22 NEW YORK STATE MUSEUM 
county, reaches an altitude of nearly 2100 feet, while the central 
portion of the province, covering many square miles, is remarkably 
flat and swampy with the general level above 1800 feet of elevation. 
On a smaller scale, this is as truly a plateau as the great South- 
western plateau already described and, interesting to note, this 
Tug Hill plateau is merely an erosion remnant of the great up- 
raised Cretacic peneplain (see chapter 5) which formerly included 
all of New York State. 
On the south and west this province slopes rapidly downward 
to the lowlands of the Mohawk valley and Ontario plain provinces, 
while on the east and north the Black river valley sharply separates 
this province from the Adirondack and St Lawrence Valley prov- 
inces. The rapid descent into the Black river valley bottom is 
everywhere 1000 feet or more over a series of high, steep terrace 
fronts (see plate 8). In passing, it should be stated that, though 
seldom recognized, this Black river depression takes rank as one 
of the few greatest valleys within the borders of the State. 
Near Boonville, and at an elevation of about 1100 feet, occurs 
the division of drainage between the Mohawk and Black rivers, 
and this divide forms the highest land connecting the Tug Hill and 
Adirondack provinces. But in spite of this partial connection and 
the close proximity of the province to the Adirondacks, the rock 
formations and structure are wholly different from those of the 
Adirondacks while they greatly resemble those of the Southwestern 
plateau. The rocks are all of lower Paleozoic (chiefly Ordovicic) 
age, with several hundred feet of limestone at the base followed by 
about a thousand feet of shales, the whole being capped by a 
resistant sandstone of Siluric age. These strata tilt slightly west- 
ward but they have never been disturbed by folding, faulting or 
igneous activity (see figure 35). 
ST LAWRENCE VALLEY PROVINCE 
The St Lawrence valley, lying along the northern boundary of 
the State, is a great, open depression of comparatively simple struc- 
ture and near sea level. Where the river leaves Lake Ontario, the 
elevation is only 247 feet, while points with elevations more than 
a few hundred feet seldom oceur within the province. As shown 
on the accompanying map (plate 9), low hills are common over 
the valley floor. The Thousand Islands form a remarkable physio- 
graphic feature of the province, where the wide, slow-moving St 
Lawrence river does not occupy any very distinct channel, but 
