Englacial Drift. — Crosby. 209 
It may safely be assumed that over the glaciated area in 
preglacial times, as now in lower latitudes, the superficial de- 
tritus or soil was chiefly the residuary product of quiet chem- 
ical decay. This is equivalent to saying that it consisted 
chiefly of clay and sand. Hard angular fragments of rock, 
such as are so common in the till, were wanting; and the 
rounded bowlder-like nuclei of decomposition must have been 
of rather rare occurrence at points near the surface of the 
ground, or above the lower limit of the frost. 
If we may assume a thickness of residuary detritus com- 
mensurate with that which now mantles the Southern States, 
it is, perhaps, probable that over considerable areas solidifica- 
tion by freezing would fail to reach the firm, un decomposed 
rocks beneath. However that may be, it is certain that with 
increased thickness the ice-sheet became a more and more 
efficient protection to the ground against the climate of the 
Ice age; and the steady efflux of heat from the earth's inte- 
rior would thus tend to gradually loosen the hold of the frost 
upon the rocky substratum. 
Observations in the Arctic regions and at high altitudes 
show that the ground may become frozen to a depth of several 
hundred feet ; and K. S. Woodward's theoretical discussion* 
of the subject not only corroborates these observations, but 
indicates that the downward penetration of the frost may be 
comparatively rapid. Thus, if the mean annual temperature 
of the surface should fall from 60° F. to 10° P. the ground 
would become frozen to a depth of 400 feet in less than 1,000 
years. Russell suggestsf that the great depth of frozen soil 
reported at Yakutsk. Siberia (382 feet), and at other arctic 
stations, may be due in part to the surface accumulation of 
ice through the growth of the tundra, and that possibly the 
rate of diffusivity of temperature assumed by Woodward is 
too high ; but he does not seriously question the main conclu- 
sion with regard to the efficiency of frost penetration. 
It is well known that glaciated areas arc not, in general, 
those of most extreme cold. A humid climate is the first es- 
sential ; and, given that, a temperature low enough through- 
out the year to insure precipitation chiefly in the form of 
*Bull. Geol. Sue. of America, i. 130-132. 
tlbid. 
