320 The America?i Geologist. May,i899 
sheet and of drumlins; (2) englacial or intraglacial drift, which 
became superglacial l)y ablation of the ice-sheet, thence being 
deposited as an upper till ; and (3) marginal moraines, in their 
diverse phases of development, so far as they consist of till or 
boulders and other drift not due to stream deposition. 
Because of changes in the course and relative strength of 
glacial currents, deposits of till of different origin and there- 
fore unlike in their rock materials and color, in the derivation 
and proportions of their boulders, etc., form in some districts, 
as on the Duluth & Iron Range railroad in northeastern Min- 
nesota, several beds in a single drift sheet, even where all are 
probably referable to one uninterrupted general stage of gla- 
ciation.* It is quite in accordance with prevailing geologic us- 
age to speak of these diverse deposits of till, overlying one an- 
other in a single sheet, as beds or strata. Furthermore, layers 
of modified drift occasionally intercalated in till may give it a 
stratified appearance, although all the till should have the same 
origin and character. On the other hand, some portions of 
our coarsest deposits of modified drift gravel and sand, with 
waterworn and rounded cobbles and boulders, sometimes up to 
two or even three feet in diameter, as in many eskers and 
kames in the hilly and mountainous region of New England, 
present sections having no evident stratification or assorted 
condition through thicknesses of 5, 10, or 20 feet. It is thus seen 
that the other terms based on evidence of stratification, wlfich 
have been proposed for these great classes, as unstratified drift 
and stratified drift, are not fully satisfactory. 
Modified Drift. 
The grand two-fold classification of the drift was very clear- 
Iv recognized by Dr. Edward Hitchcock in 1857, its water-laid 
formations being collectively described and named as follows: 
When drift has been acted upon by waves, or currents of water, the 
boulders are reduced in size, they are smoothed and rounded, their 
stride are generally obliterated, and all the materials are redeposited in 
regular layers, being sorted into finer and coarser deposits, according 
to the velocity of the currents. These I call modified drift. t 
*Geol. and Nat. Hist. Survey of Minnesota, 22d Annual Report, for 
1893, PP- 4i'45' ^^''th four sections. 
flllustrations of Surface Geology, p. 4. This paper, accepted for 
publication, January, 1856, forms pages viii, 155, with 12 plates, in Smith- 
sonian Contributions to Knowledge, vol. ix, 1857. 
