CALCULATING THE SLIDING VELOCITY OF THE RAINY LOBE OF THE LAURENTIDE ICE SHEET USING MASS BALANCE CALCULATIONS AND LARGEST CLAST SIZE IN LODGEMENT TILLS

Lodgement tills are important in glacier reconstruction because their properties can be used to infer parameters such as glacier sliding velocity, ice thickness, and hydrology. These tills provide important calibration parameters for mass balance studies and numerical simulation. In Minnesota, lodgement tills of the Rainy lobe of Late Wisconsin glaciation exhibit significant changes in sedimentology between tills associated with LGM and those deposited late during ice retreat. These changes include a systematic increase in the modal size of clasts in the tills from the LGM to final ice retreat. Clast size in lodgement tills is directly related to the basal sliding velocity of ice. Small clasts drag on the bed and “lodge”, whereas large clasts simply plow into the basal sediment, but the stress on them is too large for deposition. Therefore, we can use the size of clasts in lodgement tills to estimate sliding velocity at various times in a glacial advance.

This investigation uses field-based measurements of modal clast size in till, along with the analyses of Iverson and Hooyer (2004) and Weertman (1959, 1964) to estimate sliding velocity. Clast size is determined photogrammetrically from field exposures along the path of the Rainy Lobe. Calculated sliding velocity is compared with independent calculation of the velocity based on mass balance. These calculations provide an important calibration parameter for comparison to balance velocity calculations from mass balance studies of numerical simulation.