Lake Whittlesey was a proglacial lake that was an ancestor of present-day Lake Erie. It formed about 14,000 years ago. As the Erie Lobe of the Wisconsin Glacier retreated at the end of the last ice age, it left melt-water in a previously-existing depression area that was the valley of an eastward-flowing river known as the Erigan River that probably emptied into the Atlantic Ocean following the route of today's Saint Lawrence River. The lake stood at 735 feet (224 m) to 740 feet (230 m) above sea level. The remanent beach is not horizontal as there is a ‘hinge line’ southwest of a line from Ashtabula, Ohio, through the middle part of Lake St. Clair. The hinge line is where the horizontal beaches of the lake have been warped upwards towards the north by the isostatic rebound as the weight o