STRATIGRAPHY AND STRUCTURE CASTLETON AREA VERMONT

Although some of the rocks referred to above are poorly dated faunally
and some are dated only by inference, the stratigraphic evidence is here
accepted as establishing the existence of the Taconic overthrust.
Facies evidence: As has been emphasized repeatedly, the rocks of the
Taconic sequence are of a predominantly argillaceous facies and are
surrounded on all sides by carbonate rocks of the Valley sequence. Both
sequences range in age from Lower Cambrian to Middle Ordovician. If
the two sequences now lie approximately where they were deposited,
it is difficult to explain how shales and limestones could have been laid
down, as it were, side by side. Theories of land barriers separating discrete
depositional basins which are alternately submerged and uplifted,
e.g., that of Ruedemann (1930), are on the whole unconvincing. From
the presence of two dissimilar facies in close association we may more
reasonably infer that one has been overthrust into contact with the other.
The overthrust hypothesis: Taken together, the lithologic, structural,
stratigraphic, and facies evidence favors only the overthrust hypothesis.
The fault plane, however, is not easily located in the area investigated.
Crushing, gouge, slickensides, silicified zones, and other common indications
of faulting are completely wanting. In Ira, where impure banded
quartzite lies at the base of the thrust sheet, the fault has topographic
expression in fairly continuous 25- to 50-foot cliffs. Elsewhere it is concealed
in the phyllites of the eastern slope of the Taconic Range. The
fault was placed between the outcrops of the heterogeneous strata of the
Taconic sequence and the outcrops of the fairly homogeneous Hortonyule
slate. The actual fault plane was nowhere observed.
On the western side of the Taconic sequence nobody doubts the existence
of overthrusting. Kaiser (1945, P1. 1) mapped fault breccia along
the Taconic overthrust at places in the Whitehall quadrangle. Ever
since Walcott (1888) demonstrated the Bald Mountain thrust at Schuylerville,
New York (Figure 1), geologists in the Hudson Valley region
have shown on their areal geological maps (Cushing and Ruedemann,
1914; Ruedemann, 1930, 1942a; Goldring, 1943) one or several western
border faults between the Taconic sequence and the autochthonous
rocks. The partisans of the Taconic overthrust have merely extended the
western zone of faulting entirely around the Taconic Mountains, thereby
making the whole mass a thrust outlier.
Suggestive evidence of the presence of the Taconic overthrust comes
from the area north of the Taconic Range. It is supposed that before
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