An Introduction to Ground Water Hydrology

Section 1 - General Geology of Maine's Ground Water Sources

Maine obtains useful supplies of ground water from two sources of very different geologic origin -- unconsolidated surface sediments deposited by glaciers over the last two million years, and underlying consolidated bedrock formations that began forming hundreds of millions of years ago.

The bedrock formations that form the foundation of Maine and New England come from the same variety of sources active in the world today, including volcanoes (lava and ash), intrusion of molten rock (granite and gabbro), and weathering and erosion of landforms (sandstone and mudstone). Regardless of their various origins, however, these bedrock formations have very similar ground water bearing characteristics because of metamorphism and crustal deformation that has left them first brittle and now highly fractured. Metamorphism, caused by high heat and pressure associated with deep burial in the crust, changed the texture and mineralogy of the original formations giving us today the hard schists and gneisses that are seen nearly everywhere in Maine and New England except where there are granitic rocks.

Like the numerous granites and gabbros that cooled slowly from intrusions of molten rock several miles beneath the ancient crust, the metamorphic rocks are water bearing only where they are fractured. This is quite in contrast to bedrock formations in other parts of the country, for example along the Atlantic coast south of New York City. Sandstone formations in this region are unmetamorphosed and therefore retain their original high potential for ground water storage and transport in the open spaces and channels among the sand grains.

Unconsolidated sediments that overlie the bedrock formations are largely products of continental glaciers that spread across Maine and New England as far south as Long Island, New York. Much of what is seen today was deposited during the last 100,000 years by the most recent period of glaciation that ended in Maine around 10,000 years ago.

Advance of the mile thick ice across the land left widespread deposits of mixed clay, silt, sand, cobbles, and boulders called till. The ice sheet's annual, and eventually complete, melting left more restricted deposits of sand and gravel that are important sources of ground water today.

An unusual event occurred in Maine as the climate warmed and the ice sheet melted away. The weight of the ice had so depressed the underlying bedrock formations in Maine's coastal region that the ocean flooded the area for a period of perhaps 600 years until the land surface rebounded again. Throughout this area of temporary marine transgression, glacio-marine silt and clay deposits now cover the glacial till and sand and gravel deposits. The clay and silt are not a source of abundant ground water in Maine, but are important because their low permeability has a strong influence on the occurrence and quality of ground water in the underlying glacial and bedrock aquifers.

(A more complete description of glacial events in Maine can be found in the Surficial Geology Handbook for Coastal Maine, by Woodrow B. Thompson, Maine Geological Survey, 1978.)

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Last updated on March 25, 2009

Contents

An Introduction to Ground Water Hydrology

Section 1 - General Geology of Maine's Ground Water Sources