Between the Berkshires and the rest of the state lies the Connecticut River Valley, known within Massachusetts as the Pioneer Valley. This ancient rift valley appeared in the Mesozoic Era when North and South America broke away from Europe and Africa. Dinosaur footprints near Mount Tom bear witness to that era, and series of basalt and sedimentary rock ridges (collectively known as the Metacomet Ridge) including Mount Toby, Mount Holyoke, Mount Tom, and others extending south to Long Island Sound and the valley's abrupt thousand-foot (300 meter) western escarpment illustrate the tectonic forces. More than a hundred million years later, as the Pleistocene epoch ended, receding glaciers left moraines that dammed the Connecticut River, creating Lake Hitchcock. Lacustrine silt deposits replaced soil scraped away by the glaciers, leaving behind deep, productive soil after the river breached the obstructing moraine and the lake disappeared.