48 TACONIC PHYSIOGRAPHY. 
.1 
and organic sedimentation ; then the formation in these stratif Jj 
sediments and their crystalline basement of a series of great para 111 
folds, diminishing in altitude from east to west, which caused I 
retreat of the sea. This folding resulted in faults, metamorphis j 
and secondary structures of several kinds. There were three peril I 
of folding — one at the close of the Lower Cambrian, affecting Ij 
central portion of the basin ; another at the close of the Ordovici J 
more far-reaching in its effects, and a third seems to have ocean f 
in post-Silurian time (Devonian or Carboniferous), as shown 
the Rensselaer Plateau and Bird Mountain. The various materi I 
thus collected, folded, altered, and traversed by structural plai | 
became exposed as great longitudinal ridges and valleys to strei 
erosion, and that erosion was retarded as it approached base-level ij 
was accelerated by uplifts. The first anticlinal ridge became, 
places, denuded of all its sediments, although these amounted 
several thousand feet, and the ancient sea floor became exposed ( j 
PI. Y,A ). The anticlinal ridges west of this were carved into vallt j 
and the svnclines remained as ridges, but in some instances 1 
original forms persisted with modifications. Erosion operate 
laterally completely across these synclinal and anticlinal mountai 
and also sculptured them on all sides into forms bearing lit 
resemblance to their structure. Eventually, completely buried unc 
the continental glacier which moved both southward and southea 
ward, the surface features became still further modified by the shin 
tering of ledges, the removal of blocks, the gouging and scouri: 
action of bowlders and gravel held in the ice. The melting of tl 
vast body of ice gave rise to streams which freshly eroded the surf a 
scattered morainal material, and formed glacial lakes. 
The Taconic landscape is thus the result of erosion acting up 
rock material of various composition and structure. 
The amount of this erosion depended on the relative solubility a 
hardness of the material. The resulting forms depended largely 
the direction and pitch of the axes of the folds, their anticlinal 
synclinal form, faulting, jointing, and cleavage. Some of these h 
tors, however, must have counteracted others ; hardness may have be 
rendered of no avail by details of structure, while erodibility 
physical processes in one rock may have been balanced by greal 
solubility in another. Some of the schist masses appear from tin 
general outlines to have behaved as if largely homogeneous in stri 
ture and composition. 
The chief agents of erosion were the mechanical action of streai : 
through the material they transported or rotated, of frost operatij 
in fissures due to structure, of ice operating through bowlders a 
gravel, of roots growing into structural openings and acting 
