THE DRIFT-DEPOSITS OF THE NORTHWEST. 295 



the mass an impervious and uniform hard-pan. This has generally- 

 been denominated " unmodified drift." It is that which escaped the 

 assorting action of the water issuing from the glacier. In other places, 

 this unmodified drift would be superficially assorted, showing the effect 

 of running water after its deposition or in the act of deposition. Prob- 

 ably very much of that portion of the drift that lay in the course of 

 the broad Mississippi, yet south of the limit of the glacier, would be 

 superficially worked over, losing much of its clay. We actually see 

 vast tracts on the Upper Mississippi, and even in the latitude of St. 

 Paul, in which the surface consists, to a considerable depth, principally 

 of stratified, sand and gravel. He would also find parallel ridges of 

 drift-materials, consisting largely of the coarser portions, and showing 

 stratification where water passed over or through it in being deposited. 

 Some such ridges would still retain the most or all of the original 

 clayey portions. This would be the case where the drainage was not 

 powerful. Such ridges mark the places at which the retreat of the ice 

 was temporarily stopped by a period of greater cold, the slow advan- 

 cing of the ice under the propulsive forces already named serving to 

 heap up a greater amount of detritus all along the ice-margin. These 

 ridges are known as moraines, and they occur in all parts of the drift- 

 latitudes. They are developed on a very grand scale in Northwestern 

 Ohio. 



There is still one important point in this discussion that must not 

 be omitted. It is plain to see that, in some parts of the Northwest, 

 the advance of the continental glacier would be up gentle slopes, in- 

 stead of descending an incline. These slopes, of course, present ob- 

 structions to the movement of the ice in those directions. It is true, 

 also, that the continental glacier would tend to level the country and 

 obliterate such northward slopes. But, in the later part of the ice 

 period, the valleys would be the last relinquished, and would be deeper 

 dug by isolated branches or spurs from the main ice-sheet, which would 

 conform in their direction to the contour of the valleys they might oc- 

 cupy. All glaciers, however, whether continental or local, would avoid 

 an ascent if there were any other passage. Now, when a glacier, pro- 

 pelled by a force exerted far to the north, meets with a gentle slope 

 toward the north, the water which issues from its foot Will be con- 

 fined in a lake about the foot of the ice, and will rise to the height of 

 the lowest outlet. Into this lake may flow streams of considerable 

 size, bringing their sediment from the south, east, or west, according 

 to the topography. Here we should have, then, a constant accession 

 of drift from two sources, the chief of which would be, of course, the 

 glacier itself. As this drift is brought under the influence of standing 

 water, its fine parts are floated away by currents and waves, to be 

 spread over the bottom of the lake in horizontal laminations, the prin- 

 cipal portion, and notably the bowlders, sinking at once to the bottom 

 unassorted. Thus, by the continued slow withdrawal of the field of 



