124 G. H. Stone — Glacial Sediments of Maine. 



adjacent country. The fact, then, that the finest matter has 

 been washed out of a mass of glacial-drift does not necessarily 

 prove that it is not true moraine-stuff. In Maine, the till 

 being largely derived from slates, limestones and feldspathic 

 rocks naturally contains a large proportion of clay. A sandy 

 till is here almost always a water-washed till, and it is gen- 

 erally easy, by a comparison of a suspected deposit with the 

 till of the neighborhood, to determine whether the deposit has 

 been modified by water or not. Such a comparison would not 

 be so easy over a large area of quartz sandstone. It is there- 

 fore difficult to always apply the test of composition to distin- 

 guish till from glacial sediments. 



3. The test of transportation. According to this test, all 

 matter brought to its final position by ice movements would 

 be termed till, while that which was brought to its final 

 position by the water of glaciers would receive the name 

 glacial sediments. This test at once distinguishes from the till 

 all the sedimentary clays containing drift boulders and some- 

 times fossils which have been named boulder clays, and which 

 have often been confounded with the true morainal till, — to 

 the great detriment of glacial science. 



The first and second of the above mentioned tests leave the 

 kames and osars in the same class with terminal moraines. 

 On the whole, the test of transportation is the best single test 

 whereby to distinguish the till from the glacial sediments. 

 Besides the three tests named, there are several others which 

 are often of great importance. In the field the method of 

 transportation of a given mass of drift has to be determined 

 by its structure, its composition, the shape of the individual 

 drift fragments and their markings, the shape of the mass, etc. 



The sediments transported by the waters of the ice-sheet 

 were deposited in various situations. 



1. In channels bordered on both sides by ice, partly in sub- 

 glacial tunnels, partly in channels open on the top to the 

 air. Such channels varied in breadth from a few feet up to a 

 mile or more. They were often locally enlarged so as to form 

 pools or lakes. Sediments dropped in them now form two-sided 

 ridges or plains. 



2. In channels bordered on one side by ice, on the other by 

 the land. The sediments left in such a channel now form a 

 terrace along a hill -side. 



3. In channels bordered for most of their length by ice, 

 but in some parts of their courses extending across a whole 

 valley. The rivers that flowed in such channels were so far 

 confined by ice walls that we must term them glacial rivers, 

 yet for a few miles in the midst of their courses, they were 

 bordered by land on both sides like ordinary rivers. 



