Thirty thousand years ago, the streams and rivers in this area flowed north. A ridge extended across what is today the Muskingum-Morgan County line about seven miles south of the Philo Lock. When the Wisconsin Glacier moved down from Canada twenty-five thousand years ago, it blocked the north-flowing streams. A large lake formed in Muskingum County. Eventually the water spilled over the ridge and flowed to the Ohio River, carving the Muskingum River Valley. When the glaciers retreated, the drainage divide moved north to an east-west line thirty to fifty miles south of Lake Erie. The Tuscarawas, Walhonding, Scioto, Muskingum, and other area rivers and streams have continued to flow south ever since.