Isostatically Induced Fluvial Origin of the Norridgewock Sand Plain, Central , USA

by

Robert E. Nelson1

Newton W. Krumdieck1, 2

Daniel F. Belknap3

and Ramsey Meigs 1, 4

______

1 Department of Geology, Colby College, 5804 Mayflower Hill, Waterville,

Maine 04901-8858, USA; [email protected]; (ph) 207-859-5804, (fax) 207-859-5868

2 current address: 443 Taborton Road, Sand Lake, New York, 12153, USA;

[email protected]

3 Department of Earth Sciences and Climate Change Institute, University of Maine, Bryand

Global Sciences Center, Orono, Maine, 04469-5790, USA; [email protected]; (ph) 207-

581-2159, (fax) 207-581-2202

4 current address: 2504B Chapel Hill Rd, Durham, NC 27707; [email protected]

Norridgewock Sand Plain, Maine, USA .. 2 ..

A Abstract

Thick Last Glacial Maximum (LGM) ice covered all of modern Maine, U.S.A, resulting in

hundreds of meters of isostatic depression. The rapidity of post-glacial retreat, with

consequent invasion by the sea up to 150 km inland, left an important record of

glacioisostatic changes and induced glaciofluvial and post-glacial adjustments.

Previous work has shown that a migrating proglacial isostatic forebulge resulted in early

Holocene tilting of the 300-km2 basin, such that its outlet shifted from a

Penobscot River tributary in the north, to a southern discharge into the upper

of north-central Maine. Our work indicates that the resulting headwaters erosion and 60-80%

increase in upstream Kennebec discharge resulted in sufficient downstream alluviation to

force river avulsion over a low divide into a new drainage. This left behind a broad, sandy

plain underlain by buried channels, and a former outlet course lined with discontinuous

terraces as much as 12 m above modern stream levels. The modern river flows in its post-

diversion channel for some 26 km, and cascades over a major falls, before rejoining its

former course.

These results are comparable to changes previously documented in the lower Rhine-Meuse

valley in the Netherlands, and may be applicable to understanding otherwise enigmatic

stream courses in similar settings in other parts of the world (e.g., northern Washington state

and adjacent British Columbia, Scandinavia and the Baltic States, Scotland and Northern

Ireland).

Norridgewock Sand Plain, Maine, USA .. 3 ..

Key words: avulsion, glacio-isostacy, Kennebec River, Moosehead Lake, Presumpscot

Formation, sand sheets

A Introduction

The Norridgewock sand plain of southern Somerset County, central Maine, USA (Fig. 1), is a

large, planar feature extending over an area of approximately 15.5 km2 (labeled "The Plains"

on Figs. 1 & 3a; Fig. 2). Large areas of level ground like this are relatively uncommon in

interior portions of the state, which was completely glaciated throughout the Last Glacial

Maximum (LGM). However, unlike other superficially similar features [e.g., the Kennebunk

Plains of southern Maine (Tary et al., 2001), the Brunswick-Topsham sand plain (Crider,

1998; Weddle, 2001, 2002) or Pineo Ridge of eastern Maine (Borns and Hughs, 1977; Tary

et al., 2007)], it is neither clearly an outwash plain nor a glaciomarine delta feature, nor is it

underlain at the surface by postglacial marine silts and clays of the Presumpscot Formation

(Bloom, 1960).

In the Waterville, Maine, 15-minute quadrangle (scale 1:62,500), this feature has been

mapped as a glaciofluvial unit by Smith (1986), whereas in the adjacent Norridgewock

quadrangle, Weddle (1987) mapped it as a regressive sand of the Presumpscot Formation,

which generally consists of postglacial marine silts and clays. This latter is consistent with

well data from the northern portion of the plain that did not penetrate the full sediment

thickness (Adamik et al., 1987), as well as earlier interpretation of drill logs from boreholes

that penetrated the sedimentary cover to bedrock (Mueser et al., 1971).

Norridgewock Sand Plain, Maine, USA .. 4 ..

These early boreholes (Mueser et al., 1971) were drilled in a preliminary site evaluation for a proposed paper mill that ultimately was built elsewhere. They showed a sequence that, from the base, consisted of Devonian granitic bedrock, till, outwash, marine silts and clays, and overlying interbedded sand and silt/clay units, capped with a thick sand – a typical postglacial regressive sequence in an area that has experienced approximately 120 m of postglacial isostatic rebound (Thompson and Borns, 1985; Belknap et al., 1987; Thompson et al., 1989).

In some boreholes, the entire Quaternary sequence exceeds 50 meters in thickness (Mueser et al., 1971), although there are Silurian metasedimentary bedrock outcrops near the margins of the plain that indicate considerable relief in the buried bedrock surface.

The Kennebec River, which borders the sand plain to the north, is by discharge the sixth- largest river in the continental USA to flow directly into the ocean, with a mean annual discharge at the mouth of ~ 261 cms (cubic metres per second) (Fenster et al., 2005).

However, when it encounters the Sand Plain, the river makes an abrupt 90-degree turn, from a flow direction of S40°E to one approximately N45°E, to flow approximately parallel to regional bedrock strike (Fig. 1c; Osberg et al., 1985). This is also where the river ceases flowing essentially alongside a major esker complex that originates near The Forks and continues more or less in a straight line to come adjacent to the river once again in the

Augusta area (Fig. 1b; Borns and Hagar, 1965). There are no apparent bedrock controls in the channel bed or on the cutbank at the bend, and the cutbank itself consists of unconsolidated sediments. Thus, this change in flow direction has also been enigmatic.

At the southern end of the sand plain, the north-flowing 3rd-order Martin Stream makes a similarly abrupt change in course, first changing flow direction to the east and then to the southeast (Fig. 1c). The nature of the channel also changes dramatically, from one tightly Norridgewock Sand Plain, Maine, USA .. 5 ..

confined where it flows through till and the sediments of the Presumpscot Formation, to one

more broad, shallow and braided as it enters a floodplain reach hundreds of metres wide.

This study was undertaken in hopes of understanding better the drainage history of this

portion of central Maine, and of the ultimate origins of the Norridgewock sand plain.

A Methods

Preliminary field evaluations began with study of topographic maps (scale 1:24,000 and

1:62,500) and aerial photography, including archival materials. Natural and artificial

exposures (roadcuts, railroad cuts, borrow pits) accessible within and on the perimeter of the

sand plain were investigated to determine the nature and orientation of strata and depositional

environments. A hand auger was used to bore through the surficial sand at numerous

localities to determine the depth through the surficial sand to the uppermost gray silts and

clays of the underlying Presumpscot Formation.

In an attempt to obtain remote geophysical imaging of stratigraphy of the sand plain, we

conducted ground-penetrating radar (GPR) transects using a Sensors and Software

PulseEkko10 with unshielded 100 MHz antennae and 1000 volt transmitter. Five GPR lines

were run across a section of the Plain (Fig. 4), three in roughly a NE-SW direction and two

trending approximately NW-SE. Lines were laid out along straight tapes and flags, on

roughly level agricultural fields and gravel roads. No survey leveling was felt to be necessary

in this setting. GPR Line 5 passed immediately adjacent to the locality of borehole BH4 of

Mueser et al. (1971). Post-processing of the GPR data was with IXeTerra software, which

allows filtering, georeferencing, and leveling (if needed), and adjustments for Norridgewock Sand Plain, Maine, USA .. 6 ..

electromagnetic velocity. A velocity of 0.06 m/ns, typical for saturated sand, and attenuation

of roughly 0.1 dB/m (from the GPR manual and best output results), were chosen. Digital

images were then interpreted on the basis of electromagnetic reflection character (a: intensity

based on grain size differences, compositional differences, water content; b: internal and

external geometry; and c: successions) and output with Canvas10 graphics.

Subsequent to the acquisition and interpretation of the GPR data, additional study was

undertaken down the lower reaches of Martin Stream (lower-central portion of Fig. 2). Sand

samples were taken from selected exposures for grain size analysis and mineralogical

determination.

A Results

Despite the superficial appearance of being a topographically horizontal feature, the

Norridgewock Sand Plain is a gently inclined surface sloping to the SE at ~2 m/km. Over

much of the area of the northern and central plain, the thickness of the surface sand exceeded

the maximum depth penetrable by auger (~6 meters), but in the south the unit gradually thins

and pinches out. Lowland surface exposures beyond this consist of Presumpscot Formation

marine silt and clay, with till exposures farther to the south and on higher slopes.

The riverbank exposure where the Kennebec River makes the abrupt turn to the NE in

Norridgewock shows minimal fresh exposures, but is principally mantled with sandy, muddy

colluvium, with scattered boulders to 30-50 cm across. Exposures nearby along a railroad

grade (location S on Fig. 1c), however, show well-bedded gray silts at an elevation of Norridgewock Sand Plain, Maine, USA .. 7 .. approximately 54 m a.s.l., or 18 m below the main surface of the sand plain. This is consistent with the borehole logs of Mueser et al. (1971) and Adamik et al. (1987).

Typically ~80% of the sand in the plain and in the terraces along Martin Stream ranges from

0.125–0.500 mm in diameter, though at the mouth of Martin Stream the lowest terrace is underlain by finer material, with over 75% of the grains smaller than 0.355 mm. Occasional small (up to 1 cm diameter) pebbles are also encountered. Lithic fragments were typically

55–75% of the larger grains, with quartz comprising 15–40% of the remainder. Other identifiable grains consisted of feldspars (up to 8%) and biotite, usually < 2%. This contrasts with coarser sand from the Embden Formation of Borns and Hagar (1965), which in a sample from Embden, showed 78% of the grains in the 0.25–1.0 mm size range, with 18% of the grains being coarser than 1 mm, including 5.5% granules 2–4 mm in diameter. Embden

Formation grain mineralogy was more dominated by lithics than what was found at

Norridgewock.

GPR lines that were recovered along generally NE-SW transects showed complicated subsurface stratigraphy (Fig. 4) including multiple strong reflectors, none of which was expected or indicated in the borehole stratigraphy but which is nonetheless consistent with that subsurface sampling. Both lines 2 and 5 (Fig. 4) showed broad troughs cut into the underlying Presumpscot Formation, each approximately 200 meters wide and 4–5 meters deep, with bases 8–10 meters below the modern ground surface. Both have apparently been back-filled with cross-stratified sandy deposits with multiple thin gravel layers.

The Martin Stream floodplain in the region of the southern margin of the sand plain is broad and the channel is braided at high flow regimes. Farther downstream, the channel gradually Norridgewock Sand Plain, Maine, USA .. 8 .. becomes more constrained and meanders more tightly. Abandoned cutbanks in elongate, rounded hills interpreted as paleo-dunes expose cross-bedded fine sands; slip-face orientation in one excavated exposure indicate a paleowind direction from the SW, while a second showed a NW paleowind direction. Other plain-proximal cutbank exposures reveal sequences of ripple cross-bedded sands interpreted to have been deposited in a fluvial setting.

Farther downstream, the channel and floodplain become still narrower. At a site just downstream from the confluence of Alder Brook, some 9 m above stream level, a cluster of large, partly rounded granitic boulders occurs at the surface, surrounded by fluvial fine sand

(Fig. 5). The largest boulder measured in the cluster is in excess of 4 m in greatest dimension.

Above this on the hillside, 2-3 m higher than the highest of the boulders, is a planar flat surface also underlain by fine sand, interpreted here as a high terrace (Fig. 6A). This terrace thus stands some 11–12 m above the modern course of Martin Stream.

Near the mouth of Martin Stream, at its confluence with the modern Kennebec River, three distinct terrace levels may be identified. The higher of these stand at 3–4 m (Fig. 6B) and 5–6 m above the modern level of Martin Stream, and the lowest, found nearest the mouth, occurs

1.5–2 m above the modern Kennebec River surface (Fig. 6C). Bedrock outcrops in the stream bed have presented significant downcutting of Martin Stream below the two higher terraces.

The extensive dune area ("D" on Fig. 2) probably originated as a result of NW winds from the broad alluvial floodplain now abandoned.

Early 20th century construction of the Shawmut Dam on the Kennebec River, some 5.5 km south (downstream) of this site, has artificially raised the river level by some 7 m, however.

The mouth of Martin Stream, shown as an estuary on modern maps, was free-flowing in a Norridgewock Sand Plain, Maine, USA .. 9 ..

deep ravine prior to this dam construction, and thus the lowest of these terraces (Fig. 6C)

would have been as much as 8–10 m above the natural river level.

A Discussion

We interpret the two wide troughs shown in the GPR lines to be fluvial channels of an

ancestral Kennebec River. Sedimentation in this paleoriver gradually infilled the channel,

such that it was forced over a drainage divide and down a new lower course, leading to its

current position.

However, this interpretation presents some problems. What was the source of the vast

volumes of sediments that infilled the channel, and if the Kennebec River flowed into the

area currently occupied by the Sand Plain, where did it exit? The existence of a giant paleo-

eddy, in which the water recoursed back on itself to continue down the current river course, is

highly unlikely. The extension of the plain in a low area at the northeastern margin, by

Bigelow Hill Road (Fig. 1c), is underlain by Presumpscot Formation marine silts at the

surface, and shows no evidence of fluvial downcutting.

Farther to the south, to the west, and to the east, the terrain generally rises significantly, and

the upland surface consists of uneroded till or bedrock outcrops. The only possible exit for

fluvial discharge in this area is down what is now the valley of Martin Stream. However, the

present cross-sectional area of the modern valley of Martin Stream is insufficient to

accommodate the discharge of the Kennebec River.

Norridgewock Sand Plain, Maine, USA .. 10 ..

However, it may not have been necessary to accommodate the full flow of the modern

Kennebec in this channel. Balco et al. (1998) demonstrated the impact of a migrating

glacioisostatic forebulge on the basin of Moosehead Lake, which is the headwaters of the

modern Kennebec River. Early in postglacial time, Moosehead Lake drained northeastwards

across a low divide (the Seboomook Outlet, Fig. 1b), into the upper reaches of the West

Branch of the . By about ~9750 14C ybp, the tilting basin began draining

through East Outlet at the head of the Kennebec River (Balco et al., 1998), and the

Seboomook Outlet was abandoned. Kelley et al. (2005, 2010 in review) discuss the

consequences of this alteration for the evolution of the Penobscot River and upper

Penobscot Bay, as well as the Kennebec River drainage. Both East Outlet and West

Outlet from Moosehead Lake are currently controlled artificially by dams that maintain lake

level at 313.6 meters (1030 feet) a.s.l., approximately 4 m above natural lake level. West

Outlet was, in pre-dam days, apparently only occupied at times of high spring runoff (Balco

et al., 1998).

This is analagous, at least in part, to the significant shifts in the lower Rhine-Meuse

Pleniglacial and Lateglacial drainage as a result of differential glacio-isostatic rebound of the

proglacial forebulge, based on the sedimentary record in the southern North Sea Basin that

was reported by Busschers et al. (2007).

B Discharge Calculations

Analysis of historical (1931–present) mean annual discharge (MAD) records for the

Kennebec River shows that absent the input from Moosehead Lake (represented by the

discharge measured at The Forks), the modern discharge of the Kennebec River when it Norridgewock Sand Plain, Maine, USA .. 11 .. reaches Norridgewock would only be ~50–60% of its current value. Although there are some minor streams above The Forks that contribute to Kennebec discharge, the bulk of that discharge originates in the two outlets from the Moosehead Lake basin, and includes all flow from the into the lake. Downstream from The Forks, the Dead River, the

Carrabassett River, and the Sandy River are the major tributaries to the Kennebec before it reaches Norridgewock.

At Bingham, the Kennebec discharge includes all input from the Dead River, Austin Stream and other lesser tributaries. Subtracting the discharge measured at The Forks yields a projected discharge for the Kennebec River at this site that does not include Moosehead Lake input. Both the and Sandy River enter the Kennebec below this station, at

North Anson and Madison, respectively. These are the only major tributaries to the Kennebec before it reaches Norridgewock.

Although there is a gauging station currently operational at Madison, long-term data are not available. However, an estimated discharge for the Kennebec at Madison over the historical record period may be calculated by adding the historical discharge record for the Kennebec

River at Bingham to that for the Carrabassett River at North Anson and the Sandy River at

Mercer, immediately to the west of Norridgewock. This, then, should approximate the discharge of the Kennebec at Madison and immediately downstream at Norridgewock. If one then subtracts from this the discharge at The Forks, one would get an estimated MAD for a

Moosehead-Lake-free (MLF) Kennebec as it arrived at Norridgewock.

The maximum calculated MLF MAD for the Kennebec River at Norridgewock over the period of historical records is approximately 170 cms (cubic metres per second, = ~6000 Norridgewock Sand Plain, Maine, USA .. 12 ..

cubic feet per second [cfs]; the US Geological Survey uses cfs in all its discharge

measurements) in 1996, with a mean MAD over the period of record of slightly over 98 cms.

However, this maximum discharge was still only 56% of the actual calculated total discharge

at this site, which included some 133 cms (4700 cfs) recorded at The Forks. The mean

increase in discharge attributable to Moosehead Lake outflow is 75% over the period of

record, with one extreme year (1941) in which the Moosehead Lake drainage more than

doubled the total flow at Madison.

B Fluvial erosion/sedimentation

The tipping of the Moosehead Lake Basin to force this new discharge through the East Outlet

would have resulted in drainage across a landscape that had a significant veneer of till and

other glacial debris: the West Outlet dam is underlain by some 15 m of till that overlie older

Quaternary lacustrine sediments (Balco et al., 1998). Although the East Outlet dam currently

is underlain by bedrock at or very near the surface (Balco et al., 1998), probes to bedrock on

either side of this outlet show ~6 m of silty or sandy till (Locke, 1998). It is reasonable to

assume, then, that at least some 6 m of till and other sediment has been eroded as a result of

the new spillway development, and possibly more.

The input of such an increased discharge and sediment load into the Kennebec Drainage

would have had dramatic impacts downstream. Farther south in the Kennebec River valley,

the postglacial marine silts and clays of the Presumpscot Formation rise to an elevation of ~

120 m (400 feet), but along with the early postglacial Embden Formation and North Anson

Formation show significant erosional scour in the central valley (Borns and Hagar 1965). Norridgewock Sand Plain, Maine, USA .. 13 ..

This erosional scour may reflect the initial impacts of the sudden increase in discharge resulting from the Moosehead contribution.

In our study area, however, the ancestral Kennebec River was flowing through a broad topographic low, some 3 km wide, between Dodling and Bigelow Hills (Fig. 2), perhaps created in part by deposition of sediments eroded upstream by the increased discharge. Here, even the main channel was some 200 m wide, but a maximum of 4–5 m deep (Fig. 4). We propose that alluviation in this channel and floodplain system gradually raised the level of the river channel to such an elevation that flow spilled over a low drainage divide into an adjacent stream valley to the northeast, analagous to the documented course shifts of the Kosi

River in India in response to extreme sedimentation from the Himalaya (Wells and Dorr,

1987) but quite distinct from avulsion events in areas of general tectonic and climatic stability (e.g., Phillips, 2009)

This adjacent stream channel would have been a small (probably 3rd- or 4th-order) right tributary to the south-flowing Wesserunsett Stream, which enters the modern Kennebec River at Skowhegan (Fig. 1), but which would have been the main trunk stream in this area prior to the Kennebec avulsion; prior to the Kennebec diversion, Wesserunsett Stream would have joined the ancestral Kennebec at Hinckley (right margin of Fig. 1c). The new Kennebec

River widened and eroded its new channel downwards until it was halted by a bedrock high, forming a cascading falls at what is now the town of Skowhegan.

Martin Stream, previously a north-flowing right tributary to the Kennebec, was blocked by the alluviation of the Kennebec drainage, and diverted to the east down the course of the old Norridgewock Sand Plain, Maine, USA .. 14 ..

river valley. At first encountering unconsolidated fine sands of the alluviation event in the

downvalley channel, it eroded rapidly, leaving few remnants of the older features behind.

Smaller surviving remnant features would have been subject to natural downslope

modification processes, or rapidly eroded with the widespread nineteenth-century

deforestation and agricultural activities. However, near the mouth, where Martin Stream

rejoins the new Kennebec, the deposits were sufficiently deep and widespread that some

remnants were preserved.

A Conclusions

The Kennebec River prior to the onset of the Holocene flowed through a different course in

this study area than it does today (Fig. 7), probably splitting into two channels to flow around

Burrill Hill and the small unnamed hill east of Tobey Brook. Substantial alluviation, forced

by the postglacial tilt of Moosehead Lake to drain into the headwaters of this river system

and the ensuing upstream erosion, raised the level of the river until it was forced over a low

drainage divide into an adjacent drainage.

This new drainage course took the river over a previously buried bedrock high, resulting in

the falls at which the town of Skowhegan is located today. Along the old channel, the

alluviation to the north blocked the northward flow of Martin Stream, which was deflected to

the east and continues to flow down the abandoned Kennebec Valley. Downcutting through

the alluvial deposits of the ancient Kennebec has resulted in the formation of multiple

terraces above the stream channel in those areas where any record of the former river is

preserved at all. The large boulder patch found below the highest of these terraces (Fig. 5)

was likely eroded from till by the ancestral Kennebec, subsequently buried in the alluvial Norridgewock Sand Plain, Maine, USA .. 15 ..

event, and then re-exhumed by the downcutting of Martin Stream through the fine sands of

the alluvium.

Study of this sandy fluvial deposit provides insights into processes acting on the rapidly

changing post-glacial landscape at the edge of a major continental ice sheet. Alluviation and

fluvial incision were driven by sequential events caused by isostatic uplift both locally, and in

the broader regional changes to slopes and river drainages. This is comparable to processes

already documented as having altered the course of the Rhine-Meuse River system in Europe

(Busschers et al., 2007), and thus may potentially be a more common phenomenon than

previously recognized. As such, this study can be applied to other post-glacial isostatically

altered landscapes elsewhere and at other times.

A Acknowledgments

Supported by a funds from the Natural Sciences Research Fund of Colby College, and by the

Colby College Department of Geology. Peter Leach assisted with the GPR work while an MS

student at UMaine. The Maine Geological Survey graciously provided access to archival

aerial photography in their collection. Many thanks to the Scott Family for permission to

conduct GPR investigations across their lands. Thanks also to Mr. & Mrs. Alex LeHay for

permission to access the sand pit on their property in Embden, and to Merle Lloyd & Sons of

North Anson, Maine, and Mr. Gene Ripley of Norridgewock, for permission to work on their

properties. An early draft of the manuscript was improved greatly through constructive

criticism by T. K. Weddle of the Maine Geological Survey. Norridgewock Sand Plain, Maine, USA .. 16 ..

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Maine. Augusta, Maine: Maine Geological Survey Open-File Map 87-23, scale 1:62,500.

Norridgewock Sand Plain, Maine, USA .. 20 ..

Weddle, T.K., 2001, Surficial geology of the Brunswick quadrangle, Maine: Maine

Geological Survey (Department of Conservation), Open-File Map 01-484, scale 1:24,000

Weddle, T.K., 2002, Surficial geology of the Brunswick 7.5’ quadrangle, Cumberland and

Sagadahoc Counties, Maine: Maine Geological Survey (Department of Conservation), Open-

File Report 02-52, 10 p.

Wells, N.A., and Dorr, J. A. , Jr., 1987: Shifting of the Kosi River, northern India. Geology,

15:204-207. Norridgewock Sand Plain, Maine, USA .. 21 ..

Figure Captions

Figure 1. (A) Index map of Maine showing approximate inner margins of postglacial isostatic marine incursion (shaded area), and location of the Norridgewock Sand Plain; (B) index map of Kennebec River drainage showing important places mentioned in text (after Robinson,

1998), including Seboomok paleospillway from Moosehead Lake; (C) map of the approximate extent of the Norridgewock sand plain, showing abrupt course changes of both the Kennebec River (in the north) and Martin Stream (at the southern margin of the plain).

Base of (C) from the Norridgewock and Waterville, Maine, topographic quadrangles, scale

1:62,500, edition of 1943; contour interval is 20 feet (6 meters). The mostly abandoned

Schoolhouse Road functions today mainly as a track for off-road vehicles (see Fig. 5). "B" marks location of boulder lag shown in Fig. 4; "D" denotes an area of extensive dunes, some still active; "R" is a site of fluvial ripples in fine sand; "T" at far right indicates area of terraces near mouth of Martin Stream at modern Kennebec River junction.

Figure 2. Ground-level view to the SW across the central portion of the sand plain from the base of Burrell Hill towards Dodling Hill, approximately parallel to State Route 137 (see Fig.

1 for locations).

Figure 3. (a) Reference map showing where within the sand plain our GPR data were acquired, and (b) locations of specific lines run; topographic map base for (a) from the USGS

Norridgewock, Maine, 7.5-minute topographic sheet, scale 1:24,000. The dirt track shown on

(a) is indicated on (b) by the solid gray line along lines 1 & 2 (SE corner). "Green pole barn" indicated in GPR line map is the second building from the south of the five that are shown in Norridgewock Sand Plain, Maine, USA .. 22 .. the boxed area on the topographic map. BH-4, etc., refer to specific boreholes of Mueser et al. (1971). Borehole BH-4 was immediately adjacent to our GPR line 5 (Fig. 4).

Figure 4. Ground-penetrating radar (GPR) data (above) and interpreted structure sections

(below) for lines 2 (A, vertical exaggeration = 10x) and 5 (B, vertical exaggeration = 15x).

(See Fig. 3 for locations.) [Line 1 was an abortive line less than 20 m in length.]

Figure 5. Granitic lag boulders to 4 m across above Martin Stream valley at southern margin of the sand plain, 9 m above modern channel (locality "B" on Fig. 2); note seated figure in upper right for scale. Boulders are surrounded by clean, stratified fine-medium sand.

Figure 6. (A) Terrace surface 2-3 m above the granitic lag boulders shown in Fig. 4, some 11-

12 m above Martin Stream; track is remnant of abandoned Schoolhouse Road (Fig. 1). (B)

Terrace surface 3-4 m above Martin Stream near the mouth; the 5-6 m terrace lies above this on the opposite side of the valley, but is too densely forested to photograph well. (C) Lowest terrace surface at the mouth of Martin Stream, lying 1.5-2 m above the modern level of the

Kennebec River at high water (shown). Prior to dam construction on the river in the early

20th Century, this terrace was approximately 6 m higher above the stream level than it is today.

Figure 7. Interpreted paleochannel of the Kennebec River through the Norridgewock Sand

Plain and down the modern valley of Martin Stream, to its confluence with the modern

Kennebec River channel. Topographic base from the Norridgewock and Waterville, Maine, topographic quadrangles, scale 1:62,500, edition of 1943, and Skowhegan, Maine, topographic quadrangle, scale 1:62,500, edition of 1913; contour interval is 20 feet (6 meters) Norridgewock Sand Plain, Maine, USA .. 23 ..

Fig. 1

Norridgewock Sand Plain, Maine, USA .. 24 ..

Fig. 2

Norridgewock Sand Plain, Maine, USA .. 25 ..

Fig. 3

Norridgewock Sand Plain, Maine, USA .. 26 ..

Fig. 4 Norridgewock Sand Plain, Maine, USA .. 27 ..

Fig. 5 Norridgewock Sand Plain, Maine, USA .. 28 ..

Fig. 6 Norridgewock Sand Plain, Maine, USA .. 29 ..

Fig. 7