RIVER ACTION, AND DRIFT AGENCIES. 89 



In our country, as well as in northern Europe, the force appears to have had a 

 southern direction. Consequently, the northern sides of our hills and mountains, 

 as well as their tops, are rounded and striated, while their lee side is rough and 

 precipitous. 



If any one wished to be impressed with the extent and power of this agency, let 

 him compare the rounded appearance of the mountains and hills of New England, 

 with the pointed and jagged aspect of those of Wales, or the Alps, especially above 

 the line where this agency has operated. He will see that the amount swept away 

 must have been immense; nor will his conviction of the quantity be lessened 

 when he examines the loose detritus covering the surface of northern countries, 

 most of which was originally drift. 



4. Erosions by Rivers. 



To this action I have given more attention than to any other form of erosion. 

 At the outset, however, I found myself embarrassed by the difficulty of distin- 

 guishing between river action and the other denuding agencies above described. 

 So far as I know, very few definite rules on this subject have been proposed by 

 geologists. I give the following as the result of my own examinations. They are 

 not in all cases as decisive as I could desire. But they seem to me sufficiently so 

 in general, to enable one to discriminate between the different agencies, and they 

 have led my mind to some interesting conclusions from phenomena which I had 

 hitherto overlooked. 



Marks by which River Action can be distinguished from Drift Agency. — 1. By the 

 direction in which the denuding force has acted. Since the drift agency in several 

 countries took a southerly course, we may conclude erosions made in other 

 directions to have resulted from rivers. Or more specifically, having determined 

 the course taken by the drift in a given district, we may refer other marks of 

 aqueous action to rivers or the ocean. I ought to make an exception of those few 

 cases where the marks of ancient glaciers have been found in the same countries 

 as the drift phenomena, with which they agree, except that the striae made by 

 glaciers follow down the valleys in whatever direction they run. In every other 

 respect the glacier marks correspond to those of drift, and can be distinguished 

 from fluviatile action by the marks that follow. 



2. Drift agency has eroded the northern slopes of mountains; but rivers have 

 cut channels through the rocks in every direction, or where they have pressed 

 against the sides of hills, they have formed steep precipices. 



3. The drift agency has but slightly conformed to the minor irregularities of 

 surface, but has operated like a huge plane, or rasp, to remove protuberances. 

 Whereas rivers have insinuated themselves into the minutest anfractures and 

 cavities, smoothing the uneven surface — the depressions almost as much as the 

 protuberances. 



4. Drift agency has covered the eroded surfaces with striaa and furrows of vari- 

 ous sizes, from the finest scratches, to troughs several inches deep. But save in a 

 few spots, where ice with gravel frozen into it has been crowded over the surface, 



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