The tour passes a section of prairie dotted with small mounds of earth covered with prairie grass. The origin of these bumps, scattered over the surface of a half-dozen prairies just south of Puget Sound, has been the subject of speculation for nearly a century. Symmetrical in form, the mounds round out from slightly flattened tops to circular bases; a few of them are surmounted by young trees. They are composed chiefly of gravel with a layer of black soil and rise as high as eight feet. Captain Wilkes, during his expedition of 1841, dug into a number of them, thinking they might be burial mounds, but no traces of human remains were found. Two renowned naturalists, Louis Agassiz and Joseph LeConte, made a study of them and arrived at different conclusions. One believed they had been built by fish for spawning nests during an early inundation; the other was sure they had been caused by erosion following the recession of the ice sheets. Many other explanations have been advanced, ranging from one that they were Native American-built buffalo decoys to another which held that they had been forced up by gas and oil pressure. The most generally accepted is that of J. Harlan Bretz; in his Glaciation of the Puget Sound Regions he concludes that the silt blowing across the surface of the ice sheet covering the Puget Sound area had collected in depressions, and that, as the ice melted, this accumulation fell through to the gravel beneath, forming the mounds.