Near the end of the Ice Age, the Wisconsin Glacier expanded south and west into northern Illinois, pushing rock and soil along its leading edge like a mile-high bulldozer. Over the centuries it grew and shrunk several times, reshaping the landscape with every wave. Its final retreat ten thousand years ago left behind ridges of land called moraines, including the Tinley Moraine along Salt Creek. The large boulders at Fullersburg Woods Forest Preserve are glacial erratics carried from the north and east by the glacier and dropped when it receded, and many there are granite from Michigan and Canada. About twenty-five thousand years ago the glacier expanded into DuPage County from the northeast, and about ten thousand years ago it shrank back for the last time.