The park beach is the ideal place to study the rocks of the park, for
here the rocks are best exposed and can be easily examined at close
range. The attitude of the rock layers can be seen on a walk down the
ramp. By attitude is meant their relationship to an imaginary horizontal
plane, which for our purposes is the level of the lake. Are the rock
layers parallel to the surface of Lake Champlain or do they slant or dip
into it? If the layers were parallel to the surface they would not
“dip.” The dip of the park strata (layers) is seen in Figure 2. Dip is
expressed in the number of degrees _down_ from the horizontal and here
the dip is toward the west and is measured to be between 8 and 11
degrees. The dip is always measured perpendicular to an imaginary
horizontal line on a rock layer called a “strike line.” The average
“strike,” or compass direction of the “strike line” is 22 degrees east
of _north_.[1]
_Sedimentary rocks_[2] crop out on the park beach. These were originally
lime mud resting on the sea bottom. Under continued pressure from the
overlying sediments resulting from continued deposition and burial, the
muds were slowly compacted and cemented into the hard limestones and
limy shales which we see today. The many layers of rock were then
tilted. Tilted layers tell the geologist of giant earth movements which
took place since their formation. The story of these movements will be
developed later in this pamphlet.
As you look at the tilted rock layers from the ramp, can you tell which
layers are the oldest, that is, those first deposited as lime muds on
the sea bottom? A basic geologic law, the Law of Superposition, states
that if a series of sedimentary layers have _not_ been overturned, the
oldest is on the bottom and the youngest on top. Assuming that the
layers which you are looking at have not been overturned, those on your
left (south) are the oldest and those on your right (north) the
youngest. Looking at Figure 2, taken from the ramp-bottom toward the
north, the layers in the foreground are older than those in the
distance. Let us take a close look at the individual layers of rock and
delve deeper into the story that they have to tell.
[Illustration: Fig. 3. Block diagram illustrating _dip_ and
_strike_.]
STRIKE DIRECTION
ROCK LAYER
SURFACE (PLANE) LAKE LEVEL
HORIZONTAL
DIP
ROCK LAYER
Search the top of a few layers and you will notice many shell and other
animal impressions. Do they represent animals which lived hundreds of
millions of years ago or were they washed onto these rocks from
present-day Lake Champlain? If you try to make a collection of the shell
impressions you will see that they are a part of the rock and therefore
must represent remains of animals that were buried in the ancient lime
muds. These preserved _remains_[3] are called fossils. The geologist who
specializes in the study of fossils is called a _paleontologist_. You
may ask, “What can fossils tell me about the past?” In the first place,
fossils tell us at what time in the past the sediments in which they are
found were deposited. In this way, the _relative age_[4] of the rock
layers found in the Park can be learned. Secondly, the environment or
surroundings in which these ancient sediments were deposited can be
reconstructed from the types of fossils contained within them. Certain
animals living today are quite similar to those in the Park rocks and
their environment in today’s sea can be used to reconstruct the
environment of animals which lived in the past. The characteristics of
the rocks and their relation to adjacent rocks are considered in any
reconstruction of past environment. In the third place, the study of
fossils is a mainstay of the theory of evolution. That is to say,
changes in fossil forms collected from groups of successively younger
rock layers document the theory that life has evolved little by little
since its first appearance on Earth. Finally, it should be mentioned
that some animals found as fossils are not living today and have not
lived, to the best of our knowledge, for millions of years. Why did
these forms of life die out? What set of circumstances led to their
extinction? The answers to these questions are not easy to find and they
are highly speculative.
[Illustration: Fig. 4. Standard Geologic Time Scale.]
GEOLOGIC TIME
ERA YEARS AGO PERIODS EVENTS
CENOZOIC 70,000,000 CENOZOIC MAN (1½ MILLION)
MESOZOIC 125,000,000 CRETACEOUS END OF DINOSAURS
165,000,000 JURASSIC FIRST BIRD
200,000,000 TRIASSIC FIRST DINOSAUR
PALEOZOIC 230,000,000 PERMIAN END OF TRILOBITES
260,000,000 PENNSYLVANIAN
290,000,000 MISSISSIPPIAN
330,000,000 DEVONIAN
360,000,000 SILURIAN
420,000,000 ORDOVICIAN ROCKS OF D.A.R. STATE
PARK
500,000,000 CAMBRIAN
BEGINNING OF FOSSIL RECORD
1 BILLION
2 BILLION
3 BILLION
4 BILLION
PRECAMBRIAN