56 DEIFT DEPOSIT. 



DKIFT. 



We think it will not be difficult to give almost any inhabitant of Vermont an accurate 

 idea of drift. For in almost every part of the State occur accumulations of bowlders, or 

 large blocks of stone, with the angles more or less rounded, lying upon the solid ledges, 

 or upon, or in the midst of, a mixture of smaller fragments, with gravel and sand ; the 

 whole mingled confusedly together, and evidently abraded by some powerful agency from 

 the rocks in place, and driven along pell mell often to great distances : for if the bowlders 

 and fragments be examined, they will for the most part be found not to correspond to the 

 ledges beneath, but to others many miles perhaps to the north or northwest. 



It is probable that if the whole State were denuded of its loose surface materials, the coarse 

 mass of detritus which we have just described would be found almost everywhere, spread 

 over the solid ledges. But as a matter of fact in most of the valleys and the level parts 

 of the State, the surface trod upon consists of the same kind of materials in a finer state- 

 in the form of gravel, sand, clay and soil. These are evidently for the most part drift, 

 worked over and over again, and comminuted, mostly by water, and hence we call them 

 Modified Drift. They lie above the true drift, and are the result of operations of pos- 

 terior date. But where rivers or other erosive agencies have cut through the modified 

 drifts, they generally disclose that which is unmodified beneath. 



From these circumstances it happens that the unmodified drift does not form an ex- 

 tended surface deposit in but few instances, although almost everywhere it shows itself. 

 Hence we have not thought it best to give it a place on the Geological Map. It is, how- 

 ever, spread over the higher portions (not the very crests and peaks) of the Green 

 Mountains, especially in the southern part. And it is curious that it should be found so 

 abundant at so great elevation. But we have evidence that no mountain in Vermont was 

 high enough to escape the agency by which drift was produced, although some of the high- 

 er parts and ridges are too precipitous to retain the abraded materials upon their tops and 

 sides. 



But what was so remarkable an agency ? We have often wondered why this question 

 is not oftener put by intelligent men over the whole northern part of our country ; for it is 

 not in Vermont alone, but from Newfoundland to the Rocky Mountains, the same phenom- 

 ena prove the former presence of the same powerful agency. Geologists have long and 

 earnestly sought it out, and though they are not as well agreed on this as on most of the 

 principles of their science, they have of late made a near approach to unity of opinion. 

 But it will be desirable to go more into details as to the facts before venturing upon the 

 theory. 



FORM, SIZE, AMOUNT, AND POSITION OF BOWLDEES. 



Sometimes we find bowlders with angles as sharp as if just chipped off from a ledge, 

 and not at all exposed to the grinding action of other bowlders driven along by a force 

 behind. But generally their rounded and often smoothed edges prove them to have been 

 subject to such attrition for a long time. Indeed in many cases it is only the hardest 

 kinds of rock that have been able to resist the grinding action to which drift has been sub- 

 ject, while the softer kinds have been ground to powder. Bowlders of the softer kinds of 



