The Yahara chain of lakes resulted from glacial action about 40,000 years ago, when a massive ice sheet one-quarter mile thick at Madison gouged hills and widened valleys as it moved westward. As the climate changed, the glacier retreated and left behind great masses of rock and debris, and blockage in the channel of the pre-glacial Yahara River formed the four lakes Mendota, Monona, Waubesa, and Kegonsa. The largest of the four, Mendota covers 15.2 square miles, reaches a greatest depth of 84 feet, and has an average depth of 40 feet. Before white settlement, numerous Winnebago Indian villages and camps occupied Mendota's shores, and about 350 effigy mounds built by the Winnebago have been identified around Lake Mendota, with 1000 in the four lakes area. A bird effigy on the Mendota hospital grounds is one of the finest and has a wingspread of 624 feet. The Winnebago called the Four Lakes Region Tay-cho-pe-ra and called Lake Mendota Wonk-shek-ho-mik-la. In 1849, Madison surveyor Frank Hudson gave Mendota its official name after the Sioux Indian term meaning "mouth of the river."