THE GEOLOGY OF
D.A.R. STATE PARK

MT. PHILO STATE FOREST PARK

SAND BAR STATE PARK

SANDBAR STATE PARK

MT. PHILO STATE PARK

BUTTON BAY STATE PARK

D.A.R. STATE PARK

LAKE CHAMPLAIN

Milton

Burlington

Shelburne Falls

Hinesburg

Vergennes

Bristol

Port Henry

Champlain Bridge

Middlebury

Crown Point

Shoreham

INDEX MAP

THE GEOLOGY OF D.A.R. STATE PARK

INTRODUCTION

D.A.R. State Park is located in western Vermont on State Highway 17, approximately 1 mile north of Lake Champlain (toll) Bridge (see map, Fig. 1). The park, which fronts on Lake Champlain, contains undeveloped acres on the east side of the Highway. Tenting, leanto camping, picnicking and swimming are adequately provided for during the summer months.

This park, more than most others, not only awakens the visitor’s curiosity about the past history of the Earth, but satisfies it. The story of an ancient sea and the life which existed in it can be read from the rocks exposed in D.A.R. State Park. You can read this story for yourselves. This Pamphlet is designed as an aid to a more complete understanding of the observations which you make. “Reading the rock record” is not difficult, but the geologist does have the advantage of possessing a certain trained scientific approach to these problems. This method of approach, the “tools of the trade,” will now be passed on to you.

THE GEOLOGY OF THE PARK

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.

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 8 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.

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

THE FOSSILS

Many groups of invertebrates are represented in the fossils of D.A.R. State Park. Plate 1 will help you to identify these fossils. The name, phylum (major group) and age of each fossil are provided in the explanation of the plate. The following paragraphs describe each phylum represented in the Park rocks.

Arthropods. D.A.R. State Park rocks contain trilobites with the following imposing names: Cryptolithus tesselatus (crip-toe-LITH-us tessell-AH-tus[5]), Isotelus gigas (ice-so-TELL-us GIG-us) and Flexicalymene senaria (flex-eye-cal-ah-Mean-ee sen-AREA). These fossils are figured on Plate 1, 1-A, B, C, D; 2; 3. Within the Park Cryptolithus tesselatus is very common wherever fossils occur. Generally only the cephalon or head portion of this trilobite is preserved. The cephalon is 11 easily recognized by three concentric rows of pits arranged around the brim. Cryptolithus is an excellent index fossil[6] for the Park rocks. The arthropod phylum is characterized by animals with jointed legs, segmented bodies and a jointed outer armour of chitin.[7] For examples, the crabs, lobsters, spiders, scorpions and insects are arthropods. Trilobites appear early in the fossil record but they did not survive beyond the Paleozoic Era.

Brachiopods. Brachiopods are abundant in the Park rocks (see Plate 1, 4A, B; 5; 6; 9; 10; 11; 12). These invertebrates are small marine animals which generally live in waters no deeper than 600 feet. The two valves of their shell are joined at the back (posterior) end of the body along a hingeline of interlocking teeth and sockets. The shell of the brachiopod is opened or shut by muscles attached to the inside of each valve. Brachiopods are found in the oldest rocks containing definite and abundant fossils. Brachiopods are still living today.

Bryozoans. Bryozoans or “moss animals” are very small marine animals which live in colonies. The bryozoans construct their mutual home or colony of lime which is commonly preserved for the fossil record. Large colonies of the fossil Prasopora (prah-sop-OR-ah) are commonly seen on the weathered surfaces of many of the rock layers in the Park (see Plate 1; 7A, B, C; 8 and Figure 5). Individuals of one genus common here, can be recognized by their chocolate drop shapes. Bryozoans first appear in lower Paleozoic rocks and are still living today in clear well-circulated shallow to deep marine water. Considering all of the fossils found in the Park rocks, the past environment is thought to have been a relatively shallow and warm sea.

THE ROCKS AND THEIR HISTORY

Approximately 75 vertical feet of the Glens Falls Limestone occur along the Park beach. The rocks are black[8] or blue-black on a fresh surface, gray or grayish-white on a surface which has been exposed to the weather. Most of the individual beds or layers are 5 to 7 inches thick (see Fig. 6) with the thickest being just under 5 feet. The beds are separated by thin “partings” of rock, many of which contain abundant fossils. The beds consist of massive limestone, shaly limestone or limy shale; the partings are generally limy shale or shaly limestone.

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.

SUGGESTED READING

Beerbower, J. R., 1960, Search for the Past, Prentice-Hall, Englewood Cliffs, N.J.

Collinson, C. C., 1959, Guide for beginning fossil hunters, Educational Series 4, Illinois State Geological Survey, Urbana, Ill.

Dunbar, C. O., 1959, Historical geology, John Wiley and Sons, New York.

Fenton, C. L., 1937, Life long ago, The John Day Co., New York.

Goldring, Winifred, 1931, Handbook of paleontology for beginners and amateurs, part 2, Handbook 9, New York State Museum, Albany, New York.

—— ——, 1950, Handbook of paleontology for beginners and amateurs, part 1, Handbook 9, 2nd Edition, New York State Museum, Albany, New York.

Moore, R. C., 1958, Introduction to historical geology, 2nd Edition, McGraw-Hill Book Co., New York.

Shimer, H. W., 1933, Introduction to study of fossils, The Macmillan Co., New York.

Simpson, G. G., 1953, Life of the past, Yale University Press, New Haven, Conn.

Stokes, W. L., 1960, Essentials of earth history, Prentice-Hall, Inc., Englewood Cliffs, N.J.

Welby, C. W., 1961, Bedrock geology of the Central Champlain Valley of Vermont, Vermont Geological Survey Bull. 14.

—— ——, 1962, Paleontology of the Champlain Basin in Vermont, Vermont Geological Survey Special Publication 1.