During the Ice age, which was about 10 to 30,000 years ago, give or take, so much of the water was locked up in glaciers that the sea level was 500 feet or so lower. So the Atlantic and Pacific Coasts were way way out there.
If you look at a Google map, and i'm guessing on this, you can see some light blue on the Atlantic Coast, and I"m guessing those were the coasts during the Ice Age.