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UNDERLYING BEDROCK
The Housatonic Valley Region Connecticut date from geologically ancient times, before the continent of North America assumed its present form. Its rock-ribbed hills are the eroded stumps of what were once lofty mountain peaks. The crystalline bedrock of the hills consists of two general types, the dominant noncarbonate and the lesser carbonate type.

As the mountains wore down and subsequent uplifts of the earth’s crust occurred, a carpet of ocean floor was thrust up among older rocks to form the marble or carbonate rock strata that makes up some of the valleys: Great Swamp and Titicus Valley in Ridgefield, Mill Plain in Danbury, East Swamp in Bethel, and the great “rift” or valley which extends north from West Redding through Bethel, Danbury, Brookfield and New Milford.

Then during the comparatively recent ice age, which affected the Region as recently as 15,000 years ago, great glaciers advanced and retreated over the Region, scouring its hills, filling valleys with stratified drift sediments deposited by meltwaters, and draping the upland areas with glacial till. See federal definition, then see these deposits mapped for area towns. The final surface topography and drainage system (see regional topographic map) of the Region were therefore shaped by the moving ice and running water of the last glacier.

HYDROLOGY
The entire water resource of the Housatonic Valley Planning Region is derived from precipitation, both that which falls directly on land and water surfaces and that which falls elsewhere and flows into our area via rivers and streams.

The U.S. Geological Survey has estimated that 30% to 50% of the 46 inches of mean annual rainfall in the local area returns to the atmosphere in the form of evaporation and transpiration from plant life. The remainder enters surface streams and groundwater reserves, eventually flowing out of the Region towards Long Island Sound and the Hudson River via the Housatonic, Norwalk, Saugatuck, Croton and other rivers.

Groundwater represents that portion of the “hydrologic cycle”– evaporation, condensation, precipitation, runoff – in which water enters the soil and surface rocks and is transmitted to a point of discharge, such as a stream or well. The movement of water through the ground’s unsaturated zone varies in relation to soil characteristics, but is generally downward through permeable soil to the upper boundary of the “saturated zone” or water table.

In permeable soils, the saturated zone may rise and fall in response to recharge from rainfall, or discharge to streams and wells. But the configuration of the water table tends to roughly parallel the surface of the ground above it.

Consequently, groundwater flows “downhill” within a drainage basin, just as do the surface waters above. They generally rise upward, however, at the bottom of a valley to discharge into a flowing stream or pond through its bottom sediments. Drainage basins are therefore the key land areas in analysis of groundwater aquifers.

Groundwater occurs in three types of aquifers: the till aquifer, stratified drift aquifer, and crystalline bedrock aquifer. Stratified drift and crystalline bedrock in the saturated zone below the water table are capable of yielding usable quantities of water throughout most of the Region. Most wells are drilled into the bedrock, but in lowland areas stratified drift may be more accessible to shallow dug or driven wells.

TILL AQUIFER
Till is the compact sediment covering most hills, slopes and uplands. It is an unsorted, unstratified material composed or rock particles of all sizes including stones and boulders. This sedimentary material was deposited directly, as a mantle on the bedrock, by the glacier.

Much of the bedrock in the area is overlain by till that is less than 10 feet thick, although locally till may exceed 100 feet in thickness. Thickness of till varies from 0 to 200 feet, but till in this Region is usually shallow, frequently from 10 to 50 feet deep. Its poor hydraulic conductivity limits wells to very modest yields, typically a few hundred gallons per day in a shallow domestic well.

Historically till was a major source of water for individual domestic and farm supplies but has been almost entirely supplanted by drilling beyond it and into the crystalline bedrock aquifer. Inadequate yields with respect to modern requirements, the susceptibility to pollution, and the economic ability of homeowners to pay for drilled bedrock wells are the principal reasons for the general abandonment of the thin till layer as a water supply source.

STRATIFIED DRIFT AQUIFER
The stratified drift aquifer, typically a layered deposit of gravel, sand and silt in river valleys, is the only ground formation sufficiently productive to meet large volume water needs such as public water supply wells.

Stratified drift is an unconsolidated sediment composed of interbedded layers of gravel, sand, silt and clay. These materials were deposited during the deglaciation of the area and are generally restricted to the valley areas that served as drainageways for glacial melt water or were the sites of temporary glacial lakes. The stratified drift commonly forms an infilling of the preglacial bedrock valleys.

Both stratified drift and till contain open spaces or pores between individual grains, this in contrast to bedrock which contains open spaces along cracks or fractures. Below the water table, such pores and fractures are filled with water. Stratified drift and till have greater porosities than fractured bedrock, and, where saturated, they contain significantly more water per unit of volume.

Well yields, where the stratified drift has favorable hydraulic characteristics because of coarse texture and major depth or “saturated thickness” may produce from 50 to 2,000 gallons of water per minute. But sustained pumping of wells tapping stratified drift can lower the water table beneath adjacent stream and lake beds, inducing recharge from these surface water bodies to the adjacent aquifer.

BEDROCK AQUIFER
The bedrock of the Housatonic Valley Region is principally metamorphic and igneous crystalline rocks such as gneiss, schist and granite, and tends to be close to the ground surface. Groundwater is transmitted in these hard rocks through fracture systems, or cracks, both horizontal and vertical, within several hundred feet of the surface.

Studies in Connecticut indicate that at depths greater than 300 feet below the bedrock surface, water-bearing openings are few and the rock is relatively impervious. The size and distribution of these water-bearing cracks is irregular, and therefore bedrock well water yields differ significantly from one site to another, but generally do not exceed 100 gallons per minute and frequently yield less than 10 gallons per minute.

But, the Region’s carbonate or limestone bedrock area typically is more fractured and tends therefore to be a somewhat more productive water source, yielding up to 300 gallons per minute to individual wells, though generally less. A few public water supply wells in Ridgefield tap such more productive carbonate bedrock.

Bedrock is nonetheless highly important as a source of water for small individual uses such as private dwellings and small commercial or institutional establishments. Like the limited till aquifer, the deeper bedrock aquifer is widespread but generally provides only small supplies of water.