STRATIGRAPHY AND STRUCTURE CASTLETON AREA VERMONT

was first recognized by Wolff (1891) at Pine Hill in Rutland and Proctor
and is named from that locality. As Dale (1894) brought out, the conspicuous
ridge between the Green Mountain front and the Taconic
Range from Pittsford to Danby is a consequence of this fault. The resistant
Cheshire quartzite and Mendon series hold up a line of hills
which in this quadrangle include Pine Hill and Boardman Hill and the
hills between Chippenhook and the Otter Creek. The Rutland valley is
developed in the Lower Cambrian dolomites that overlie the Cheshire
in the Pine Hill thrust sheet. According to Dale, the total airline distance
along which the fault extends does not exceed 18 miles.
Mapping of the thrust was not always easy because of the similarity
of the Hortonville slate that generally underlies the fault to black
phyllites of the upper part of the Mendon series that belong to the
overthrust plate. The Cheshire quartzite is not generally found at the
base of the thrust sheet. A reason for this may be that slippage between
the two graphite-bearing phyllites was most easily accomplished.
The dip of the thrust plane is unknown, although Dale and Bain conceived
of a fairly steep eastward inclination. Bain (1938, fig. 12) showed
the thrust plane as repeatedly folded and in some places thrust on itself.
Deformation at the base of the thrust plate is well exposed at the crest
of Boardman Hill. Here the upper Mendon is a gneiss, greatly contorted
and shattered and with a conspicuous development of boudinage. Quartz
veins are abundant. The axial planes of drag folds in the gneiss dip at
low angles to the east and may be roughly parallel to the fault plane here.
The stratigraphic throw is as great as 10,000 feet where the Mendon
series lies on Hortonville slate and as small as a few hundred feet where
Mendon rests on Cheshire. It is evident from a glance at the geologic
map that the Pine Hill thrust truncates the east limbs of the Chippenhook
and Pittsford anticlines in a similar fashion. The fault is genetically
a break thrust. Compressive stress formed fractures across the east limbs
of the two folds and pushed the dislodged sheet westward. A probable
klippe of Cheshire lying on Hortonville west of Center Rutland was first
located by W. M. Cady. The minimum net-slip along this thrust is 2
or 3 miles, but without data on the true dip of the fault no maximum
figure can be given.
Minor faulting: There can be little doubt that some small-scale
thrusting has taken place within the Taconic sequence, but in no place
can it be proved conclusively. Certainly no proof of the sort of thrusting
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