In more recent times, massive ice sheets at least one kilometer thick ravaged the state's landscape and sculpted its terrain. The Wisconsin glaciation left 12,000 years ago. These glaciers covered all of Minnesota except the far southeast, an area characterized by steep hills and streams that cut into the bedrock. This area is known as the Driftless Zone for its absence of glacial drift. Much of the remainder of the state has fifty feet (15m) or more of glacial till left behind as the last glaciers retreated. Gigantic Lake Agassiz formed in the northwest 13,000 years ago. Its flat bed now is the fertile Red River valley, and its outflow, glacial River Warren, carved the valley of the Minnesota River and the Upper Mississippi downstream from Fort Snelling. Minnesota is geologically quiet today; it experiences earthquakes infrequently, most of them minor.