MICKELSON, David M., and CUTLER, Paul M., Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton St., Madison, WI 53706.  mickelson@geology.wisc.edu

The theory and field documentation of subglacial conditions and processes, and how these
processes control ice and sediment dynamics, have undergone substantial change in the last 30
years. In the early 1970's, basal sliding on a hard or soft bed, accompanied by transport and
deposition from deforming or stagnant debris rich ice, was a dominant model. Issues of meltout
versus lodgement processes dominated the literature. Based on preliminary observations of
deforming unfrozen subglacial sediment in Iceland, the hypothesis of wet deforming beds
blossomed in the 1980's, leading to ideas that thick subglacial deforming layers may be responsible
for the fast flow of Antarctic ice streams and low-profile Pleistocene ice lobes. Evidence of wet
deforming beds in the Pleistocene record was reported and disputed, and it remains unclear what
field evidence definitively distinguishes wet from frozen bed deformation, nor is it clear whether
there are extensive areas of modern glacier beds where thick deforming layers exist. Recently,
ploughing of subglacial sediments and decoupling at the ice-bed interface have been proposed as
important components of basal motion over soft sediments, and the thermal state of these
sediments with regards to landform development and Heinrich events is being analyzed through
theory and modeling.

Understanding of the subglacial hydraulic system and realization of its significance to ice dynamics
has evolved greatly over the last 20 years. Basal water flow is now intimately linked with theories
of subglacial motion. Furthermore, large volumes of sediment are known to have been
transported by water, though how much by more-or-less steady flows versus catastrophic floods
is not resolved. Unravelling the control of bed conditions on the behavior of ice sheets is a crucial
facet of Quaternary geoscience. Great strides are now being made by glacial geologists and
glaciologists working together, combining field observation, theory development, laboratory
experimentation, and numerical modeling.