(Posl=GIacial Geology of Ann Arbor — Wooldridge. 37 
have brought these bowlders here must have carried the gravel 
on which they rest. I could only account for their presence 
by supposing them the residue of some mass of bowlder clay 
which towered above the water close by, and had fallen upon 
these gravel beds as a land-slide, the finer materials of which 
the waves had carried away. A little farther w^est, where the 
western approach to the Michigan Central depot has recently 
been excavated, the cutting is made in bowlder clay which 
might be the stump of such a mass as could produce a bowlder 
bed in the way supposed ; this clay seems to rise from beneath 
the gravel nearly to the bight of the beds which we have ex- 
amined. 
The other gravel pit available for study is in the north- 
western part of the city. Here the principal face of the exca- 
vation fronts the southeast while its southern extremity bends 
round to face the northeast. This southern part of the cutting 
is excavated beneath an older gravel pit, and here deeper beds 
are exposed than elsewhere. The northern part of the cutting 
shows a section extending from the natural surface down to the 
beds exposed farther south. The total depth exhibited I esti- 
mated at 35 feet. These deeper strata are composed of sand, 
thin bedded, the layers succeeding each other at intervals of 
less than half an inch, having a very perceptible dip to the 
southward, and sprinkled thinly with pebbles ranging from the 
size of a butternut to that of the sand itself. These strata have 
the appearance of beds deposited on the slope of a lake bottom 
below the action of ordinary waves down which a few pebbles 
have slid from a higher level. Some of these beds have been 
cut away on the south side and refilled with similar beds having 
the same general slope as before. The junction between these 
sand beds and the gravel overlying them was hidden by a talus 
at the foot of the cutting in the northern part of this excava- 
tion, beneath which these sand beds enter with a rising slope 
of 10° or 12° toward the north. 
Above this talus is a series of thicker beds of gravel similar 
to those examined in the other excavation. These beds are 
nearly horizontal and show oblique laminations dipping toward 
the south as before, but at this point a broad valley opens to the 
southward, while a short distance to the north rises the highest 
hill in the vicinity of Ann Arbor. 
