Complex sentence: The shoreline was formed when the lake was higher during the last Ice Age when ice blocked the Straits of Mackinac and after the straits were freed the lake receded and left behind a sand ridge at about 640' where the shore resided.
Split into simpler sentences: The shoreline was formed when the lake was higher during the last Ice Age, while ice blocked the Straits of Mackinac. After the straits were freed, the lake receded and left behind a sand ridge at an elevation of about where the shore resided.