Lower Rock River geologic and geographic setting

The Lower Rock River basin includes the portion of the Rock River below Fort Atkinson and its tributaries. The Bark River originates in Washington County and traverses the southern Kettle Moraine region before emptying into the Rock at Fort Atkinson. From the northwest, the Yahara River drains much of central Dane County toward its intersection with the Rock just south of Edgerton. Most of the basin lies in the Eastern Ridges and lowlands Geographic Province. A small portion in western Rock County is in the Western Upland Province.

A dominant feature of the basin is the Green Bay terminal moraine and interlobate moraine of the Green Bay and Lake Michigan Lobes, also known as the Kettle Moraine. The moraine runs southeast through Dane County and sweeps across northern Rock and Northeastern Walworth counties before turning north through Waukesha. Surficial deposits to the south are primarily outwash, isolated patches of older till and loess over residual bedrock materials.

To the north are sandy tills with patches of outwash and lacustrine deposits, the largest of which is located east of Whitewater. Aside from the heavily wooded topographic high of the moraine and the wetlands associated with lacustrine deposits, the landscape is rolling and intensely farmed.

The bedrock geology of the basin includes all but the very oldest Precambrian and the youngest rocks of the state, Devonian shale and limestones. Cambrian sandstones underlie glacial deposits in Dane County and in the deeply incised ancient Rock River valley in Rock and Jefferson counties. The entire Paleozoic sequence dips along the Precambrian surface and thickens to the south-southeast where the total thickness of Cambrian rocks exceeds 1,800 feet near Beloit.

Rocks of the Ordovician system overlie the Cambrian sandstones. These rocks are a repeating cycle of Limestones (dolomites), sandstones and shales. Rocks of this age are present over much of the remaining area of the basin. In the very eastern reaches of the Bark River, there is Silurian dolomite. The Silurian Escarpment location coincides with the Kettle Moraine glacial feature.

Glacial deposits

Glaciers of the Pleistocene Era were formed by the continental accumulation of snow and reached a maximum thickness of two miles. The ice sheet that spread over Canada into Wisconsin during this time transported a great deal of rock debris called drift. As the ice melted, large amounts of sand and gravel were deposited forming outwash plains.

The glacial drift that forms the surface layer of the Rock River basin is either stratified or unstratified. Stratified drift is from ice-contact deposition or from outwash ("outwash drift") carried by glacial meltwater. This stratified drift occurs primarily in areas of lakes and impoundments in river headwaters. Unstratified drift consists of ground moraine, a sheet of unsorted sandy clay till laid down directly by ice; end moraine, ridges of boulder-strewn till laid down by ice during pauses in glacial advance or retreat; and unpitted or pitted outwash, or lake deposits.

Pits were formed in the outwash where buried blocks of ice melted; many of these pits are now lakes or wetlands. Large lakes formed in front of the glaciers and clayey lacustrine deposits settled out in these basins. These glacial deposits were transported by glacial ice or meltwater within the last million years. The most recent deposits were left as the Green Bay and Lake Michigan Lobes retreated from the area around 10,000 years ago.

The entire basin falls within the glaciated part of the state, although much of the older till in southern Rock County has been eroded away. South of the terminal moraine the glacial deposits consist of patchy old clayey tills, pitted outwash to the southeast and deep alluvial sand and gravel in the river valley. The moraine is composed of course gravel, sand and stony till. To the north are lacustrine deposits, spotty outwash and sandy till. A large drumlin field -- hills elongated in the direction of ice flow -- and several recessional moraines cross eastern Dane and Jefferson counties.

Aquifers

Most of the basin is covered by a layer of glacial deposits. Glacial deposits often supply sufficient amounts of water for domestic purposes where they are thicker and more extensive. Within the basin, sand and gravel deposits are abundant, particularly in the river valleys. Wells finished in the unconsolidated materials can supply sufficient amounts of water for domestic supplies and municipal supplies in the river valley. A well at Janesville yields 5,450 gallons per minute (gpm) for 24 hours with only seven feet of drawdown. Aquifer tests in Beloit indicated a transmissivity of 1,200,000 gpd per foot. The underlying sandstone has a transmissivity of about 33,000 gpd per foot sustaining yields up to 1,200 gpm. The dolomite aquifers are much more variable and will range from 0-50,000 gpd per foot depending on the amount of fracturing and karst development.

Wells in the basin are finished, or draw water from three of the four aquifers of the state. Sand and gravel wells are distributed throughout the basin but are less common in Dane County. Most of these wells are shallower and the majority are domestic with the exception of high capacity wells in the alluvial deposits in Rock County. The shallow wells are susceptible to contamination. Many subdivisions have histories of nitrate and other human-caused contamination.

The Eastern Dolomite Aquifer is confined to the very eastern part of the basin. The aquifer is well protected from surface contamination by fairly thick clayey till deposits. Moving west, bedrock wells are finished in the upper part of the sandstone aquifer.

The upper sandstone aquifer is comprised of the Sinnippe group (dolomites), the St. Peter sandstone and the Prairie du Chien dolomite. This aquifer is the most commonly drilled to in the basin. The distribution of wells in this aquifer is even throughout the basin except where the formations are absent, such as in the Yahara and Rock valleys.

The lower sandstone aquifer is primarily tapped by municipal systems in the western and southern parts of the basin. In Dane County, where the Cambrian rocks are nearer the surface, many domestic wells also draw water from these formations.

Sand and gravel aquifer

The sand and gravel aquifer is a relatively shallow aquifer consisting of permeable sediments of unconsolidated glacial deposits. The aquifer is thickest in the buried bedrock valleys of the basin. It is locally important as a source of groundwater for private use and public supplies where thick saturated unconsolidated deposits exist. Within the basin, deposits of high permeability sand and gravel are found in the Kettle Moraine area and in the outwash plains and alluvial filled valleys in Rock County.

The deposits range up to 400 feet thick in several areas. These thick deposits consist mostly of alluvial sands and gravel that are excellent aquifers. Thinner deposits are generally sandy tills with less extensive localized outwash, which are adequate producers for domestic uses. Groundwater collects and moves in the pores and open spaces between the grains of silt, sand and gravel.

Silurian dolomite aquifer

The Silurian dolomite is found only in the far eastern tip of the basin. The rock dips to the southeast. Bedding planes and fractures have a great influence on groundwater flow. Many of the fractures have been enlarged by the dissolution of the carbonate rock matrix. This secondary porosity is referred to as "karst." Since the dolomite is buried quite deep beneath glacial deposits the fracture flow is not as great of a concern in this basin as in other less protected areas.

Sandstone aquifer

The Sandstone is made up of the Cambrian and Ordovician sandstones and dolomites. It is separated from the Silurian aquifer by the Maquoketa shale, which subcrops in a north-to-south line through western Waukesha and central Walworth counties. The younger Ordovician system has more dolomite formations than the Cambrian system. This makes the hydrologic properties somewhat different than the lower system as well as making the shallower system more vulnerable to contamination. Yields are variable depending on the formation and well construction, but they are always sufficient.