Confirmation of the elevation of lake shorelines can be found on the landscape by locating deltas that form at the point where glacial drainage channels enter lakes. There are several excellent examples of this in Cheboygan, Montmorency, Alpena, and Alcona counties, Michigan. Larsen has several papers. I've written up one in Geologiska Foreningens 1989 v 111. Given that the ice advanced either uphill or parallel to the slope of the northern Adirondacks it seems quite likely that supraglacial ponds would form and that sediments would be preserved.
Several possibilities come to mind.
As the ice moved from the north it plowed some debris into a hardpack layer known as lodgement till. This is usually a relatively thin layer of dense material. Then during retreat, as the ice front moved north, there were often lakes trapped infront of the ice either by debris dams to the south or possibly remnant pieces of ice. As you might know, the Champlain Valley contained a much larger lake during ice retreat known as Lake Vermont. Subsequent deposition left lake beds that can be found on till in places.