Wisconsinan Age is the most recent glacial period to impact the Upper Iroquois Watershed. The first two retreats of the Lake Michigan and Lake Erie Lobes of the Wisconsinan Age glaciers deposited the Iroquois, Shelbyville, and Crawfordsville/Chatsworth Moraines and established the current topography of the watershed about 20,000 years ago. A glaciated plain was created where a variety of unconsolidated deposits are present including dune sand, lacustrine sediments, outwash plain sediments (sand and gravel), and till. The glacial deposits on these end moraines, recessional moraines, ground moraines, outwash plains and lacustrine plains range in thickness from a few feet to more than 100 feet. Many of the depressional potholes within the watershed are the remnants of old kettle holes.
The study of Indiana’s geology is commonly divided into the study of the rocks that are sometimes at, but mostly below, the earth’s surface and the unlithified materials that occur at the earth’s surface.
The term “bedrock geology” describes the study of the rocks at and below the bedrock surface. Different types of geological bedrock differ in their resistance to erosional conditions and therefore can have an effect on a landscape region. Geological bedrock in a watershed can also be an important factor that affects the conditions and physical, chemical and biological processes occurring in the watershed’s streams. Water in its natural state is never pure, absorbing minerals and salts from the land over which it passes. For example, some contaminates such as arsenic and radon are released into groundwater and surface water through natural processes of erosion and sedimentation. Radon, which is a by-product of uranium erosion, is found in the earth’s crust and is undetectable. However, if it enters a drinking water supply it can increase the risk for cancer.
Since the physical, chemical and biological conditions in a watershed are often directly or indirectly related to bedrock and underlying geologic formations, the bedrock can often provide an explanation why a stream has certain characteristics, especially the composition of the streambed. For example, bedrock consisting of limestone will act as a buffer to acidic waters. This chemical property can be advantageous in areas which receive more acidic precipitation due to air pollution. The limestone bedrock helps stabilize pH levels, reducing the impact of acid rain.
Bedrock geology of the Upper Iroquois watershed is comprised mostly of limestone, dolomite, and shale bedrock. These materials can be grouped into four basic areas or regions of the watershed.
1. The Borden Group lies within the Mississippian bedrock system and is primarily located south of U.S. Highway 24 in the southern part of the watershed. This bedrock group consists mostly of siltstone with lenses of crinoidal limestone in the upper part. There is much cherty and silty limestone and dolomite in the southwest.
2. The New Albany Shale rock unit lies in the central part of the watershed, north of U.S. Highway 24. It is mostly black and greenish gray shale. It is part of the Devonian-Mississippian bedrock system.
3. The Muscatatuck Group consists of dolomite and limestone and lies in the northern part of the watershed. It is part of the Devonian Bedrock system.
4. The Wabash Formation is part of the Silurian bedrock system. This unit lies in the northwest area of the watershed, primarily west of U.S. Highway 41. It consists of limestone, dolomite and argillaceous dolomite.
Depth to bedrock varies throughout the watershed and can range from a few feet to more than 100 feet below the soil surface. Of note, is a 1-2 mile stretch of the Iroquois River running through Rensselaer where the bedrock is very close to the soil surface. Bedrock in this area is typically within two feet, or less, of the soil surface of the floodplain, and the rock itself forms the sides and bottom of the river. The Hydrogeological Atlas of Aquifers in Indiana shows the bedrock in Rensselaer.