Rhythmic Accunudation of Jforaines. — Upham. 413 
ent on the repetition of secular variations of climate V or was 
it due to regular varying physical conditions of the wasting 
border of the ice-sheet, in combination with the exposure and 
deposition of its previously inclosed drift? 
Rapid Formation of Moraines shown by theik Relation 
TO Lake Agassiz. 
My studies of the glacial lake Agassiz convince me that 
the moraines were, in a geological sense, very rapidly formed. 
Four or five large moraines in northern Minnesota and North 
Dakota were accumulated, in succession, contemporaneousl}^ 
with the formation of the highest or Herman beach of this 
glacial lake. The time occupied in the accumulation of any 
one of these moraines was probably no more than a few de- 
cades of years; for the entire duration of lake Agassiz, mark- 
ed by a series of thirty shore-lines, appears, from the total a- 
mount of wave erosion and resulting deposition of beach 
gravel and sand, to have been only about a thousand years.* 
Ten times as much wave action during the Postglacial period 
is shown by the shores of the great Laurentian lakes. 
■ Derivation op the Marginal Moraines chiefly from 
Enolacial and Superglacial Drift. 
Such rapid accumulation of the marginal moraines, their 
exceptional abundance of boulders, and their occurrence as 
steep hills and ridges w^hich rise abruptly to bights of 100 to 
300 feet, all point to their derivation chiefly from drift which 
was inclosed in the lower part of the ice-sheet. When abla- 
tion had thinned the ice to its basal third or fi)urth part, in 
which drift was held and borne forward, the ice surface, in 
its being further melted, became overspread by a covering of 
drift. In a former paperf I have attempted to show how this 
drift could be amassed in large marginal moraines during a 
few years or decades, as is implied on each side of lake Agassiz. 
Retardation of the Ice Movement by 
Englacial Drift. 
The growth of a forest on the border of the Malaspina ice- 
sheet, to a distance of three to five miles from its edge, with 
hundreds of feet of ice beneath the superglacial drift and its 
*U. S. Geological Survey, Monograph xxv, 189G, pp. 240-244. 
f.-^M. Geologist, vol. xvi, pp. 107-113, Aug., 1895. 
