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 striz; 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 : 
