Glacier IVork. — Scott. .257 
and yet in a veritable valley region of the ^Mississippi. The re- 
gion has been known and studied for more than half a century 
and has afforded glaciologists a most direct means of attacking 
the great problems of continental glacier work and drift classi- 
fication ; * it is chiefl}- the results of an investigation of the 
region immediately south west of the great lakes, which has 
led to the belief in at least five distinct divisions of the drift. 
Both in America and Europe the glacial accumulations show 
conclusive evidence of climatic oscillations, resulting in several 
distinct epochs. Professor Chamberlin has shown that the drift 
may be divided as follows: "Wisconsin Till Sheets (earlier 
and later); Interglacial deposits (Toronto perhaps),; lowan 
Till sheet; Interglacial deposit; Illinois Till sheet (Leverett) ; 
Interglacial deposit (Buchanan, Calvin); Kansan Till sheet; 
i\ftonian beds, interglacial; Albertan Drift sheet (Dawson) 
Geikie recognizes in the glaciation of Europe six epochs, 
though the last two were cjuite insignificant compared with the 
others. He says, "From the Pleiocene down to the close of 
Pleistocene times we have the record of a continuous series of 
geographical and climatic changes. Early in the cycle the gla- 
cial and interglacial phases attained their extreme development. 
The climax once passed, each successive cold and genial epoch 
declined in importance. In a word, the climatic and geographi- 
cal changes became less and less marked as the cycle drew to 
a close." 
The greatest of all drift deposits, the glacial till, was doubt- 
less mainly deposited under the thinned edge of the ice mass. 
It consists of a great sheet of heterogeneously intermixed boul- 
ders, gravel, sand, and clay, which is more or less firmly com- 
pacted according to local conditions and parental origin, with 
which is usually associated more or less stratified material. The 
component rock fragments are usually rounded, and sometimes 
polished and striated. 
Another form of till has been recognized as overlying the 
so-called true till. Such has been regarded by some as the de- 
posit of englacial or superglacial material, because of its loose- 
ness, angularity of rock fragments, porosity at the base, and 
higher oxide of iron compounds, and is thought to have been 
dropped at the tinie of final melting of the ice, upon the true 
sub-glacial till. 
*See Preliminary Paper on the Driftless Area of the Upper Mis- 
sissippi Valley, bv T. C. Chamberlin and R. D. Salisbury. Ann. Report 
U. S. Getjl. Sui-v-ey. 1884-S5. 
