EVIDENCE FOR EARLY PRE-ILLINOIAN GLACIATION IN SOUTHCENTRAL INDIANA

An ongoing mapping project has geolocated more than 70 isolated glacial boulders south of the glacial boundary in Brown County in southcentral Indiana. The glacial erratics extend as far as 6 mi/9.6 km beyond the maximum extent defined by glacial tills. All the erratics are within the rugged terrain of the Norman Upland where narrow streams are deeply incised into siltstones of the Mississippian-age Borden Group. The large majority (>95%) of the boulders are quartzite with a maximum length of 42in/106.7cm. They rest on bedrock and local gravels in narrow stream channels near major drainage divides. No finer-grained glacial materials are present nearby.

Several lines of evidence suggest that the glacial boulders were deposited by an early glacial episode that extended beyond the limit of glacial tills of later pre-Wisconsin glaciations:

• Their large size and distance from the margin of the till deposits.
• Their location in the upper reaches of small drainageways upstream from the main fourth- and fifth-order glacial sluiceways of the North Fork Salt Creek. More than 86 percent of the erratics are located in first and second order streams. If placed by ice rafting a closer location to the main streams would be expected.
• The absence of finer grained glacial materials in the vicinity of boulders suggests a long residence time for the erratics, sufficient to allow finer grained glacial materials to weather to constituent minerals or be washed away.

In a regional context, the Brown County erratics share similarities with other locations in southeastern Indiana (Washington, County), northern Kentucky and southwestern Ohio where crystalline erratics have been reported beyond the glacial margin defined by pre-Wisconsin glacial tills. Collectively these erratics suggest an early pre-Illinoian glacial episode that extended farther than any subsequent episode in the southeastern Midwest. There is indirect evidence for a mid-Matuyama glaciation circa 1.3 Ma based on cosmogenic dating of sediments deposited in Mammoth Cave in central Kentucky. Though not intended for application at the regional level, mid-Matuyama Laurentide Ice Sheet reconstructions of maximum thickness and glacial extent are consistent with an early glaciation extending so far south in the Midwest.