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SCIENCE 



[N. a Vol. XXXIX. No. 997 



appointed assistant professor of forestry in 

 the University of California, in the new de- 

 partment of which Walter Mulford, now pro- 

 fessor of forestry at Cornell, is next August, 

 to become the head. 



Dr. Albert N. Gilbertson has charge of 

 the instruction in anthropology at the Uni- 

 versity of Minnesota in the absence on leave 

 of Dr. A. E. Jenks. 



Dr. Oscar Perron, of Tiibingen, has been 

 called to a professorship of mathematics at 

 Heidelberg. 



DISCUSSION AND COBSESPONDENCE 



WHAT WAS THE CAUSE Or THE ESKERS ? 



To THE Editor of Science: Eakers are fea- 

 tures of the earth's surface well known to all 

 students of glacial phenomena. They are more 

 or less well defined ridges composed of mix- 

 tures of sand, gravel, clay and boulders, hav- 

 ing a direction generally parallel to that of the 

 movement of the latest ice sheet that covered 

 the region where they occur, or normal to the 

 front boundary of the sheet, and they often 

 have a length of many miles, though entire 

 continuity rarely exists throughout the length 

 of any one such ridge or series of ridges hav- 

 ing such relations as to be considered as one 

 esker. In some cases such ridges have a 

 striking uniformity in height and cross sec- 

 tion, with an abruptness of side slopes and an 

 alignment that suggest an artificial embank- 

 ment like that for a railroad or a levee. 

 Other forms that have been called eskers are 

 flattened and spread out, broken into detached 

 ridges that often depart from parallelism, and 

 these are frequently associated with knolls and 

 irregular hummocks and valleys that would 

 not be considered as related in any way to 

 esker forms if they stood by themselves. Esk- 

 ers in the United States have been described 

 and illustrated in several publications of the 

 United States Geological Survey, as well as in 

 various papers and geological text-books. 

 They are numerous and extensive in the east- 

 ern part of that portion of North America 

 that was covered by the latest ice sheet, par- 

 ticularly in Maine, New Brunswick and the 



eastern Canadian provinces. Several examples 

 on a smaller scale are found in the Great Lakes 

 region of the United States. The writer has 

 examined more especially the eskers near 

 Circleville, south of Norwalk, and near Ken- 

 ton, in Ohio; the one north of Muncie, Indi- 

 ana, the fine example near Kaneville, HI., and 

 the strikingly uniform and conspicuous esker 

 ridge at Mason in southern Michigan. Casual 

 examination has also been made of similar 

 ridges in Ontario, Canada. 



The theory to account for these ridges which 

 is most often met with is that they were 

 formed by stream action, in crevices or in 

 tunnels under the ice, during the period of re- 

 cession or withdrawal of the ice sheet. From 

 the published descriptions and views and 

 sketches of eskers and from the examinations 

 above referred to, I became satisfied that this 

 theory was untenable, although there are some 

 evidences that stream action has had a second- 

 ary and modifying effect on the final esker 

 forms in some cases. I concluded that the 

 eskers resulted primarily and principally from 

 cracks in an ice sheet of moderate thickness 

 covering approximately smooth and level areas 

 of considerable extent; these cracks becoming 

 the locus of the accumulation of the esker ma- 

 terial from the lateral " shove " of the sepa- 

 rated parts of the ice sheet imder the influence 

 of seasonal changes of temperature. This ac- 

 tion resulted in the upheaval and breaking of 

 the ice along the initial crack, and the melting 

 of the resulting broken ice at a rate greater 

 than that of the main ice sheet due to increase 

 of exposed surfaces, with the accumulation of 

 the general surface earthy material as well as 

 that imprisoned within the ice itself along a 

 more or less well-defined line. This earthy 

 material remained, of course, after the ice 

 disappeared, and it was often modified to a 

 greater or less extent by flowing water during 

 the melting of the ice. I prepared a tentative 

 memorandum setting forth this view some 

 three years or more ago, but it was not pub- 

 lished. The illustrated supplement of the New 

 York Times of November 23 contains a photo- 

 graphic view of a " pressure ridge " in a sheet 



