Marion County
Glacial Geology — Unconsolidated Deposits
Anthony H. Fleming

Unlike the underlying Paleozoic bedrock that is present at depth everywhere in Indiana, the glacial deposits in Indiana have never been buried at a sufficient depth to be lithified , or consolidated, into rock and are thus comparatively soft. As a result, they can be easily excavated by hand (fig. 1) or machine at most places, as well as by running water. The topography at places like Fort Benjamin Harrison and Holliday Park (fig. 1) shows that even a small, intermittent stream can incise a precipitously steep ravine in the comparatively short span of geologic time since these deposits were laid down. This distinction is embodied in the term "unconsolidated deposits," commonly applied to glacial deposits and other more recent sediments produced by streams and wind. A comparable term sometimes used to describe the deposits of glaciers, streams, and wind is "surficial deposits," which refers to their presence at the surface of the modern landscape.

Figure 1.
Left-unconsolidated glacial materials are soft and can be excavated by hand. Right-An intermittent stream has incised a steep ravine at Holliday Park in Marion County. Photos by A. H. Fleming.

In certain situations, however, "unconsolidated deposits" sometimes deviate from the script, particularly in places where groundwater discharges to the surface from sand and gravel deposits. Groundwater deep below the surface commonly contains dissolved calcium, which immediately becomes insoluble in the presence of oxygen when the groundwater reaches the surface. At places where the groundwater contains large concentrations of this element, deposits of calcium carbonate ( calcite ) may form in springs and seeps, cementing the gravel into conglomerate , such as this ledge exposed above a fen at Holliday Park (fig. 2). Such cemented glacial deposits are resistant to erosion and have other properties similar to bedrock, which they superficially resemble.

Figure 2.
Cemented glacial gravel at Holliday Park in Marion County. Photo by A. H. Fleming.

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