This outcropping is an erosional high, rock smoothed by glaciers. Continental glaciers advanced and retreated across this region many times during the last million years before leaving for good about 12,000 years ago. This extremely hard quartzite preserves a unique record of glacial history. Individual glaciers, each carrying a bottom layer of rocks and moving in a separate direction, made the crisscrossed lines visible here, while other rock retains evidence only of the most recent glacier. The many ripple marks seen here were formed by currents in an ancient river. Shallow river currents were rapidly covered by mud and then subjected to heat and pressure for millions of years. The river's deposited sands gradually solidified into sandstone and then into the quartzite seen today.