North-Central Section - 48th Annual Meeting (24–25 April)

Paper No. 3
Presentation Time: 8:40 AM


KARAFFA, Marni D. and GRAY, Henry H., Indiana Geological Survey, Indiana University, 611 North Walnut Grove Avenue, Bloomington, IN 47405,

The precise position and timing of the southern-most bounday of glacial ice in Indiana—the pre-Wisconsin boundary—has remained elusive to many gologists for well over a century. The recent availability ofLiDAR imagery and the advancement of age-dating techniques have provided an impetus to reexamine the landscape and deposits along this enigmatic zone in the hopes of refining both its position and time.

Beanblossom Valley, a main drainage (91 square miles) for northern Monroe and Brown Counties in Indiana, is a small area that may provide insight into at least a portion of the southern margin. The Beanblossom Valley occupies a reeentrant in Indiana's southern glacial margin that was bounded on its east and west margins by two tongues of ice flowing southeastward and southwestward, respectively. This location, at the apex of the unglaciated south-central part of Indiana, gives the valley a unique record of glacial history during the pre-Wisconsin through the late Wisconsin. During pre-Wisconsin time, the mouth of the valley was blocked, more than once, by ice extending to the southwest, while the lobe extending to the southeast eventually ocuupied the upper reaches of the valley where it deposited outwash. Sediment sequences of thick sand deposits at the mouth grading up-valley into silts and clays interfingered with outwash are suggestive of a finger lake setting. Certain topograpic features within the Beanblossom Valley, and other drainages along the southeastern margin, such as interbasin cols, abandoned high-level valley segments, bedrock valley narrows, and high-level discontinuous patches of glacial sediments, collectively provide evidence of ice-marginal drainage and ice-marginal position. These topographic features and sedimentary deposits suggest a complex, discontinuous network of ice-marginal drainages, with meltwater draining directly away from the ice into preexisting bedrock valleys in the becrock-controlled uplands of south-central Indiana. This is in contrast to a simpler, integrated network that defines the southwestern glacial boundary. Further detailed examination of these features and sediments and the collection of datable materials will be key to redefining the pre-Wisconsin boundary in south-central Indiana.