DRIFT BURIED GRAVEL DEPOSITS. 83 



whose outlines resemble those of boulders (as if the gravel had been cemented 

 into masses by frost and then moulded into boulders, and afterwards deposited 

 in the frozen state. By the characters just given, these accumulations can 

 be readily distinguished from those of true beaches. They are commonly 

 overlain by a few feet (perhaps ten or twenty) of stony clay or other ma- 

 terials of the upper till. Occasionally the covering may reach several times 

 this thickness. 



The external form of these deposits, with their clay mantle (which last is 

 dependent upon the form of the underlying gravels), may be that of undula- 

 ting plains, or these undulations rising to the magnitude of ridges and hills. 

 In this case, the ridges rise in succession one above the other, until they 

 reach an altitude of a hundred feet, or even more, above the plains which 

 are commonly in front of them. They may occupy a breadth of several 

 miles across the country. The ends of the ridges often overlap, and at other 

 times send out spurs, and enclose kettle-like depressions, which are liable to 

 be confounded with or not separated from those of the next group. These 

 ridges form a considerable proportion of the so-called moraines of America. 

 These slightly covered sand and gravel deposits are not so commonly devel- 

 oped below the altitude of 700 feet above the sea as at higher elevations, 

 for the lower country is more apt to consist of terraces, cut in the drift, and 

 of lacustrine deposits and beaches. But these accumulations cap the ridges 

 of the great chain named the Oak hills, which extend for over a hundred 

 miles in length, parallel to the northern side of Lake Ontario, at an eleva- 

 tion of from 900 to 1,200 feet above the sea. Farther west, such are also 

 the capping materials of the country, which is 1700 feet above the sea. The 

 same holds true for Michigan and other States. 



II. — The gravels of this group are not only well water-worn but also well 

 washed and free from earthy matter. Indeed, they are sometimes free from 

 the finer sand. The pebbles are often coarser than in the lower beaches, in 

 some cases forming accumulations of almost cobble stones. There are 

 occasional boulders in the mass, but these are more common upon the surface. 

 The materials are mostly of local origin, with a small proportion of trans- 

 ported crystalline stones. None of the materials have been derived directly 

 from the subjacent Paleozoic rocks, but secondarily from the assortment of 

 the stony boulder clays. The gravels with their accompanying beds of 

 sand, when these are present, are stratified as in beaches, without anything 

 of the tumultuous structure of the last group. Still, there may be false 

 bedding, as in beaches; and when the deposits assume the form of ridges, the 

 layers may dip in opposite directions, as in barrier beaches. The materials 

 of this group are never covered with drift deposits, but often rest upon the 

 till, or against hills of the tumultuous accumulations already described. 

 In 'external form, the gravel deposits differ greatly, and it is upon this 

 character that they are divided into the three series. 



