Part of the sub-continental divide runs through the eastern edge of Waukesha County, separating the Great Lakes watershed from the Mississippi River watershed. The divide is a ridge of land, created by thawing and receding glaciers 10,000 years ago. Water that falls west of the ridge flows into the Fox River and other small waterways joining into the Mississippi River and eventually the Gulf of Mexico. Water that falls east of the ridge flows towards Lake Michigan, part of the Great Lakes watershed, flowing eventually through the St. Lawrence Seaway. The divide lies north-south through eastern Waukesha County.