Glaciologist checking in here.
Retreat is south to north. The advance maxes out 22-18 thousand years ago. While I don't recall the exact history for Maine, it's probably fully deglaciated by 12-14 thousand years ago.
The general cycle is that snow falls on the ice sheet. The weight of the snow compresses it to ice. The ice flows under its own weight (similar to watching honey level itself off on a spoon). The ice flows from the accumulation area (where snow falls) to the ablation area (where it melts and sublimates). The melting happens almost exclusively in the summertime.
As climate warms, the summers get warmer. Warmer summers mean longer summers for the ice sheet -- the time of melting is lnoger. The amount of melt exceeds the amount of snowfall causing the ice sheet to retreat.
This is where it might get confusing: the ice is flowing south, but the ultimate extent (what we refer to as the margin) is moving north.
Because the summertime melt is high during retreat, there is a lot of "excess" water that carves the landscape. We almost never see the growth phase of the ice sheet on the land surface because the retreat phase is so strong and it's the last thing to happen.
I hope this helps.