluvium was River caused some change in the South Platte-Arkansas deposited, for the Nussbaum lies only about 10 mi (16 km) divide and in the course of the modern South Platte River. west of the scarp of the Ogallala south of Cedar Point and The low area southward from Fort Morgan to River Bend about 350ft (107 m) below the eroded top of the Ogallala. formerly was capped by the Ogallala Formation, but the Thus, the Ogallala scarp had already retreated about 50 mi Ogallala and a considerable thickness of underlying beds (80 km) from the mountain front in the 2 million years were since removed by erosion. All the streams that enter the between the end of Ogallala deposition 5 m.y. ago and South Platte River from the south from Brush westward deposition of the Nussbaum about 3 m.y. ago. Since including Beaver, Badger, Bijou, Kiowa, and Box Elder Nussbaum time the retreat has been much slower-about 10 Creeks head in the divide between the Arkansas and South mi (16 km) in 3 m.y. The sharply increased rate of erosion in Platte Rivers. Apparently, because of greater activity of these early Pliocene time is not surprising because this also is the streams than of those south of the divide, the divide has been time of greater canyon cutting in the mountains. consistently migrating southward since Ogallala time Discussions with Coffin, Paul A. Schneider, Jr., and interrupted only by an occasional stream capture. This divide . Joseph A. Sharps, all of the U.S. Geological Survey (oral area is noted for periodic torrential rainfalls that result in commun., 1979) brought up the fact that Big Sandy Creek is catastrophic flash floods. The large amounts of sediment underfit in the reach between River Bend and the Arkansas generated by these flash floods in late Pleistocene time River. Inasmuch as Big Sandy Creek is underfit, it ultimately dammed the South Platte River, temporarily raised its profile was the stream that was captured rather than the stream that at the mouth of each of the tributaries (especially Bijou did the capturing. Big Sandy Creek probably was originally a Creek), and periodically forced the river against its north much larger stream flowing eastward from near the mountains valley wall. Mapping by Gardner ( 1967) suggests that the as shown by both the coarse character of its valley fill and its river had been forced against its south valley wall in Louviers underfit valley. The valley fill is 70ft (21m) thick and consists Alluvium time as it was in the rest of its reach in eastern of gravelly alluvium that yields large quantities of Colorado, but that the southern migration was reversed groundwater to wells (Coffin, 1967). during deposition of the Broadway Alluvium when the river The drainage history of the River Bend area probably was was forced to move 6 mi (10 km) northward and cut a new even more complicated than previously suggested and may channel in bedrock. have involved three stream captures. A sharp northward bend A stream capture involving Big Sandy Creek has been of East Bijou Creek about 11 mi ( 18 km) west of River Bend suggested by several authors including Smith ( 1940, p. 147), suggests to me that the upper part of East Bijou Creek Pearl (1971, p. 29) and by Donald L. Coffin of the U.S. formerly was a tributary of a stream farther east, possibly Geological Survey (oral commun., 1960). Smith inferred that Beaver Creek or Big Sandy Creek. I propose that this upper Big Sandy Creek had captured a northeastward flowing part of East Bijou Creek flowed eastward across what is now a stream near River Bend; Pearl (1971) proposed that Big divide 325 ft (99 m) higher than Big Sandy Creek and joined Sandy Creek had captured the upper part of the ancestral ancestral Beaver Creek 2 mi (3 km) west of River Bend; from Arikaree River near River Bend. He inferred that the Arikaree there ancestral Beaver Creek flowed northward across what is River at the close of Ogallala deposition was a major river that now a divide 175 ft (53 m) higher than the loop of Big Sandy originated in the mountains and flowed east down the Creek west of River Bend and joined the modern course of northeast-trending reach of Big Sandy Creek to join the Beaver Creek (fig. 2A). The upper part of ancestral Beaver course of the present Arikaree River. Creek later was captured near River Bend by the present Big As stated earlier, I don't believe that an ancestral Arikaree Sandy Creek and the increased large flow which created the River existed at the end of Ogallala time, but instead, that all of broad valley of modern Big Sandy Creek all went the well-defined streams on the Ogallala originated after the southeastward to the Arkansas River (fig. 2B). Finally, the start of dissection in Pliocene time. Indeed, only one of these upper part of this Big Sandy Creek, which is now the upper streams, the South Fork of the Republican River has been part of East Bijou Creek, was captured, possibly after Slocum incised into the westward-facing scarp of the Ogallala time, by modern East Bijou Creek about 11 mi ( 18 km) west of Formation. The other streams arise from tributaries that start River Bend (fig. 2C) and the water was diverted from Big east of the scarp. This suggests to me that only the South Fork Sandy Creek to the South Platte River. This capture severely of the Republican River had any possibility of being an diminished the flow of Big Sandy Creek and created the established stream that was beheaded after Ogallala underfit stream we see today. These postulated events deposition ended. Also, I believe that all drainage across the probably all took place in Pliocene and early Quaternary time. Ogallala was beheaded by cutting along the mountain front The Nussbaum Alluvium was deposited in Pliocene time long before creation of present Big Sandy Creek. Both the about 3 m.y. ago and the Rocky Rats Alluvium was deposited southeastward and the northeastward reaches of Big Sandy starting in Pleistocene time about 1.4 m.y. ago. 10 104° early Quaternary time. A cooling climate was causing ~~ lJ ) 0 Agate l glaciation in the mountains to the west. The Verdos Alluvium I and the Pearlette ash type 0 were deposited. The South Platte I ;( River was captured near Barr Lake by a tributary of the Cache :,-j' Bulck 0 I ~ .. I Ia Poudre River and its former channel along Beebe Draw was I abandoned. The river moved southward several miles again ~ I ELBERT I UNCOLN as the Slocum Alluvium was deposited. The river reached its present position in late Pleistocene (Bull Lake) time or a little 0 River Bend ! .·-...-.l._..,llmon bit later. Drainage in the quadrangle was strongly influenced 39°15' 8lg I "- ~·· by southeast trending wind-carved valleys and by linear 1I "~~"'- ... ) eolian deposits. j ,-v··· I ".-.... ~." In Illinoian time, southeasterly winds started to erode sedimentary rocks in the area, particularly the outwash being I carried by the South Platte River. Eolian activity continued I through parts of Illinoian, Bull Lake, Pinedale, and Holocene I time. The Illinoian deposits (Loveland Loess) and the Bull I Lake(?) and Pinedale deposits (Peoria Loess) are composed I I of wind-blown silt; the Peoria Loess forms definite I southeastward trending ridges. Eolian sand was deposited in ELBERT I UNCOLN linear dunes during Pinedale and Holocene time. River Bend ~--- 0 39°15' REFERENCES CITED Blackstone, D. L., Jr., 1975, Late Cretaceous and Cenozoic B history of the Laramie Basin region, Southeast Wyoming, 104° in Cenozoic history of the southern Rocky Mountains: ,,/'C~ee'f. I .,.; 0 Agate Geological Society of America Memoir 144, p. 249-279 . J.: ;.-i I Bryan, Kirk, and Ray, L. L., 1940, Geologic antiquity of the :'! I ~ Lindenmeier site in Colorado: Smithsonian Miscel ;·· Bulck 0 (J I I laneous Collections, v. 99, no. 2, pub. 3554, 76 p. ,8.: ( I Burbank, W. S., Lovering, T. S., Goddard, E. N., and Eckel, IX!\ ELBERT I UNCOLN E. B., 1935, Geologic map of Colorado: U.S. Geological East ( I Survey, scale 1:500,000. River Bend ,/·.,-...... __r·' 0 Clark, John, 1975, Controls of sedimentation and ,. ...,/ 39°16' _,..· provenance of sediments in the Oligocene of the central ,• Rocky Mountains, in Cenozoic history of the southern Rocky Mountains: Geological Society of America c ... ~· 10 Memoir 144, p. 95-117. 1 1 I I 1 1 I 'I I Coffin, D. L., 1967, Geology and ground-water resources of 10 20 KILOMETERS the Big Sandy Creek valley, Lincoln, Cheyenne, and Figure 2.-Diagrams showing postulated stream-course Kiowa Counties, Colorado with a section on chemical changes involving East Bijou, Beaver, and Big Sandy quality of the ground water, by C. A. Horr: U.S. Creeks. Dashed lines indicate inferred stream courses Geological Survey Water Supply Paper 1843, 49 p. that no longer exist. A, upper part of East Bijou Creek and Condra, G. E., and Reed, E. C., 1959, The geological section ancestral Beaver Creek are part of Beaver Creek of Nebraska: Nebraska Geological Survey Bulletin 14A, 82 p. [revised from 1943 edition by E. C. Reed]. drainage. B, Big Sandy Creek captures upper part of Corbett, M. K., 1968, Tertiary volcanism of the Beaver Creek. C, East Bijou Creek captures upper part of Big Sandy Creek. Specimen-Lulu-Iron Mountain area, North-Central Colorado: in Epis, R. C., ed., Cenozoic volcanism in the In Quaternary time, southward migration and Southern Rocky Mountains: Colorado School of Mines downcutting of the South Platte River continued. The Quarterly, v. 63, no. 3, p. 1-37. downward cutting, the result of changed baselevel, has Dalquest, Walter W., 1975, Vertebrate fossils from the Blanco continued to the present time, interrupted less than 10 times local fauna of Texas: Texas Tech Museum Occasional when baselevel was stabilized long enough to permit the river Papers no. 30, 52 p. to widen its valley floor at progressively lower topographic Denson, N. M., 1969, Distribution of nonopaque heavy levels. 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A., 1975, Late Cenozoic sedimentation and Colorado and Kansas: U.S. Geological Survey deformation in northern Colorado and adjoining areas, Miscellaneous Investigations Series Map 1-944, scale in Cenozoic history of the southern Rocky Mountains: 1:250,000. Geological Society of America Memoir 144, p. 179-209. Smith, H. T. U., 1940, Geologic Studies in southwestern ___ 1977, Volcanic ash beds in continental deposits of Kansas: Kansas State Geological Survey Bulletin 34, 212 the Southern High Plains: Their bearing on the time of p., 34 plates. the Blancan-Irvingtonian faunal transition: Geological Smith, R. 0., Schneider, P. A., Jr., and Petri, L. R., 1964, Society of America Abstracts with Programs, 1977, v. 9, Ground-water resources of the South Platte River basin no. 7, p. 1034. in western Adams and southwestern Weld Counties, Matthew, W. D., 1901, Fossil mammals of the Tertiary of Colorado: U.S. Geological Survey Water Supply Paper northeastern Colorado: American Museum of Natural 1658, 132 p. History Memoir 1, p. 355-447. Soister, P. E., and Tschudy, R. H., 1978, Eocene rocks in the McKenna, M. C., 1965, Stratigraphic nomenclature of the Denver basin: Rocky Mountain Association of Miocene Hemingford Group, Nebraska: American Geologists, Field Conference, Guidebook, p. 231-235. Museum Novitates, no. 2228, 21 p. Stanley, K. 0., 1976, Sandstone petrofacies in the Cenozoic Naeser, C. W., lzett, G. A, and Wilcox, R. E., 1973, Zircon High Plains sequence, eastern Wyoming and Nebraska: fission-track ages of Pearlette family ash beds in Meade geological Society of America Bulletin, v. 87, no. 2, p. County, Kansas: Geology, v. 1, p. 187-189. 297-309. Obradovich, J. D., Izett, G. A, and Naeser, C. W., 1973, Stanley, K. 0., and Wayne, W. 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