It is in the broad, shallow depressions of the ground moraine that many of the lakes and more of the marshes of southeastern Wisconsin are located.
The rolling, undulating topography characteristic of ground moraines is well shown about the City of Baraboo and between that point and the lake, and at many less easily designated points about Merrimac.
In thickness the ground moraine reaches at least 160 feet, though its average is much less--too little to obliterate the greater topographic features of the rock beneath. It is, however, responsible for many of the details of the surface.
The marginal portion of the ice sheet was more heavily loaded--certainly more heavily loaded relative to its thickness--than any other. Toward its margin the thinned ice was constantly losing its transportive power, and at its edge this power was altogether gone. Since the ice was continually bringing drift down to this position and leaving it there, the rate of drift accumulation must have been greater, on the average, beneath the edge of the ice than elsewhere.
Whenever, at any stage in its history, the edge of the ice remained essentially constant in position for a long period of time, the corresponding submarginal accumulation of drift was great, and when the ice melted, the former site of the stationary edge would be marked by a broad ridge or belt of drift, thicker than that on either side. Such thickened belts of drift are _terminal moraines_. It will be seen that a terminal moraine does not necessarily mark the terminus of the ice at the time of its greatest advance, but rather its terminus at any time when its edge was stationary or nearly so.
From the conditions of their development it will be seen that these submarginal moraines may be made up of materials identical with those which constitute the ground moraine, and such is often the case. But water arising from the melting of the ice, played a much more important role at its margin than farther back beneath it. One result of its greater activity may be seen in the greater coarseness which generally characterizes the material of the terminal moraine as compared with that of the adjacent ground moraine. This is partly because the water carried away such of the finer constituents as it was able to transport, leaving the coarser behind. Further evidence of the great activity of water near the margin of the ice is to be seen in the relatively large amount of assorted and stratified sand and gravel associated with the terminal moraine.
