Correspondence. 1 27 
was marshy, consisisting of mud which was not penetrated by digging 
to a depth of four feet. 
Perhaps it should be added that the remarkable hight of the beach 
deposit described as adjoining this lake in Aastad is explained by the 
fact that the shores to the right and left of the beach approach each 
other toward the original inlet till the distance between them is only 
about twenty rods at the place where the beach was formed. This un- 
doubtedly caused a concentration of the force of the waves as they 
rolled in from the northwest, a distance of a little more than half a mile. 
Fergus Falls, Minn. X. P. Nelson. 
The Drift Mounds of Olympia and of Long Island. — I was much 
interested in reading the paper on the "Drift Mounds of Olympia" in 
the June number of the Geologist, as I had just paid a second visit 
to the so-called sand-duDes at Easthampton, Long Island, and I am 
inclined to think that they have a common origin. 
In 1891, September number of the American Geologist, I spoke of 
these peculiar ridges as being formed by streams of water flowing from 
the front of the ice-sheet, and this view has been confirmed by subse- 
quent investigation; but I am inclined to think now, that said streams 
were in a sense subglacial ; that is, the upper part of the glacier seems 
to have advanced some distance south of the so-called terminal moraine. 
As early as 1885,* I had partly conjectured this, but misled by the com- 
mon idea that the backbone of the island marked the southern limit of 
the ice-sheet, I was unable to account for certain phenomena connected 
with the drift ridges and clay deposits that make up the south side. 
The study of the englacial till however, has led me to see that this up- 
per drift extended further southward than the subglacial till, as the bot- 
tom drift is called, and yet, the absence of large boulders on the south 
side of the island is difficult to account for, as they seem to end at least, 
with the marginal kames. The clay, however, covering the stratified 
deposits is continuous from the central ridge to the ocean, but thins out 
from about two feet to a few inches. 
Most writers have imagined that this frontal plain, with its ridges on 
deltas, was formed underneath the waters of the ocean when the sea 
stood over this part of the island, and that it was subsequently ele- 
vated. I do not know how many ups and downs our poor island has had 
in the hands of theoretical geologists, but there is no appearance of os- 
cillation at least, since the age of ice. 
In regard to the drift mounds of Olympia, G. O. Rogers rejects Profes- 
sor Le Conte's theory of their origin, and formulates one similar to my 
own. with this difference, that the streams that assisted in forming the 
mounds were superglacial instead of subglacial. Mr. Rogers may be 
correct, but if the Olympian mounds are in any way related to the hum- 
mocky ridges on the south side of Long Island, I would question his 
theory very much, They were doubtless formed similar to kettle hole 
depressions with their interlacing ridges, but were not the depressions 
a s w e ll as the mound ridges, formed beneath the ice? These are no 
*8ee my pamphlet on "The Geological Formation of Long Island, 1885,'' p. 8. 
