■894] 365 [Grabau. 
in tlie background of the Glen Iris ampliitheatre shows them 
to be composed entirely of loose material. Although, so far as I 
know, no borings have been made, the erosion and surface con- 
foi-mation of these hills seem to furnish pretty conclusive evidence 
of the absence of rock. Climbing to the top of the hills, which 
rise over three hundred feet above the river bed beyond the first 
falls, and looking towards Castile, it is possible to trace the con- 
nection between this point and the Wyoming Valley, which proves 
to be in the same direction with the Genesee Valley south of 
Portageville. It seems, then, very evident that Glen Iris is a 
portion of the Genesee Valley cut off from it by the "Portage 
Ridge," while the Wyoming Valley is a further continuation of 
this valley cut off from it by the more extensive drift deposit 
between Glen Iris and Castile. The modern Genesee, leaving its 
preglacial valley at Portageville, passes through a short gorge cut 
in the left bank; enters this valley again some distance beyond, 
crossing it at right angles from left to right in Glen Iris ; and 
continues its way in a rocky gorge to Mt. Morris. Rock appears 
in the bed of the river below the first falls, this marking approxi- 
mately the preglacial depth of the valley, which was about sixty 
feet lower than the bed of the river at Portageville or about nine 
hundred and ninety feet above sea level ; hence the Genesee 
Valley at Portageville has been filled to a depth of about sixty 
feet. 
Dansville- Rochester Valley. — From the foregoing considera- 
tions it becomes evident, that the Genesee was not the original 
occupant of the Dansville-Rochester Valley; nor had it anything 
to do with the formation of this valley, althougli, as will appear 
later, it had considerable to do with filling it. This valley is 
entirely distinct from the Genesee Valley and owes its existence 
to another river which flowed parallel to the Genesee at a distance 
of less than twenty miles to the eastward, and which was, like 
the Genesee, ouly one of the many northward flowing, preglacial 
rivers that characterized central New York, most of which, 
becoming permanently dammed up, have left their valleys filled 
with water, forming the "Finger Lakes." Of the character of 
the Dansville-Rochester Valley, Professor Hall writes as 
follows^ : — 
"The deep depression known as the Genesee Valley extends from 
Kochester, southward, as far as Dansville. Following the same direction 
1 Kept. 4th H'eol. (iistr., 1843, p. 343-344. 
