LInmestone Belts of Westchester County, New York. 29 
ordinary facts. The underdipping side of the limestone area is 
the steep side of the valley, because of the undermining whic 
the position of the limestone stratum favored ; and, for the same 
reason, the river channel (7) is made to hug the same side, and 
leave the other for wide strips of marshy land, with sometimes 
ond-like broadenings of the stream. This point was observe 
y Percival in his survey of Western Connecticut and the 
adjoining portions of New York State. 
But the pitch of the beds has not been the only cause of this 
form of the valleys. The throw of the waters against the 
right bank of a stream (the western if flowing south, or north- 
ern if flowing west), in consequence of the earth’s rotation, 
must have had its effects, and may account for the cases in 
which the western side is the steep one, notwithstanding a ver- 
tical or even a high eastern pitch. During the progress of the 
Glacial era, the subglacial streams would have felt this throw 
and worked in accordance with it; and afterward, when the 
Glacial flood, from the melting, was at its height, the rushing 
waters would have swept the earth away from the same side, 
and transferred much of it to the opposite. 
Not unfrequently the profile of a valley and its marsh at 
bottom are, for long distances, all the evidence there is in sight 
to suggest the presence of limestone beneath; and hence come 
uncertainties as to the true limits of a limestone belt, which only 
deep diggings through the alluvium or stratified drift of the 
valley can remove. 
The mapping of the limestone areas has other difficulties in 
consequence of the frequent intercalations of beds of mica 
has come out to view through denudation. If the fold is one 
having a horizontal Axis, the denudation, if alike in all parts, 
would reduce it to two limestone bands and one of schist inter- 
mediate ; but if its axis is inclined—the common fact—the 
two limestone bands may be separated by schist only for part 
of its length. This point is illustrated by several of the belts, 
and is explained in connection. with the description of belt 
No. 1 beyond, and further exhibited on the map of Westchester 
limestone areas, accompanying this memoir. 
