Delta-Plain at AndovCr, Mass. — Mills. 167 
feeding esker merges into the plain. The periphery of the 
plain is sharply and distinctly preserved, in its nearly circular 
outline. Those delta plains whose outlines are so distinctly cir- 
cular, and whose feeding eskers merge uninterruptedly into the 
head of the plain, have been given the more definitive name of 
esker "fan."* The converging radii of the plain representing 
the fan, and the feeding esker the handle. The deposition of 
the plain- took place in a small amphitheatre walled in to the 
east, south, and west, by low-lying granite hills. While to the 
north, the temporary lake was held up by the ice-front itself, 
completing the basin. Along the northern border of the plain, 
or ice-contact side, the outline in detail is exceedingly cuspate, 
showing the normal irregularity of melting along the decaying 
ice- front. 
The basin was about one mile and a half in length, from the 
ice-contact phase to the opposing upland on the south, and in 
width, from east to west, approximately one mile. In the 
southern upland border of this glacial lake, is a notch-like open- 
ing. Through this notch the lake probably found its first over- 
flow. The drainage led away to the south, and eventually into 
the Atlantic. When the time arrived that the ice had entirely 
disappeared from the head of the Shawsheen valley, a reversal 
of drainage took place. Then the pent up waters would find 
an easier way of escape to the north, and could empty into the 
larger valley of the Merrimac, and the lake was rapidly 
drained. This notch in the southern upland has been greatly 
deepened by post-glacial erosion of a north flowing stream. 
At the head of the delta plain, where esker and plain unite, 
a compact, stratified deposit of gravel is found. The debris 
laden stream, issuing from the ice-front into a zone of static 
water, found its velocity at once checked. With a checking of 
velocity is found its corollary in a loss of carrying power. 
Shorn of its initial transporting power, the stream became at 
once unable to carry further its burden and must perforce de- 
posit some of its load. Such being the fact it would naturally 
drop first that which was the most difficult to handle ; namely, 
the coarser and denser material. As the velocity, or transport- 
ing power, stands in a ratio to distance from point of issuance 
from the ice-tunnel, so, we find a relative decrease in size of 
* Bull. Essex Inst., vol. xxxix, 1897. J. B. Woodwoki h. 
