Glacial Geology in America, — FaircJiild. 167 
the thickness of the ice over New England and the middle 
states. Later observations have shown that the thickness of 
ice was sufBcient to bury the highest mountain peaks within 
the ice field. Explorations of the Greenland ice cap have 
yielded data concerning the curving slope of the margin of 
the ice sheet which seem in the hands of Chamberlin and 
•Upham to give a reliable basis for the comparison with ancient 
ice sheets. Air. McGee estimated the thickness in northeast- 
ern Iowa as "500 feet a few miles from the margin, to an in- 
definite but not very much greater thickness in the interior of 
the ice bod}-." 
Direction of /7fzt'.— Previous to 1879 W J AIcGee was 
able, by the study of topographic forms of the drift in north- 
eastern Iowa, to determine the direction of ice flow without 
evidence from strise. It is now recognized that drumlins and 
eskers are more trustworthy criteria of general direction of ice 
movement than the glacial strise, which are m.ore subject to 
the effect of local causes. Peripheral moraines are demon- 
strations of the direction of flow at the margin of the ice sheet. 
By these several criteria it is now demonstrated that the ice 
movement followed the trend of the larger depressions and 
valleys. As early as 1863 Dana argued, from direction of 
strise, for distinct glacial flow in the Connecticut and Hudson 
valleys. The verv decided localization of movement in the 
basins of the Laurentian lakes was shown cartograpliicallv by 
Chamberlin in 1877. 
Lobing of Margin. — This is closely related to the flow i:i 
the longitudinal valleys, and is produced bv the concentration 
or massing of the marginal ice in such depressions. This 
was also shown by Chamberlin in 1877 in his map of terminal 
moraines; those of the ice lobes filling the basins of the great 
lakes having a conspicuously looped, festooned or cresccntric 
character. Further study has emphasized the lobate form 
of the periphery of the ice sheet, at least during the recessional 
phase. 
Driftless Areas. — The surprising and apparentl\- incx])lica- 
ble phenomenon of an area unafTected bv ice action, but 
wholly surrounded by glaciated territory, was brought to no- 
tice by J. D. Whitney in 1862. Much doubt was felt regard- 
iner the nature and even the existence of this driftless area 
