Table !«—Geologic units in the lower Androscoggin River basin and their water-bearing characteristics
Geologic unit	Thickness (feet)	Character	Water-bearing characteristics
Alluvium	0-27*	Sand, silt, and clay, some gravel, of river flood plains.	Occurs in thin, discontinuous deposits in smaller valleys but forms a mappable geologic unit in places, particularly along Androscoggin, Little Androscoggin, and Nezinscot Rivers. Yields water to a few wells where deposits are extensive and contain gravel. Subject to flooding.
Eolian deposits	0-30*	Fine to medium sand and silt. Some sand deposits form fixed, vegetated dunes but several areas of active dunes exist Silt (loess) in places occurs as a discontinuous layer as much as 2 feet thick, overlying glacial deposits.	A source of small quantities of water to numerous dug and driven wells. Fineness of grain size and generally high topographic position preclude obtaining large yields from this unit.
Swamp deposits	0-32*	Partly decomposed organic matter—leaves, moss, heath plants and grass (peat) — and some intermixed silt, clay and sand. Includes some tidal marsh deposits along Androscoggin River below Brunswick.	Not known to yield water to wells in lower Androscoggin Basin. May contain a considerable quantity of water which may be released slowly to underlying permeable deposits or to streams flowing through or issuing from them. Contained water is likely to be acidic, highly colored, or high in nitrate or other organic matter.
Outwash	0-105*	Stratified sand and gravel deposits in valley trains or outwash plains. Contains some silt and clay. May overlie or interfinger with marine deposits.	Yields small to moderate quantities of water to wells. Largest reported yield is 60 gpm. Probably larger quantities are available to properly constructed and developed wells under favorable circumstances. Water is of good quality.
Marine deposits	0-173*	Dark-blue to gray silt, clay and very fine sand. Tan colored where weathered. Contains layers of sand and gravel, a few inches to a few feet in thickness.	Clay yields water to wells very slowly. Sandy layers yield small quantities of water to dug wells. One drilled well is reported to obtain 7 gpm from marine deposits.
Ice-contact deposits	0-150	Well-stratified to poorly stratified deposits of sand, gravel, and cobbles, with some silt, clay, and boulders. Landforms include kames, kame terraces, kame fields, kame deltas, and eskers.	The source of the largest supplies of ground water in the lower Androscoggin Basin. Under most favorable conditions - where deposits are adjacent to a body of surface water for recharge - yields of more than 1,000 gpm (gallons per minute) are obtainable.
Early stratified drift	0-144*	Deposits of stratified sand and gravel that underlie marine deposits. Occurs in the subsurface.	Yields as much as 1,000 gpm to properly drilled wells under favorable conditions. Not as widespread as ice-contact deposits.
Till	0-160*	A heterogeneous mixture of clay, silt, sand, gravel, cobbles, and boulders. In some places is very sandy, and resembles ice-contact deposits or outwash except for lack of stratification. In other areas is rich in clay and very dense or is very bouldery. In some exposures upper few feet appear to have been washed by water.	Till is widespread and is the source of water to many dug wells, and to a few drilled wells. Sustained yield of most dug wells is less than 1 gpm. Dug wells are likely to go dry in the summer. One drilled well in till is reported to yield 20 gpm.
Bedrock		Igneous and metamorphic rocks, including granite, pegmatite, gneiss, and schist.	Rocks are dense and impermeable. Yields small to moderate amounts of water to wells from joints and fractures. Yield of 416 wells ranges from 0 to 85 gpm. Median yield is 5 gpm and average yield is 8 gpm. Depths of 600 wells range from 25 to 550 feet. Median depth is 158 feet and average depth is 157 feet.
* Maximum values from drillers' logs.
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