398 
James D. Thompson, Jr. 
Origin. There are two ice invasions with one of which the 
formation of the Locust Grove Esker must be associated: the 
Illinoian and the Wisconsin. Drift from both these invasions 
occupies the region south of Newark/ We find that the drift of 
the Wisconsin invasion extends out over the Illinoian drift in this 
area, and in some places we find them intermingled. 
The drift of any particular region is always characterised by 
local material. In some cases it is difficult to determine to which 
of these two invasions certain drift materials should be attributed. 
Sometimes we note the Illinoian drift overlain by early Wisconsin 
drift, which in turn is overlain by later Wisconsin drift. 
The till of the Wisconsin invasion is generally composed of 
yellow gravelly clay, somewhat loose in texture. The later Wis- 
consin drift is practically unweathered, while that of the earlier 
periods is more or less weathered and rusted to some depth. 
The till of the Illinoian period is also a gravelly clay. Usually 
where the till is 20 feet or less in thickness, it is yellow in color 
and shows evidences of prolonged weathering; where thicker 
than 20 feet the lower part of the Illinoian is generally a bluish 
gravelly clay, more or less metamorphosed. 
Conclusion. The Locust Grove Esker is evidently not of Illi- 
' noian origin. The till immediately beneath it, however, is too 
highly weathered to be of the late Wisconsin period, and we may 
be justified in concluding that the esker is of early Wisconsin 
origin. 
^ Lcverett, Monograph XLI, U. S. Geol. Survey, 1902, pi. XIII. 
