EXCAVATIONS. 
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CHAPTER IX. 
Embracing excavations or denudations , valleys; lakes; scratches upon rocks which 
have been smoothed; and mounds. 
Excavations, Valleys. 
No part of the Union presents such obvious and extensive excavations as New-York, and 
it is owing to this circumstance that such facilities are offered for the study of its rock masses. 
Reversing the order in which the excavations or denudations have been made as to time, the 
first, being the lowest depression of the district, is Lake Ontario. The accordance of the 
rocks on both its sides, and the dip of the rocks, are altogether favorable to its being a lake 
of excavation. No other cause could have given rise to it, but the sinking of the mass which 
forms its area; a supposition which is not in harmony with the nature of its outlet, and dip 
of its rocks on all its sides; the parts of the whole being in perfect conformity to each other, 
and in accordance with a lake of excavation. 
The next and more extensive excavation or denudation, is the Mohawk valley and the 
Great level; these comprehend all that part which extends north from the Helderberg range, 
to where it thinned out, or originally terminated, and which is as yet wholly unknown. As 
the Marcellus shales and the Hamilton group rise in hills, and in some points quite near the 
edge where the range commences, it is certain that from the top of the range to the bottom of 
the valley of the Mohawk river, including the Great level, was not the whole height from 
where the excavation commenced, but that it ascended yet higher. 
The third and last series of excavations, and the highest and first that were made, are the 
north and south parallel valleys and their lakes to the south of the Helderberg range; they 
are coextensive with the district, and disposed over its surface at intervals of a few miles. 
The assertion that these valleys were first excavated, is founded upon the fact, that in 
every one of them, and over their hills, the larger portion of their rolled stones are of northern 
origin, consisting of primary rock, grey and red sandstone; the latter sometimes showing its 
Fucoides harlani, and amongst them occasionally some of the harder varieties of Pulaski 
sandstone with its peculiar fossils. These stones are in such prodigious number, that their 
existence where seen can only satisfactorily be accounted for by the extension of the rocks 
north, which, by the dip of the rocks, would gradually bring them upon the same plane; just 
