3. Rocks were tilted during the late Ordovician Taconic Disturbance and
the younger rocks and part of the Glens Falls Limestone were removed by
erosion. Erosion continued for some 350 million years.
4. During the Pleistocene Epoch, which started some 1 million years ago,
glacial ice overrode the beveled layers of the Glens Falls Limestone.
Hard rocks frozen to the underside of the glacial ice produced scratches
or striations in the exposed layers of the Glens Falls Limestone.
5. Glacial lakes Vermont form as the glaciers retreat northward. In
between the glacial lakes Vermont and present Lake Champlain, marine
waters flooded the valley and formed an arm of the Atlantic Ocean. Clay,
silt, sand and gravel were deposited on glaciated Glens Falls Limestone
(Fig. 1a).
6. Present-day Lake Champlain formed when relatively greater uplift in
the north dammed the Champlain valley.
The rock types found in the Park lead to certain conclusions regarding
the environment which existed during their formation. Most of the rocks
are composed of lime (limestone) or a mixture of lime, fine sand and mud
(limy shale or shaly limestone). The mineral pyrite (FeS₂) is present in
many of the rocks. Most of the rocks contain abundant amounts of organic
matter. The sediments which make up these rocks were carried to the
Ordovician sea by streams flowing primarily from the east. As these
streams entered the quiet sea waters the larger followed by the smaller
particles began to settle to the bottom. Lime was slowly precipitated
from the warm sea water and pyrite formed under stagnant bottom
conditions. Organic material accumulated on the bottom and intermixed
with the sediments. The poor life-sustaining qualities of much of the
bottom waters prevented rapid or complete bacterial action on the
accumulated debris and the sediments remained “organic black” in color.
Slowly, as the weight of overlying sediments increased, the lower layers
were compacted and cemented into the hard limestone and shale which we
see today.
The tilt or dip of the Park rocks resulted from subsequent earth
movements. When were these rocks tilted? From the evidence presented in
the Park all that can be said is that they were tilted sometime after
hardening and before the Pleistocene glaciers overrode the region during
quite recent times (at least 10,000 years ago). Thus, there are some 350
million years of rock record missing in the Park. Can we tell what
happened during these “missing” years through a study of only the Park
rocks? The answer to this question is partially “yes,” but we must look
to the work done in adjacent areas for a more complete story.
The mere fact that there are no rocks representing these millions of
years tells us that the sea had withdrawn from the area and that the
previously deposited rocks were undergoing erosion during most or all of
the missing rock gap (the time not represented by rocks). Information
from adjacent areas, however, tells us that the Park rocks were tilted
during the Taconic Disturbance which occurred during the final stages of
the Ordovician Period. East of the Park, Taconic earth movements are
more dramatically exhibited. The rocks are tilted even more than in the
Park and are broken by faults or cracks in the earth’s crust. Some of
these faults, known as thrust faults, positioned giant slabs of rock far
from their original locations and placed older on top of younger rocks.
Following these earth movements there occurred a long period of erosion.
Many of the rock layers were stripped off and carried piece by piece by
rivers to other regions. Hundreds of millions of years passed and then,
less than one million years ago the great glacial ice sheets slowly
advanced southward over the Park area. Pieces of hard rock frozen to the
underside of the ice sheets scratched and scraped the rock surfaces
leaving these scratches or striations for us to see today (near the
northern end of the Park beach these striations are common on the
outcropping rock). The retreating glaciers created a series of lakes in
which clay, silt, sand and gravel were deposited. Today these sediments
are found resting on the beveled edges of the Park rocks.
Present-day Lake Champlain owes its existence to a general uplift of the
earth’s surface, greater in the north than in the south, perhaps due to
the removal of the heavy glacial ice sheet from the area. The greater
uplift in the north dammed the Champlain valley which slowly filled with
water. For a diagrammatic picture of the geologic history of D.A.R.
State Park, see Figure 7.