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 eastT\'ard 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. The streams in Glen Park seem to occupj' 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 Richmond, 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 did not come together. The river, forced out of its old 

 chamiel, took up a new course between the two moraines. With the melt- 

 ing of the ice sheet, the volume of water discharged by the stream would 

 be very large, and erosion of its channel correspondingly rapid. Since the 

 retreat of the ice, it has car^-ed 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 Lov.-er Silurian beds. Trilobites are not numerous, 

 though several species ai*e found. Calymene senaria is commonest. Ryn- 

 chotrema eapax, Zygospira modesta, Platystrophia biforata and Leptsena 

 rhomboidalis are the characteristic brachiopods. Streptolasma is exti-emely 

 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 ro€k 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 on. As soon as river erosion 

 ceases, the slope becomes gentler at once. There is a considerable amount 



