;JH6 The American GeoUxjist. December, 1897 
till, whether hf)tli the <-'lay beds and till may be referable to 
nearly (ionteinporaneous deposition from the waning ice-sheet, 
like the modified drift found elsewhere in and beneath drum- 
lins, and like the clay beds on the hills surrounding lake Win- 
nipesaukee. This Interpretation of the relations of the brick 
claj^s and overlying till, the latter in part amassed as drum- 
linp, seems to me better in accord with other features of the 
glacial geology of eastern Massachusetts than the view pre- 
sented in their paper (with the joint authorshij) of Prof. 
Shaler), namel}'', that the stratified clays were overridden by 
a readvance of the ice-sheet from the vicinity of Boston to its 
former limits on Martha's Vineyard and Nantucket.* 
This stratilied cla}^ formation, extensively used for brick- 
making beside the Mystic river estL'ar3\ is chiefly free from 
an}^ covering of till and boulders, which would be expected to 
extend continuously over it as a mantle if the ice-sheet read- 
vanced across this area after the clay beds were dejjosited. 
Therefore, instead of the interglacial origin suggested by these 
authors, the brick clay seems to me to be a part, like the clay 
beds in the Connecticut and Merrimack river valleys, of the 
large amount of modified drift which was deposited progres- 
sively while the ice-sheet was retreating, earliest southward 
and later northward, during the Champlain or closing epoch 
of the Glacial period. The upper limit of these clays, gener- 
ally 10 to 20 feet above the present mean tide sea level, is 
])robabl3^ the measure of the maximum Champlain depression 
of this part of our coast. Being deposited in the somewhat 
enlarged Mystic estuary close to the receding ice border and 
perhaps in part under the edge of the ice, as in the vicinity of 
lake Winnipesaukee, the stratified clays may in some locali- 
ties of small extent have become covered by tiie contempora- 
neous till formed likewise under the edge of the ice-fields, 
whether spread somewhat evenly or heaped in drumlins. 
Hence it seems more likely that the ])rick clays reach only 
slightly under the edge of the drumlins and other ascending 
slopes of till than that they continue under the entire drum- 
lin masses, as shown by Marbut in his diagrammatic sections. 
*U. S. Geo!. Survey, Seventeenth Annual Report, for 1895 9G, pp. 951- 
lOOi, with maps and sections (noticed in the Am. Geologist of last 
month, p. .328). 
