38 (Post=Glacial Geology of Ann Arbor — Wooldridge. 
Above these beds of gravel is a bed of horizontally stratified 
material, here about ten feet thick, composed of mingled clay, 
sand and gravel. No stones larger than a hen's egg appear in 
any of these beds, I was inclined to attribute this surface 
stratum to the wash from land-slides which had fallen from the 
clay hill to the north, but on further examination I find that a 
surface stratum containing much clay and larger pebbles some- 
times reaching the rank of bowlders, is spread quite generally 
over the Ann Arbor gi-avels. Some change of conditions must 
have attended the deposition of this surface stratum to produce 
this change of its character, but just what that change may 
have been I am not prepared to suggest. Elsewhere I have 
seen a similar condition of the surface stratum in yet more 
marked contrast to the sedimentary beds on which it rested, 
and 1 think that the phenomenon is general enough to be de- 
serving of special study. I will sum up the results of my 
studies of the Ann Arbor gravels by saying that I have come 
to see in them a well marked example of an ancient delta. 
When these beds were deposited lake Erie extended over the 
site of Ann Arbor, which at that time was a bay near its west- 
ern extremity. Into this bay the Huron river poured a turbid 
torrent from the, then naked, hills of bowlder clay to the north- 
westward, dropping its gravel as soon as it entered the waters 
of the lake, while its finer sediments were carried farther. 
This gravel and sediment formed the delta, now the hill; on 
which Ann Arbor stands. When the water subsided the river 
formed a new channel to the northward of an island, the north- 
western extremity of which is now the observatory hill, and 
soon cut a gorge in the loose drift materials from which ravines 
extended on either side to shape the hills which now border its 
valley. There may very likely have been a shallow body of 
water retained in the old bay south of the Ann Arbor delta, 
and one of these side ravines making connection with that 
would drain that water into the river and thus cut the valley 
through which flows the brook in the west side of the town. 
I have looked for traces of the old shore line corresponding to 
the level of the lake when this delta was formed. At a few 
points along the hills which formed the southwestern border of 
the bay at that time are what may be traces of shore terraces 
