At least four times in the last two million years, large continental glaciers covered northern North America, including much of Ohio, and in some parts of Ohio they were almost a mile thick. As these massive sheets of ice advanced and retreated many times, they ground down rock, carried it forward, and, when they melted, left behind thick gravel deposits containing clay, sand, pebbles, and boulders called till. Much of this rocky material came from Canada, and rocks moved from their place of origin by natural forces such as glaciers are called glacial erratics. More than 10,000 years ago, the final retreat of the last glacier, the Wisconsinan, formed the Darby Plains and the Big Darby Creek watershed, while water rushing from the melting ice cut stream channels such as Big Darby Creek. The creek's streambed contains hard igneous and metamorphic erratic rocks, including granite and gneiss from Canada, as well as softer sedimentary rocks such as limestone, shale, and sandstone from Ohio.