1891.] 
237 
[Upham. 
loosely on the land as the ice melted, forms the upper part of the 
till. The Piedmont or Malaspina glacier, bordering the coast of 
Alaska south of Mt. St. Elias, as described by Mr. I. C. Russell, 1 
is a very good illustration of the way that the drift which had been 
contained in the ice-sheet became at last exposed on its surface, 
when only a small depth of the ice remained. 
The supply of englacial drift thus indicated for many areas 
would be ample for transportation along drainage channels of the 
ice-sheet, flooded by its summer melting and by rains. We can 
readily believe that where these floods descending from the ice 
spread with slackened currents over the gently sloping land sur- 
face, or expanded into temporary lakes fringing the ice-border, 
their coarse sediments of gravel and sand would be quickly de- 
posited in great depth. Not more than a few years, or at the 
longest one or two decades, would be needed for their accumula- 
tion in so thick plains as are found enclosing lakes Walden, 
Cochituate, and the multitudes of other lakes of this class. 
It is desirable to add here, however, that some other portions 
of the ice-sheet are known to have contained very little englacial 
drift, presenting a remarkable contrast with the areas of its great- 
est abundance. Tracts several or many square miles in extent in 
some of the hilly and mountainous portions of New England, in 
northern New York, and in north-eastern Minnesota, have a total 
amount of drift not exceeding a few feet, that is, from two to five 
feet, in average thickness ; and most of this lies in hollows of the 
rock surface where it was deposited as subglacial till or ground 
moraine. One of these tracts comprises parts of Henderson, 
Hounsfield, Brownville, Lyme, Clayton, and other townships of 
Jefferson county, N. Y., bordering the east end of Lake Ontario. 
It has a flat, gently inclined surface, and consists of nearly hori- 
zontal beds of the Trenton limestone. A much larger area charac- 
terized by surprisingly scanty drift deposits lies north and east of 
Vermilion lake, Minnesota, consisting of Archaean schists, with 
very hilly contour and plentiful lakes in rock basins. 2 The western, 
central, and southern portions of Minnesota, on the other hand, 
have mostly a very thick sheet of drift, averaging from 100 to 150 
feet in depth. These and other such diversities in the distribu- 
1 American Geologist, vol. vii, pp. 33-38, and 141-2, Jan. and Feb., 1891. 
2 Geol. and Nat. Hist. Survey of Minn., Final report, vol. i, pp. 117, 131. Minn. 
Horticultural Society, Annual report for 1884, p. 398. 
