Davis.] 
320 
[Nov. 18, 
Within the lowland valley of the Hudson, the river has cut a 
trench, T, a mile or more wide, and of unknown depth below pres- 
ent tide- water. The side streams have followed their leader to the 
best of their ability, but their depth of cutting is naturally less than 
that reached by the large river. The trenches are for the most part 
of later date than the origin of the valley lowland in which they are 
sunk ; they are therefore of late Tertiary or of post-Tertiary bo* 
ginning. The Hudson valley should then be regarded as a valley 
lowland, excavated during Tertiary time, roughly speaking, in an 
uplifted lowland of Jurassic-Cretaceous denudation, and some- 
what dissected by river and stream trenches of moderate depth 
and of post-Tertiary date. 
The river and stream trenches, as well as the general valley 
lowland and the enclosing remnants of the ancient plateau are all 
glaciated. Some of the depth of the trenches should be ascribed 
to glacial erosion, especially when the trend is to the southward, 
as is the case with the trench worn on the monoclinal outcrop, M, 
of the weak Marcellus shales, west of the Hudson, between the 
bluffs of the hard Hamilton sandstones, Hm, and the hills formed 
on the anticlines of the firm Helderberg limestones, Hg. The 
general absence of till except in relatively thin veneers on the hill 
slopes, is clearly indicative of an ability on the part of the great 
Hudson ice-current to drag away the detritus that it brought 
from further north and to gather more waste from the ledges of 
the lowland down which it here flowed. Over many of the west- 
ern states, the preglacial rock topography is lost in the shroud of 
glacial drift which now determines the form of the surface ; in 
New England, alternation of rocky ledges and accumulations of 
drift — sometimes forming drumlins — is the rule ; but in the mid- 
dle Hudson valley, the drift is so scanty that the features of the 
surface are nearly as closely dependent on the rock structure as in 
the non-glaciated states. It is for this reason that the “Little 
Mountains”, as I have called them, formed by the erosion of the 
corrugated limestones and shales of the Helderberg series, Hg, 
west of Catskill, have been for several years past selected as one of 
the stopping places for a week’s work by the Harvard Summer 
School of Geology 1 . The Hudson valley indeed appears to have 
l See The Little Mountains east of the Catskills. Appalachia, iii, 1882, 20-83. — Th e 
folded Helderberg limestones east of the Catskills. Bull. Museum Comp. Zoology, 
Geol. Series, i, 1883, 311-329. 
