14 The Glacial Origin of Cliff's— Davis. 
THE GLACIAL ORIGIN OF CLIFFS. 
By W. M. Davis. 
The trap-ridges in the Triassic areas of Massachusetts, Con- 
necticut, New Jersey and Pennsylvania are much alike in the 
chief features of their physical history from their formation 
down to the glacial period; but when the ice came the ridges 
south of the moraine escaped the heavy rubbing that the more 
northern ones suffered, and it appears that a consequence of 
this recent difierence sti'l remains in the contrast between the 
generally bold front and steep, rocky talus of the glaciated 
ridges and the more rounded, tree-covered slopes of the others. 
As far as my observation goes in the four states named above, 
the contrast is distinct. It is particularly well marked between 
the ridges of Massachusetts and Connecticut and those of middle 
New Jersey and eastern Pennsylvania, About Meriden, Con- 
necticut, for example, the faces of the cliffs are high and almost 
or quite vertical, and the talus slopes below them are often bare 
of soil and support no plants, save lichens, for hundreds of feet 
together: the sharp-edged stones lie loosely at the angle of 
maximum slope and move easily under a climber's foot. The 
protection against sliding afforded by au occasional small tree 
is indicated in the grayer color just below it, caused by greater 
growth of lichens. On the other hand, in New Jersy, at the 
southern end of the Watchung mountains, in the neighbor- 
hood of Bound Brook, the ridge is heavily timbered, and while the 
outcrop cliff and its talus are indeed distinctly perceptible, they 
are both of gentle expression. It seems as if this difference 
may be because the talus of the northern ridges has not yet in 
postglacial time reached the normal condition of balance be- 
tween supply and loss, that must have obta ned there in pre- 
glacial time and that still prevails in the nonglaciated area. 
The strong cliffs and steep talus slopes of the northern ridges 
can hardly be ascribed to any peculiarity of their preglacial 
history. Like the southern ridges, they are the outcroj)ping 
edges of sheets of dense trap Ijing in most cases conformably 
between beds of much softer sandstones and shales. Since they 
were given their present monoclinal attitude, they have been 
reduced by pre-Cretaceous and Cretaceous erosion nearly to a 
base-level surface. Shown in vertical cross-section as bl, fig. 1; 
