36 ^ost==Glacial Geology of Ann Arbor — Wooldridge. 
built, is marked by a declivity from the foot of which spreads a 
flat valley broadening toward the southeast, in which direction 
it extends to lake Erie. The neighboring hills and uplands 
east, west, north, and across the flat to the southward, are com- 
posed of the boulder clay with here and there a bank of gravel 
or sand, due to its local erosion, but all over the city within the 
boundaries defined every excavation shows stratified gravel or 
sand, a deposit so different from anything else in the vicinity, 
and so extensive that it could not be due to any merely local or 
accidental causes. 
In the summer of 1SS7 I began an examniation of these de- 
posits. Two deep gravel pits in different parts of the city 
having at that time freshly excavated faces, offered good sections 
for study. The first of these examined is excavated in the bluff 
facing the river valley two blocks east of the Michigan Central 
R. R. depot. This section faces the north but bends around 
on either side so as to face the w^est and the east. Its lower 
part through a depth of ten or twelve feet exjDosed above the 
talus is composed of a fine, nearly vniiform and clean gravel, of 
pebbles averaging about the size of jDeas and beans. This is 
arranged in distinct beds of varying thickness, nearly horizontal 
in position but terminating in thin edges, overlapping each 
■other. Each of these beds, however, is obliquely laminated as 
if it were built out from a shore by successive accretions de- 
posited on its border, the pitch of that border being shown by 
these laminations. What surprised me was that the dip of 
these laminations is uniformly toward the south, that is toward 
the hill on which Ann Arbor is built, as if in that direction had 
been open water, while to the northward where now the valley 
of the Huron river is excavated to a depth of 50 or 60 feet be- 
low the level of these beds had been the shore. 
Above these gravel beds is a stratum of bowlders ranging 
from pebbles the size of a hen's egg to blocks that would weigh 
a quarter of a ton, with barely enough of finer materials, gravel, 
sand, and clay, to fill the interstices. This bed is a distinct 
stratum five or six feet thick deposited in conformity with the 
gravel beds beneath it, and covered by the surface soil. Rest- 
ing on the beds of finer gravel as it does, it seemed a strange 
deposit, for it is obvious that any current powerful enough to 
