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Title: The Geology of Darling State Park
Author: Harry W. Dodge, Jr.
Release Date: November 19, 2019 [EBook #60730]
Language: English
Character set encoding: ASCII
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[Illustration: Cover picture: View of Burke Mountain (center,
background). Picture taken toward the northeast from State Route
114, about 5 miles south of East Burke.]
THE GEOLOGY OF
DARLING STATE PARK
_By_
HARRY W. DODGE, JR.
VERMONT GEOLOGICAL SURVEY
Charles G. Doll, _State Geologist_
DEPARTMENT OF FORESTS AND PARKS
Robert B. Williams, _Commissioner_
DEPARTMENT OF WATER RESOURCES
1967
[Illustration: Figure 1. Map showing the location of Burke Mountain
(Darling State Park), and mountain peaks which can be seen from the
summit of Burke Mountain.]
Jay Peak 39 MILES 3861'
Gore Mtn. 25 MILES 3330'
Monadnock Mtn. 29 MILES 3140'
Haystack Mtn. 35 MILES 3223'
Belvidere Mtn. 34 MILES 3360'
Willoughby Lake 15 MILES
Bold Mtn. 15 MILES 3315'
Mount Mansfield 44 MILES 4393'
Stone Mtn. 11 MILES 2753'
Mount Washington 35 MILES 6288'
Camels Hump 50 MILES 4085'
Mount Ascutney 80 MILES 3144'
THE GEOLOGY OF DARLING STATE PARK
_By_ HARRY W. DODGE, JR.
INTRODUCTION
Darling State Park, located in northeastern Vermont (see map, "Burke
Mountain," Fig. 1), offers outstanding opportunities to the camper,
picnicker, hiker, view-seeker and winter sportsman. On a clear day the
top of Burke Mountain offers a most spectacular view of northeastern
Vermont and such distant points as Mount Ascutney (located on the
Connecticut River, some 80 miles as the crow flies, to the south). Other
prominent peaks that may be seen are Camels Hump (50 miles southwest),
Mount Mansfield (44 miles west), Jay Peak (39 miles northwest), and
Mount Washington (the highest Peak in the New England States and the
northeast, which is located in New Hampshire some 47 miles southeast of
Burke Mountain). For the traveler, the view from Burke Mountain reveals
"where he has been" and where he might "next go." See Figure 1 for the
location of points which can be seen from the overlooks atop Burke
Mountain.
Both the professional and amateur naturalist will find Darling State
Park extremely interesting. This pamphlet is devoted primarily to the
geology of the park, but the fauna and flora of this area present the
visitor with days of interesting studies. It is hoped that in the near
future pamphlets describing these aspects of Darling State Park will be
published.
THE GEOLOGY OF THE PARK
Before discussing the more detailed aspects of the geology of Darling
State Park, certain basic geologic concepts must be explained. But, even
before such a discussion, it might be best to clarify the position of
geology among the many other, and oftentimes interrelated, sciences.
The basic reason for the science of geology might be said to be twofold;
one is economic, the other related to Man's basic curiosity. In the
first, the geologist through the use of his knowledge of the earth's
rocks, locates those indispensable minerals and fuels without which our
advanced society and technology could not exist. In the second, the
geologist tries to unlock the many mysteries within the earth's crust
merely to satisfy a thirst for knowledge and to pass such knowledge on
to his fellow man. These two basic reasons complement each other and
allow continued advancements in geology, both as a pure science and as a
primary economic aid to the nation.
As found in most spheres of present-day scientific endeavor, the
geologist relies heavily on other related sciences for insight into
problems at hand. A basic knowledge, and oftentimes an advanced
knowledge, of physics, chemistry, mathematics and zoology, to name only
some, are needed before the geologist can approach many of his own
problems. It might be obvious to you by now, but a geologist will be
certainly included in the first scientific party to journey to the moon
and planets.
Within the general science of geology are several branches, to name only
a few; paleontology, sedimentology, mineralogy, petrology, stratigraphy,
petroleum geology, and structural geology. Each of these branches or
specialty-areas contributes basic data for the overall interpretation of
the past geologic history of any given geographic area. The historical
geologist takes all these clues and attempts to fit the pieces of
information together into a picture of past events.
The concept of Geologic Time must be understood before the history of
Darling State Park can be unraveled. Usually we think of time in terms
of minutes, hours, days, weeks, months and years. The geologist thinks
and talks in terms of millions or even billions of years. Time units as
short as hundreds of years are impossible to distinguish in the past
history of the Earth. When it is realized that the earth is probably 4
to 6 billion (4,000 to 6,000 million) years old, and the record of these
years is incomplete, it is easy to understand why the geologist speaks
in terms of millions of years instead of years. With modern methods of
radioactive dating the geologist hopes for finer time definitions in the
future.
In short then, the geologist interprets and puts order into millions of
years of history which can only be "read" as recorded in the rocks
beneath our very feet. Of course, just looking at the rocks does not
magically open the book of geologic history. This pamphlet is designed
to sharpen your powers of observation and to help you in your
interpretation of these observations.
[Illustration: Figure 2. Thin-section of granite from Burke
Mountain. The main minerals seen in this photograph are feldspar
(light gray center right, marked with "F"); quartz (whitish, marked
with "Q"); biotite (light gray, speckled appearance, marked with
"B"). Note the interlocking nature of the minerals which make up
this rock. Magnified 15 times, under crossed nicols.]
THE ROCKS AND THEIR HISTORY
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.
WHY IS THERE A BURKE MOUNTAIN?
Granite is a very resistant rock, that is, it wears away very slowly
under the forces of nature. The granite is worn down more slowly than
the metamorphic rocks which it has intruded. The granite has been
squeezed between and across the layers (bands) of the metamorphic rocks
(previously sedimentary rocks) now found on Burke Mountain. In a very
true sense, the granite forms a skeleton framework for the metamorphic
rocks of Burke Mountain. In other words, it holds these metamorphic
rocks up above the surrounding area of metamorphic rocks.
HIKES TO TAKE
A very interesting hike, both geologically and for nature hunting in
general, is the old fire road which begins just above the old C.C.C.
camp and the present Bell Gardens. This trail cuts off to the right, if
ascending the summit road, and runs completely around the mountain.
Shortly after the fire trail intersects the Bear Den Ski Trail, and on
the left, granite with obvious drill holes is seen (Fig. 18). Most of
the granite used as curb stones, culvert headers, and islands along the
park summit road was obtained from these small quarries. A few miles
walk along this trail proves quite rewarding to the adventurer; an old
lean-to demonstrates what a bear can do while sharpening his claws.
The Devil's Den Trail leads down the east side of Burke Mountain from
the observation tower at the summit. This trail is rather poorly marked
past a certain point, but if you wish to strike off on your own and see
some wonderful country, a hike down this trail with a swing to the north
will bring you back to the summit road.
The several ski trails on Burke Mountain are all walkable during the
summer, but they will not appear as smooth as when covered with snow,
and the trip down will take considerably longer on foot than on
"boards." The Bear Den Ski Trail is especially good for geological
sightseeing.
NEARBY AREAS TO VISIT
While at Darling State Park a visit to Lake Willoughby is well worth the
trip (Figs. 19, 20). A glance at Figure 1 will show you its general
location. Once you visit this lake, you might ask "why so beautiful a
Lake Willoughby?" There is still some question as to the origin of the
lake basin, but a combination of oriented joints and recent glacial
movement seems to fit the picture. Deeper and faster weathering along
parallel joints together with glacial movement and scour in the same
direction as the joints probably dug the elongated trench which, when
filled with water, became Lake Willoughby.
It is hoped that this pamphlet has given you the desire to enlarge your
knowledge of the science of geology. Geology is all around you wherever
you might travel and a knowledge of geology will open new roads even
while traveling old ones. Here's luck to you in your travels; try to
_see_ what you look at.
[Illustration: Figure 18. Geologic map of Darling State Park.]
Granite-Metamorphics
Observation Tower
Burke Summit Rd.
Bear Den Trail
Fire Trail
Park Boundary
Contour Line
Dip and Strike: Bed
Dip and Strike: Bed?
Dip and Strike: Joint
[Illustration: Figure 19. View of Lake Willoughby, seen from its
north shore. Mount Pisgah on the left, Mount Hor on the right. The
Lake Willoughby trough was produced by preferential erosion of a
joint system accentuated by glacial movement and erosion in the same
general direction. The lake bottom opposite Mount Pisgah is 210 feet
deep.]
[Illustration: Figure 20. Northwest View from the Burke Mountain
summit road. Picture taken just above the old C.C.C. Camp and the
present location of the Bell Gardens. Note Lake Willoughby trough in
the distance. Mount Pisgah is on the right and Mount Hor on the
left. Lake Willoughby lies in this trough.]
SUGGESTED READING
Leet, L. Don and Judson, S., 1965, _Physical Geology_, 3rd Edition,
Prentice-Hall, Inc., Englewood Cliffs, New Jersey. This is a
starter for the geology student.
Dunbar, C. O., 1959, _Historical Geology_, John Wiley and Sons, New
York. Also a beginning book, read after above book.
Dennis, John G., 1956, _The Geology of the Lyndonville Area, Vermont_,
Vermont Geological Survey Bulletin 8. This is for the more
advanced student and relates the geology of the area adjacent to
Burke Mountain and Darling State Park.
Jacobs, Elbridge C., 1941-42, _The Great Ice Age in Vermont_, Report of
the State Geologist, Vol. 23; pp. 27-47.
Stewart, David P., 1961, _The Glacial Geology of Vermont_, Vermont
Geological Survey, Bulletin No. 19.
Woodland, B. G., 1963, _A Petrographic study of Thermally Metamorphosed
Pelitic rocks in the Burke Area, Northeastern Vermont_, American
Journal of Science, volume 261, pages 354-375. For the advanced
student.
Woodland, B. G., 1965, _The Geology of the Burke Quadrangle, Vermont_,
Vermont Geological Survey Bulletin No. 28. This is a comprehensive
study of the Burke Mountain area and a must for those interested
in Darling State Park.
Footnotes
[1]The granite found in Darling State Park is white or pinkish in color
and most times is speckled with shiny black mica flakes. On close
inspection, grains of smoky to clear color are seen within the rock.
The white and pink grains are the mineral, feldspar; the shiny black
flakes, biotite mica; the smoky to clear grains, quartz or silica. A
magnified picture of a slice of granite (see Fig. 2) shows the
individual mineral grains and their interlocking nature with each
other. Granite belongs to a major family of rocks, termed _Igneous
rocks_. Igneous rocks are formed through the hardening or
lithification of molten rock-material when subjected to the cooler
temperatures at or near the earth's surface. The molten rock
material formed at some depth beneath the surface of the earth,
where temperatures were many hundreds of degrees hotter than at the
surface.
[2]Metamorphic rocks are either sedimentary (this, a third major family
of rocks which is characterized by a layered appearance that has
been retained by many of the altered park rocks) or igneous rocks
(the granite) which have been under the influence of pressure, heat,
and chemically active fluids, oftentimes resulting in chemical and
structural changes. Most of the metamorphic rocks seen in the park
are either schist, phyllite, slate or quartzite. For the benefit of
the more advanced student, the rocks of the park area are considered
a granite-hornfels complex (see Bertram G. Woodland's paper of 1963,
"A Petrographic study of Thermally Metamorphosed Pelitic rocks in
the Burke Area, Northeastern Vermont," in the American Journal of
Science, volume 261, pages 354 to 375).
[3]Sedimentary or layered rocks, the third major rock family, are
composed of pieces, grains and other materials from older
metamorphic, igneous and sedimentary rocks. These fragments have
been carried by rivers and streams to some resting place at the
bottom of a sea, lake, or stream channel. This mud, sand and gravel,
under the weight of steadily increasing overburden, and the presence
of cementing materials, slowly hardened into rock which we now call
limestone shale, sandstone, and conglomerate.
[4]A geologic formation consists of a sequence of rock layers which were
deposited under essentially the same conditions, or a series of
alternating conditions, and which can be easily distinguished and
mapped as a unit by geologists in the field.
[5]Geologic time is divided into four Eras which are designated from
oldest to youngest: Precambrian, Paleozoic, Mesozoic, and Cenozoic.
Each of these Eras is divided into geologic periods of time. The
Devonian Period is in about the middle of the Paleozoic Era and
began some 330 million years ago and ended approximately 290 million
years ago (see Geologic Time Scale, Fig. 14).
[6]A joint is a break in a rock mass which interrupts its physical
continuity. A group of more or less parallel joints is known as a
joint set.
Transcriber's Notes
--Silently corrected a few typos.
--Retained publication information from the printed edition: this eBook
is public-domain in the country of publication.
--In the text versions only, text in italics is delimited by
_underscores_.
End of the Project Gutenberg EBook of The Geology of Darling State Park, by
Harry W. Dodge, Jr.
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