From the very necessity of the case, glacial erosion diminishes as the limit of the extent of the glaciation is approached. At the very margin of the glacier, motion has ceased altogether. Back one mile from the margin only one mile of ice-motion has been active in erosion, while ten miles back from its front there has been ten times as much moving ice actually engaged in erosion, and in the extreme north several hundred times as much ice, Thus it is evident that we do not need to resort to two glacial periods to account for the relatively small amount of erosion exhibited over the southern portion of our glaciated area. At the same time, it should be said that the indications of active glacial erosion near the margin are by no means few or small. In Lawrence County, Pennsylvania, on the very margin of the glaciated area, Mr. Max Foshay[BI] has discovered very extensive glacial grooves, indicating much vigour of ice-action even beyond the more extensive glacial deposits which Professor Lewis and myself had fixed upon as the terminal moraine. In Highland and Butler Counties, Ohio, and in southwestern Indiana and southern Illinois, near the glacial margin, glacial grooves and striæ are as clear and distinct in many cases as can anywhere be found; while upon the surface of the limestone rocks within the limits of the city of St. Louis, where the glacial covering is thin, and where disintegrating agencies had had special opportunities to work, I found very clear evidences of a powerful ice-movement, which had planed and scratched the rock surface; and at Du Quoin, Illinois, as already related, the fragments thrown up from the surface of the rock, fifty or sixty feet below the top of the soil, were most beautifully planed and striated. It should be observed, also, that this whole area is so deeply covered with _débris_ that the extent of glacial erosion underneath is pretty generally hid from view. [Footnote BI: Bulletin of the Geological Society, vol. ii, pp. 457-464.] 4. The uniformity of the distribution of the glacial deposits over the southern portion of the glaciated area in the Mississippi Valley is partly an illusion, due to the fact that there was a vast amount of deposition by water over that area during the earlier stages of the ice-retreat. This has been due partly to the gentler slope which would naturally characterise the borders of an area of elevation, and partly to an extensive subsidence which seems to have begun soon after the ice had reached its farthest extent of motion.