444 Ly. H. Mudge—Central Michigan and the 
seems to me to prove conclusively that the agent of excavation 
was a powerful westerly-flowing stream. 
On section 23 of Ionia township (T. 7 N., R. 6 W.), the 
excavation of the valley has uncovered a ledge of Carboniferous 
sandstone, the only rock exposure along this line between 
Grand Rapids and the Saginaw valley. It is, perhaps, a por- 
tion of the cliff overlooking the ancient Hurorian valley. At 
its eastern or up-stream edge the rock rises abruptly to the 
height of four or five feet above the adjacent silt bottom lands. 
Over a considerable area, perhaps 40 acres, the level surface of 
the rock is exposed, or covered with only a few inches of sand, 
with occasional granite bowlders up to two feet in diameter 
scattered about. The down-stream extension of the surface is 
hidden by a gradually thickening mass of sand and gravel, and 
it is to the form and position of this mass that I wish to eall 
special attention. From the exposed rock surface it rises very 
gently to the west, or down stream, for a distance of more than 
a mile, terminating with a rounded front sloping down to the 
valley bottom. Its lateral diameter is perhaps one-half a mile. 
The river flows along the north side, with 40 rods of silt land 
intervening, while that part of the valley to the south is occu- 
pied by a long, narrow strip of swamp land, with black vege- 
table mould several feet in depth. This elevated tract, near 
its western end, attains an altitude of about 20 feet above the 
flats. A railroad cut ten feet in depth at this point shows it to 
be composed almost entirely of coarse, unstratified gravel. 
The surface is exceedingly stony, and many large bowlders are 
found along the northern margin. 
The presence and position of this elevated tract within the 
river valley possesses great significance. It can scarcely be 
doubted that the river, in the process of lowering its valley, 
denuded the rock above mentioned, which formed an obstruce- 
tion in the way of further excavation of the middle of the 
_valley. The force of the stream being thus broken and 
deflected to either side, this long elevated strip was left stretch- 
ing down the valley from the rock barrier, and received its 
evenly rounded contour from the influence of the overflowing 
waters. Only a large and powerful stream could have pro- 
duced these conditions, and it is equally plain that this stream 
flowed to the west. 
The problem is to harmonize these deductions with the fact 
of a general submergence of this region during the close of 
the glacial epoch. What and where was the source of supply 
of the powerful current that shaped and moulded the central 
valley? If no such submergence ever existed, the theory of a 
glacial ‘origin satisties all the conditions most perfectly. But 
if at the close of glacial time this surface was depressed below 
