River Valley Region
The Hudson River flows southward from headwater areas in the Adirondacks
region to Troy, New York, where it approaches sea level and becomes an
estuary (becoming brackish and under the influence of tides). The Hudson
River flows southward through the northern extension of the Great Valley,
a lowlands region underlain mostly by Lower and Middle Paleozoic shales
and carbonate rocks, with a surficial cover of Quaternary glacial and
alluvial deposits. Just south of Newburgh, New York, the Hudson River
crosses into the crystalline rocks of Highlands Province. The rocks beneath
the Great Valley are gently to steeply dipping sedimentary rocks that
locally display northward-trending faults, evidence of tectonism associated
with the Taconic and Acadian Orogenies. Outcrops along the New York Thruway
(1-87) provide many tantalizing views of folded Paleozoic formation which,
unfortunately like all interstate road cuts, are off limits for casual
examination. Even along lesser highways in the region sites that have
been host to field study in the past are now posted to keep people away.
However, with a collection of maps, articles, and guidebooks (many are
listed in the references section),
it is still possible to find ample places to safely study the geology.
Figure 55 is a map of the central Hudson Valley region.
|Figure 55. Map of the Catskill Mountains and central Hudson River
A generalized east-to-west cross section across the Hudson River Valley
in the Kingston serves to illustrate the character of the regions geology
(Figure 56). To the east the Hudson River the land steadily rises to the
highlands of the Taconic Mountains, a region noted for its great décollements
(allochthonous masses of rock associated with great thrusts faults formed
during the Taconic Orogeny, and later reactivated during the Acadian Orogeny).
The lowlands in the Hudson Valley region between Newburgh and Kingston
are underlain by folded clastic rocks, mostly shale, of Ordovician age
(Normanskill and Austin Glen Formations). Along the western side of the
Great Valley are escarpments created by resistant, gently dipping Silurian
and Devonian strata. To the south and west of Kingston, the Shawangunk
Conglomerate appears and gradually increases in to approaching 1,700 feet,
its maximum thickness, in the Swawangunk Mountains west of New Paltz.
This ridge-forming unit continues southwestward into New Jersey where
it forms the ridge crest of Kittatinny Mountain along the western margin
of the Great Valley. Above the conglomerate, the stratigraphic succession
of younger Silurian and Devonian strata is dominated by shale, limestone,
and dolomite formed from sediments deposited in migrating depositional
environments associated with a shallow inland seaway. Route 209 between
Kingston and Port Jervis follows the general trend of a valley where this
belt of strata crops out. It dips gently to steeply to the northwest,
creation a saddle between the escarpments of the more resistant, coarser
clastic Silurian strata and the younger coarser clastic strata of the
Late Devonian Catskill Group. (A "group" includes two or more
stratigraphic formations with significant features in common.) In between
are the variable stratigraphic sequences of the Early Devonian Helderberg
Group, and the Middle Devonian Tristates and Hamilton Groups (see Figure
54). The massive limestones within the Helderberg Group are responsible
for the scenic high cliffs of the Heldeberg Escarpment in James Boyd Thatcher
State Park (about a dozen miles west of Albany).
|Figure 56. Generalized east-to-west cross section through the central
Hudson Valley region.
The Catskill Group represents a great accumulation of clastic sedimentary
material along the western margin of the Acadian Highlands, the ancient
upland region that encompassed most of New England as a result of regional
uplift during the Acadian Orogeny. From the Hudson River Valley region
the Catskill sequence grows progressively thicker to the west to its maximum
thickness of over 7,000 feet along the Pennsylvania border. South and
west of the Scranton, Pennsylvania area the Catskill sequences grows progressively
thinner. During the Late Paleozoic the sequence was possibly more than
twice its current thickness, and the deposits probably extended far north
and east into portions of New England. This overburden has subsequently
been removed by erosion.