On May 30-31, 1805, the explorers traveled through the part of the Upper Missouri River that we now call the White Cliffs of the Missouri in the Upper Missouri River Breaks National Monument. The tall, pale sandstone cliffs rose 200 to 300 feet above the river and amazed Lewis, who said they looked like the ruins of giant stone buildings and walls. The men still had to work very hard, towing their heavy boats against the strong current along narrow, slippery riverbanks. In some places the banks and shallows were so muddy that the men took off their moccasins and slowly felt their way along the river bottom while pulling the boats through the shallow water.