RESIDUAL ISLAND HILLS IN BURIED BEDROCK VALLEYS

Buried valleys are part of a discontinuity between bedrock and unlified surficial deposits. Understanding the origin, geometry, paleogeomorphology, and filling of buried valleys is important because they usually contain shallow aquifers. In the glaciated, upper Midwest region, the discontinuity between bedrock and surficial deposits is generally interpreted as a landscape and the buried valley is part of an old drainage system. The valley may exhibit a normal open meandering form and graded profile generated by long term down cutting and lateral cutting or it may contain isolated hills as islands originating during large scale flow events such as outburst floods or redirection of streams. These isolated islands range in area from a few hundreds square meters to hundreds of square kilometers, and in relief from meters to tens of meters. These isolated islands partition the buried valleys and significantly reduce the volume of aquifers and groundwater yields. High resolution geophysics and closely spaced borings are required to delimit the nature of these islands to better model the hydrology of valley fill aquifers.