The most conspicuous rock found in the park is _granite_.[1] Along the
road which winds to the summit of Burke Mountain you will see several
outcrops of the white or pinkish biotite granite (Fig. 4). This granite
is well displayed in the summit parking area and along the trail to the
observation tower (Figs. 3 and 5). A walk down the Bear Den Ski Trail
also shows an abundance of granite outcrops (Figs. 6, 7, and 8).
[Illustration: Figure 3. Speckled granite with inclusion of
metamorphic rock. Black specks in granite are flakes of black
biotite mica. Metamorphic inclusion, located just above the hammer
head shows some reaction with the invading granite. Pieces of
metamorphic rock were undermined by and dropped into the granite as
it worked its way upward into these rocks. Picture taken a few yards
west of the tower on top of Burke Mt.]
While looking at some of the above-mentioned photographs, a second
family of rocks is discovered (Figs. 3, 5, 6, 7, and 8; also, Figs. 9,
10, 11, and 16). In many places these rocks have a layered or banded
appearance and in other places large lath-like crystals are common in
some of the layers. In some areas these rocks are very heterogeneous in
appearance and display distorted layers and profuse development of
lath-like crystals (Figs. 12 and 13). These rocks belong to the second
major family of rocks, the _Metamorphic_ rocks. The metamorphic rocks[2]
seen in the park were originally _sedimentary_[3] rocks. These rocks
belong to the Gile Mountain _Formation_[4] which was deposited during
the _Devonian Period_ some 300 million years ago (see Geologic Time
Scale[5], Fig. 14). So much for the two major families of rocks present
in the park, the igneous and metamorphic rocks, and how to distinguish
one from the other. Let us assume that you can now distinguish between
the granite and the metamorphic rocks.
[Illustration: Figure 4. Outcrop of biotite granite located on the
summit road between the second and third turns from the summit of
Burke Mountain and on the right side of the road if descending. Note
the “sheeting structure” or flat joint surface which slopes or dips
into the road. This flat break in the rock was probably caused by
the release in pressure of the overlying glacial ice when it melted
from this region.]
[Illustration: Figure 5. East side of parking area, summit of Burke
Mountain. Outcrop of granite with many metamorphic rock inclusions
(hammer, center of picture, rests on large inclusion). Layering or
banding in the inclusions is almost vertical.]
[Illustration: Figure 6. Outcrop located about midway down the Bear
Den Ski Trail. Alternating metamorphic quartzite and phyllite
invaded by lighter colored and speckled biotite granite. Note how
the granite cross-cuts the layered or banded metamorphic rocks. This
cross-cutting points out the fact that the layering or banding was
present prior to the invasion of the granite.]
[Illustration: Figure 7. Outcrop located about midway down the Bear
Den Ski Trail. Metamorphic quartzite and phyllite (darker color) and
invading biotite granite (light speckled appearance). Here the
granite has a more or less conformable relationship to the layers or
bands in the metamorphic rock. Compare this relationship with the
cross-cutting relationship in Fig. 6. For scale, the handle of the
geologic hammer or pick is about 12 inches long.]
Now, what is the relationship of one to the other? That is, where you
can see both of these rock types exposed together in one outcrop, can
you describe the physical contact of one with the other? For instance,
look at Figure 6, which was taken about midway down the Bear Den Trail,
here you see the granite (the white speckled igneous rock which cuts
horizontally across the picture) cutting across the distinctly layered
or banded metamorphic rocks. The granite is said to have a cross-cutting
relationship to the metamorphic rocks. In some outcrops the granite is
more or less parallel to the layers of metamorphic rock (Fig. 7). Here,
the granite is said to have a conformable relationship with the
metamorphic rocks. Still another relationship between the granite and
the metamorphic rock is seen in Figure 8. Here, blocks of metamorphic
rocks are inclosed by granite. These inclosed blocks are called
inclusions and are pieces of invaded rock which fell into or were
encircled by the invading granite.
[Illustration: Figure 8. Inclusion of layered or banded metamorphic
rock (hammer is resting on this inclusion) in lighter colored
biotite granite as seen along the Bear Den Ski Trail. The
metamorphic rocks were invaded and undermined by the granitic rocks,
with the result that pieces of the metamorphic rock were surrounded
by granite. For scale, the handle of the geologic hammer or pick is
about 12 inches long.]
[Illustration: Figure 9. Picture taken along the Burke Mountain
summit road of typical Gile Mountain metamorphic rock. Here the
rocks dip almost vertically. For those more advanced in geology,
note the pillow-like segments or boudinage structure about one foot
to the left of the chisel point of the hammer. This structure is due
to a stretching of the rock. For scale, the hammer handle is about
12 inches long.]
[Illustration: Figure 10. Banded or layered metamorphic rocks with
inter-squeezed granite (lighter colored material). This outcrop is
located on east side of the Bear Den Ski Trail and quite close to
the Burke Mountain summit road. The hammer handle, center of
picture, is about one foot long.]
[Illustration: Figure 11. Picture taken only a few yards from the
Burke Mountain observation tower, along the path to the summit
parking lot, looking northwest. Here you see metamorphic rocks
(quartzite and phyllite) with some inter-squeezed granite. Note how
the nearly vertical metamorphic rock layers bend or “wrap-around” to
the right. The highly resistant inter-squeezed granite actually
holds Burke Mountain up, or to be more scientific, it prevents these
rocks from being worn down as fast as the surrounding rocks. For
scale, see the clip board in the center of the picture.]
From these relationships, what can be said about the relative ages of
the two rock types? Which is the older, or first formed? Which is the
last formed? If you study the above relationships for a minute or so, it
will become obvious that the layered rock had to be formed _prior_ to
the emplacement of the granite. Some of the minerals now seen in the
layered or banded metamorphic rocks were formed at the time of granite
intrusion, but the basic “stuff” or partially metamorphosed sedimentary
rock was present before the granite entered the area from beneath. So,
the knowledge of the two rock types present and an understanding of
their relationship to one another tells us a story of at least two
events which occurred in the park area hundreds of millions of years
ago.
[Illustration: Figure 12. Outcrop on south Lookout, summit of Burke
Mountain. Distorted layers of Gile Mountain metamorphic rock.
Lath-like crystals developed along some of these layers during the
second period of metamorphism, that is, when the granite invaded the
metamorphic rocks. For scale, the hammer handle is about one foot
long.]
Can we find other facts in these rocks which might add to the
above-mentioned events? The answer to this question is, yes! The types
of minerals found in the metamorphic rocks coupled with the inherited
layered structure so common in these rocks, tells us that they were once
sedimentary rocks. There is other evidence which indicates that these
sedimentary rocks were slightly metamorphosed and folded prior to the
invasion of the granite. Added information indicates that these same
rocks were subjected to increasing temperatures with the invasion of the
granite and another metamorphic mineral change took place. Thus far, the
rocks have told us about four distinct events; the deposition and
hardening of the Gile Mountain Formation of sedimentary rocks, the first
period of wide-spread metamorphism, accompanied by broad folding, the
invasion of the granite, and a second phase of metamorphism with the
increased temperatures produced by this invasion (see cross-sections
illustrating the geologic history of the park area, Fig. 17).
[Illustration: Figure 13. Photograph of the outcrop beneath the
observation tower, summit of Burke Mountain. Note the heterogeneous
appearance of the granite-infiltrated metamorphic rock. Here the
metamorphic rock approaches granite itself in composition and if the
process had progressed a bit more, it would be said to be granitized
rock. Large lath-like crystals are very prominent in the rocks of
this outcrop.]
The four events which are mentioned in the preceding paragraph took
place hundreds of millions of years ago. What has happened in the park
since these events? Take a look at Figure 15, which was taken along the
road to the summit of Burke Mountain (coming down from the summit, this
outcrop is located on your right, midway between the second and third
turns in the road). Here the granite exhibits linear scratches or
striations which trend about 40 degrees east of south (general direction
in which the hammer handle points). Again, just down the road from the
midway picnic and camping area, and on your right, striations can be
seen. Here they trend about 45 degrees east of south or approximately in
the same direction as the first series of striations mentioned. These
scratches or striations occur in many places throughout the park, and in
most cases their orientation is about the same. What caused these
numerous striations?
[Illustration: Figure 14. Geologic Time Scale. The main Darling
State Park geologic events are noted on the right, opposite the
approximate geologic time when each occurred.]
ERAS PERIODS—YEARS AGO DARLING STATE PARK EVENTS
EPOCHS
CENOZOIC CENOZOIC GLACIAL STRIATIONS—“SHEETING STRUCTURE”
Pleistocene
Pliocene EROSION, JOINTING
Miocene
Oligocene
Eocene
Paleocene
70 MILLION
MESOZOIC CRETACEOUS
JURASSIC
TRIASSIC
200 MILLION
PALEOZOIC PERMIAN
PENNSYLVANIAN
MISSISSIPPIAN EROSION, JOINTING
Invasion of Granite with second stage
of metamorphism.
DEVONIAN Regional folding and first episode of
metamorphism.
SILURIAN
Deposition of the Gile Mountain
Formation.
360 MILLION
NO RECORD IN PARK
ORDOVICIAN
CAMBRIAN
550 MILLION
PRECAMBRIAN TIME——ORIGIN OF EARTH, 4 TO 5 BILLION YEARS AGO.
[Illustration: Figure 15. Glacial striations or scratches on outcrop
midway between the second and third turns in the road down from the
summit area of Burke Mountain. Striations trend about 40 degrees
east of south or in approximately the same direction that the hammer
handle is pointing. Hammer handle is about one foot long.]
[Illustration: Figure 16. Midway between the second and third turns,
descending on the Burke Mountain summit road. Metamorphic quartzite
and phyllite showing at least two prominent joints. Layers are
vertical and parallel to the front joint (one which hammer handle
touches). For the more advanced student of geology, note the
lineations parallel to the hammer handle and on the front surface.
For scale, hammer handle is about one foot long.]
Since they are still preserved in the rocks for us to see, they must
have been formed quite recently, that is, geologically speaking. What
can explain these striations and their common orientation? Did you ever
hear about the Great Ice Age, or the Pleistocene Epoch? Less than one
million years ago, in fact, some 12,000 years ago, an ice sheet many
thousands of feet thick rode over Burke Mountain in a southeastward
direction. The many boulders frozen to the underside of the ice sheet
tended to scratch the rocks over which they rode. The scratches or
striations seen in the park rocks were caused by these attached
boulders. The ice sheet also plucked and rounded Burke Mountain into the
shape it possesses today.
A look at Figure 4 shows still another event which occurred during
recent geological time. The prominent smooth fracture-surface seen to
slope or dip toward the road is called “sheeting structure” which has
its origin in post-glacial time. It is thought by many geologists that
these flat surfaces or _joints_[6], which are generally parallel to the
ground surface, were formed with the release of the weight of the
overlying glacial ice when the glacier retreated northward. So, here we
have evidence displayed in the rocks which tells of still another event
in the park’s history. It should be mentioned here, while still on the
subject of joints, that other joints do occur in the park rocks. Figure
16 shows joints which were formed earlier than the “sheeting” and which
are not parallel to the surface of the ground. These joints were
probably formed as a result of the removal of the overlying rocks
through erosion, thus releasing long-continued pressures produced by the
weight of the overlying rocks, and movement of the earth’s crust. We now
have the story of two main episodes in the park’s geological history;
one took place many millions of years ago, the other within the last
12,000 years. What happened between these two rock-documented episodes?
[Illustration: Figure 17. Geologic cross-sections illustrating the
geologic history of Darling State park. (For explanation of
cross-sections see top of page 19.)]
1. Deposition and hardening of the Gile Mountain Formation. At this
stage the layers of rock were more or less horizontal.
2. The horizontal and parallel layers of the Gile Mountain Formation
were gently and broadly folded and regionally metamorphosed. This
is the first stage of metamorphism in the park area.
3. Invasion by granite. This invasion was accompanied by local
metamorphism of the invaded rocks. This is the second stage of
metamorphism in the park area. Note the inclusions of first stage
metamorphosed Gile Mountain rocks in the granite.
4. Many millions of years of erosion took place, the forces of nature
finally exposing the granitic rocks at the surface of the earth.
5. Continued erosion caused the metamorphically reenforced Gile Mountain
rocks to wear down more slowly than the surrounding weaker rocks.
For this reason, these strengthened rocks stand higher than the
weaker rocks.
6. Less than one million years ago the glaciers advanced over the park
area. The glacial ice plucked and scratched (striated) the
underlying rocks as it slowly advanced southward. During the
retreat (northward) certain deposits were left. Present-day Burke
Mountain is much the same as it was when the glaciers left, but,
some added erosion has taken place and, because of uplift, the
Mountain stands a bit higher than it did some 10,000 years ago.
Some soil, much of which was removed by the glaciers, has since
formed on the mountain.
There are no rocks present in the park which were deposited during this
interval of time, therefore, no rock record. If no rocks representing
this time interval are present, one of two reasons must be responsible.
Either the park area was undergoing active erosion (wearing down) during
this period, or sediments were deposited during part or all of this time
interval and subsequently completely removed by erosion. Most probably,
the intervening time found the park area above the depositional
environment of the sea, when its rocks were being worn away by the
erosional forces of nature. Again, see Figure 17 for a diagrammatic
representation of the geologic history of the park.