62 GLACIAL GRAVELS OF MAINE. 



which border the streams of the Mississippi Valley and the Rocky Mountains, 

 except that they have been excavated in unconsolidated drift and within 

 a relatively short time. Below the contour of 230 feet all the higher 

 terraces which border the rivers of Maine are the result of the erosion 

 of till, blown sand, the marine sands and clays, or the glacial sand and 

 grravel. Erosion of these formations, especially of the marine clays, has 

 been effected on a grand scale. In many places the marine clays have 

 been eroded into forms somewhat resembling the "bad lands" of the West. 

 When a ravine once begins to form, it rapidly extends itself back into the 

 clay. I have observed several ravines which, within five years, extended 

 themselves from one-eighth to one-fourth of a mile and to a depth of 10 or 

 more feet. These were formed where there were no permanent streams, 

 and were wholly due to the wash of the rains. In the regions covered by 

 the marine clays the streams having constant flow are bordered by cliffs of 

 erosion, just like the narrow ravines, only the cliffs are . situated much 

 farther from one another, sometimes from 1 to 3 miles. The ravines and 

 cirques of erosion are so characteristic of the clay-covered regions that l3y 

 them one can recognize most of that part of Maine which was under the 

 sea, even when deeply covered by snow. The till, being much harder to 

 erode than the sedimentary drift, rarely shows cliffs of erosion at levels 

 above the channel proper, except where the flow of the stream in time of 

 flood is very much greater than the ordinary flow. Hence the scenerj^ in 

 the areas covered by till is very different from that of the clay regions. 

 The methods of terrace erosion will be more fully considered hereafter. 



2. Terraces composed chiefly of valley sediments. The simplest case 

 is that of the present flood-plain terraces. They rise to only a moderate 

 height above the present beds of the streams, and now and then they are 

 overflowed in time of high water. The drift of the flood plain is of very 

 composite origin. Part of it is usually the uneroded remains of a sheet of 

 di-ift laid down previous to the flow of the stream at its present level — either 

 till or the marine beds or valley drift deposited near the close of the Glacial 

 period. Part of it is of recent origin, consisting of sediment deposited by 

 the stream in time of flood or of matter brought down by the rains from the 

 higher terraces and the hillsides. Wherever deposition equals or exceeds 

 erosion, the flood plain is not nominally bordered by steep cliff's or banks of 

 erosion, but it simply extends to the sides of the valley, sometimes being 



