286 SURFACE GEOLOGY. 



has been shown that the modified drift was swept into the valleys, while 

 the upper till, which escaped this erosion, fell loosely upon the surface, 

 forming an unstratified, confused mass of boulders, gravel, and sand. 



The characteristics of this upper division of the unmodified glacial 

 drift are, — the large size of its boulders, which are usually abundant, be- 

 ing often from five to ten, and sometimes twenty or thirty feet in diame- 

 ter; the angular form of these blocks, as also of smaller fragments, which 

 have seldom been worn or rounded except by the weather; the occur- 

 rence of much of its iron in the form of sesquioxide, giving a yellowish 

 or reddish color; and the comparative looseness of the whole deposit. 

 Its thickness is quite variable, being commonly one to five feet, but 

 sometimes reaching to twenty feet or more. This upper till generally 

 forms the surface throughout the state, the only exceptions being tracts 

 of valley or lowland, where it is covered by beds of modified drift, and 

 frequent small areas, varying from a few square rods to several acres, or 

 sometimes, especially upon mountains, perhaps hundreds of acres in ex- 

 tent, where scarcely any superficial material rests upon the ledges. 



The lower till is distinguished by its smaller rock-fragments, which are 

 commonly less than two feet in diameter, and often consist of pebbles 

 not exceeding half this size, though occasionally it also contains large 

 boulders; by the glaciated form of many of these stones, which are fre- 

 quently marked with striae; by the usually clayey detritus, in which they 

 are held; by its darker and frequently bluish color, due to the imperfectly 

 oxidized state of its iron ; and by its very hard and compact structure 

 without stratification, boulders, pebbles, sand, and clay being indiscrim- 

 inately mixed, but at the same time showing traces of lamination, or per- 

 haps cleavage, in planes parallel to the surface, usually noticeable wher- 

 ever a section has been for a short time exposed to the weather. All 

 these features indicate that this division of the drift was accumulated 

 beneath the ice as its ground-moraine. Rough and angular boulders, 

 pushed along under the glacial sheet, were worn to small size, having 

 their sides planed off and striated ; and in the same manner gravel and 

 sand were pulverized to clay. Secluded from air and water, the iron 

 remained in the protoxide combinations which it had in the solid rocks. 

 Analyses of upper and lower till from Alton, by Mr. Hawes, show the 

 following percentages : 



