Agassiz fell and rose multiple times over thousands of years as the ice sheet retreated and regrew. During that time it found many paths to escape its confinement, and left behind several beach-lines.
One of the most clearly-written and illustrated professional Agassiz stories is Wright's lightly-technical 1990 geology paper, available in PDF [0]. A lot more has been learned in the past 30 years, but by that time its effects in Minnesota were quite well known.
[0] https://conservancy.umn.edu/handle/11299/57272 Wright, H.E. Jr (Minnesota Geological Survey, 1990)