These relations are suggested by Figs. 32 and 33. From them it will be seen that in regions where the thickness of the drift is great, relative to the relief of the underlying rock, the topography may be completely changed. Not only may some of the valleys be obliterated by being filled, but some of the hills may be obliterated by having the lower land between them built up to their level. In regions where the thickness of the drift is slight, relative to the relief of the rock beneath, the hills cannot be buried, and the valleys cannot be completely filled, so that the relative positions of the principal topographic features will remain much the same after the deposition of the drift, as before (Fig. 33). [Illustration: Fig. 33.--Diagrammatic section showing relation of drift to underlying rock where the drift is thin relative to the relief of the underlying rock.] In case the pre-glacial valleys were filled and the hills buried, the new valleys which the surface waters will in time cut in the drift surface will have but little correspondence in position with those which existed before the ice incursion. A new system of valleys, and therefore a new system of ridges and hills, will be developed, in some measure independent of the old. These relations are illustrated by Fig. 32. Inequalities in the thickness of drift lead to a still further modification of the surface. It frequently happened that in a plane or nearly plane region a slight thickness of drift was deposited at one point, while all about it much greater thicknesses were left. The area of thin drift would then constitute a depression, surrounded by a higher surface built up by the thicker deposits. Such depressions would at first have no outlets, and are therefore unlike the depressions shaped by rain and river erosion. The presence of depressions without outlets is one of the marks of a drift-covered (glaciated) country. In these depressions water may collect, forming lakes or ponds, or in some cases only marshes and bogs. DIRECTION OF ICE MOVEMENT. The direction in which glacier ice moved may be determined in various ways, even after the ice has disappeared. The shapes of the rock hills over which the ice passed, the direction from which the materials of the drift came, and the course of the margin of the drift, all show that the ice of south central Wisconsin was moving in a general southwest direction. In the rock hills, this is shown by the greater wear of their northeast ("stoss") sides (Plate XXXIV). From the course of the drift margin, the general direction of movement may be inferred when it is remembered that the tendency of glacier ice on a plane surface is to move at right angles to its margin.