224 
miles below Main street, Richmond, and from that point to its mouth the 
valley is generally broad. 
This gorge was formed as a direct result of the glacial phenomena of 
the region. There is evidence that at the close of the Early Wisconsin ice 
sheet period, the river occupied a channel much to the eastward of its 
present course. Wells to the south of Glen Park, nearly three miles east 
of the present river channel indicate at that point an old channel now filled 
with drift. Whe streams in Glen Park seem to occupy this same old chan- 
nel. From this and other evidence it seems probable that this old channel 
passes to the east of the city of Richinond, and connects with the present 
river valley somewhere below Test’s Mills. 
The advance of the Late Wisconsin ice sheet resulted in filling up this 
old channel with drift. There is evidence that this ice advance was from 
two directions, north-west and north-east, and that the terminal moraines 
of the two lobes diG not come together. The river, forced out of its old 
channel, took up a new course between the two moraines. With the melt- 
ing of the ice sheet, the volume of water dis ‘»rged by the stream would 
be very large, and erosion of its channel correspondingly rapid. Since the 
retreat of the ice, it has carved the present gorge. 
The rock of the gorge is Hudson River or Cincinnati limestone. This 
is a favorite collecting ground for paleontologists interested in this par- 
ticular portion of the Lower Silurian beds. Trilobites are not numerous, 
though several species are found. Calymene senaria is commonest. Ryn- 
chotrema capax, Zygospira modesta, Platystrophia biforata and Leptzna 
rhomboidalis are the characteristic brachiopods. Streptolasma is extremely 
common. 
The character of the rock is of extreme importance in the considera- 
tion of the ecology of the region. The rock is soft and very thin-bedded, 
and is rendered still more unstable by the alternation of thin beds of 
shale of a soft calcareous nature with the layers of limestone. The lime- 
stone itself is shaly and weathers very rapidly. Three inches is probably 
the average thickness of the rock layers. ‘The amount of shale varies 
greatly, even within limited areas. In general, the shale makes up about 
one third of the total rock. 
As a result of the nature of the rock, steep cliffs are maintained only 
where active erosion of the base is going cn. As soon as river erosion 
ceases, the slope becomes gentler at once. There is a considerable amount 
