NEW YORK STATE MUSEUM 
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stone. In southeastern New York (for example, the Shawangunk 
mountain) the first Siluric deposit to be laid down upon the eroded 
Ordovicic shales was the Shawangunk conglomerate. This latter 
formation, as determined by its fossils, belongs with the Salina divi- 
sion and is therefore much younger than the Oneida conglomerate 
which belongs with the Medina division (see table in chapter 1). 
Also the whole of the Clinton and Niagara formations, which are 
so well developed in central and western New York, were never 
formed in eastern or southeastern New York. Thus the Siluric sea, 
due to subsidence of the land, overspread central and western New 
York long before it reached the Hudson valley region. In fact it 
was not until late in the period that the sea encroached upon the 
Hudson valley area, and then it did not occupy all that area because 
the shore of the Siluric sea extended only as far east as the western 
slope of the Taconics. Western New York, during the late Siluric, 
was a more or less cut off basin or arm of the sea in which the salt 
beds were deposited. ‘ 
How much, if any, of the Adirondack region was covered by the 
Siluric sea? The total absence of any formation later than the 
Ordovicic shales around the northern Adirondacks and across the 
line in Canada strongly suggests that this region was upraised 
toward the close of the Ordovicic period, perhaps at the same time 
as the Taconic revolution, and continued as dry land not only 
during the Siluric but also during all the ages up to the present, 
except for a very brief local submergence during the Quaternary 
(see figure 34). In the southern Adirondack area the case is some- 
what different. The outcrops of Siluric strata beneath the steep 
front of the Devonic Helderberg escarpment immediately south of 
the Adirondacks, makes it certain that these strata and the Siluric 
sea formerly extended farther north. The difficulty comes in trying 
to decide how far northward these rocks once extended, because 
there is now not a single scrap of Siluric rocks north of the 
Mohawk river, though the cap rock (Oswego sandstone) of the 
Tug Hill plateau is probably of Siluric age. All we can .say is 
that the Silurian sea probably overspread the southern border of 
the Adirondacks and that the sediments which were deposited there 
have since been removed by erosion. To summarize: During the 
early Siluric the sea had spread over only central and western New 
York, while during the late Siluric it had extended over practically 
all the State west and south of the Adirondack region. 
The strata of Siluric age were deposited sheet upon sheet in the 
ustial manner upon the sea bottom. For our purpose we may con- 
