1879. | The Formation of Cape Cod. 491 
over these areas, and that it moved slowly in the direction of the 
striation, ploughing up, transporting, grinding and mixing to- 
gether the materials of the till, and leaving its track and course 
clearly marked upon the bed-rock. 
Cape Cod is mainly composed of another formation, called 
modified drift, because it has been transported, worn and depos- 
ited by currents of water. Gravel, sand and clay are separated 
in distinct layers and beds, instead of being mingled in one mass. 
Most portions are entirely destitute of any large boulders, and 
the pebbles of the gravel are rounded, instead of the planed or 
else rough and unworn forms which they have in the till. The 
modified drift is found in the valleys or upon the nearly level 
tracts of glaciated regions, and follows lines of drainage which 
reach beyond these limits. Formerly it was the prevailing opin- 
ion that this formation was gradually produced from the unmodi- 
fied glacial drift by the ordinary action of rains, streams and the 
sea. Further observation, however, seems to leave no doubt that 
it originated from the ice-sheet, and was rapidly deposited during 
its retreat. This had taken up in its lower portion not only 
boulders of every size, but also great quantities of decomposed 
rock, which covered the hills and plains and had been swept into 
thick deposits in the valleys before the glacial period. The riv- 
ers formed by its melting gathered such of these materials as they 
could transport, turned the blocks and pebbles over and over in 
their descent till they were made round and smooth, and depos- 
‘ited their freight in these beds of gravel, sand and clay. The 
modified drift of South-eastern Massachusetts, here to be described, 
appears to add much to our knowledge of these conditions attend- 
ing the departure of the ice-sheet. 
Another subject of great interest is presented on Cape Cod and 
the Elizabeth islands in a series of drift-hills, which appear to 
have been formed at the margin of the ice during a pause in its 
retreat, when, perhaps after some re-advance, it halted and pre- 
Served its termination nearly stationary through a long period. 
This series consists of very irregular hills, mounds, ridges and 
enclosed hollows, and is composed partly of till, with abundant 
boulders and no marks of stratification, and partly of stratified 
gravel and sand with boulders few or rare. That it does not 
indicate the extreme limit of glacial action is shown by the 
Occurrence of a similar belt of hills on Martha’s Vineyard and 
VOL. XIII,—NO, VIIIs 34 ; ede es 
