THE GEOLOGY OF THE PARK_The Geology of D.A.R. State Park, Mt. Philo State Forest Park, Sand Bar State Park

_The Rocks_

The rocks of the Park which will probably first attract your attention

are those exposed at the main western Overlook which is located in the

summit area. This Overlook is found just northwest of the Park lodge

(see Fig. 8). These rocks are light to dark red or purplish in color,

are primarily _quartzite with minor dolostone_[9] dipping approximately

35 degrees to the northeast (for an explanation of dip, see Fig. 3 and

text of D.A.R. State Park, page 6) and striking toward the northwest

(for an explanation of strike, see immediately preceding reference).

A closer look at this Monkton Quartzite outcrop shows that it is made up

of several layers of rock (see Fig. 8). These layers, strata, or beds

are not all of the same thickness, but are generally from 1 inch to 1

foot thick. If an individual layer is traced over the extent of the

outcrop, it is found that its thickness remains about the same

throughout. It is therefore said to be regularly bedded. Thin

laminations of dark red shale are abundant and commonly define

individual layers. A magnified look at a specimen of this quartzite,

under a hand lens, shows that it is composed of fine to coarse fragments

of quartz. Some of these fragments have rounded edges, but others are

quite angular. The spaces between the fragments are filled with silica

(quartz). Therefore, the rock is said to possess a silica cement.

In many places where this Monkton Quartzite has been studied, features

attesting to a shallow water origin have been found. Among these

features are mud cracks, which form under alternating wet and dry

conditions; ripple marks, which are usually found only on shallow water

bottoms; and cross-bedding, which commonly forms in shallow water areas.

The Monkton Quartzite underlies approximately a third of the Park (see

Geologic map, Fig. 9). This quartzite is between 250 and 300 feet thick

on Mt. Philo; however, the lower 50 feet or so consist predominantly of

white quartzite interbedded with dolostone. The age of the Monkton

Quartzite is considered to be Lower Cambrian (see Standard Geologic Time

Scale, Fig. 4).

[Illustration: Fig. 9. Geologic Map of Mt. Philo State Forest Park

(after C. W. Welby. 1961). Because Ogf and Oib were not definitely

identified by the author of this pamphlet and for the sake of

simplicity, these rock units have not been discussed in the text of

the pamphlet. Some dip and strike symbols have been added to Welby’s

original map.]

LEGEND

UPPER MIDDLE ORDOVICIAN

Oib Iberville shale

Osp Stony Point shale

Ogf Glens Falls limestone

LOWER CAMBRIAN

Cm Monkton quartzite

Park roads

Other roads

Contour line

Approximate park boundary

Dip and strike symbol. Layers dip 21° toward N.E.

Approximate contact of rock units.

Surface trace of thrust (low-angle) fault, carat on upthrown side

Inferred trace of thrust (low-angle) fault, carat on upthrown side

Surface trace of high-angle fault

Dip and strike of cleavage

Observation tower

A second type of rock is exposed in the south bank of the exit road

approximately 0.7 miles from the summit area. This is the _black_[10] to

bluish-black Stony Point Shale (see Fig. 10), which underlies the

Monkton quartzite. This shale, or hardened limy mud, is thinbedded and

shows abundant _cleavage_[11] parallel to the layers or beds. At this

outcrop the layers strike to the northeast and dip 20 to 40 degrees

toward the southeast. The dip and strike of the Monkton Quartzite (see

above) is not similar to the dip and strike of the underlying Stony

Point Shale. It follows, that the layers of the Monkton Quartzite are

not parallel to those of the Stony Point Shale.

[Illustration: Fig. 10. View looking south of cut-bank, south side

of exit road, about 0.7 miles down from the summit parking area.

Here the layers dip 20 to 40 degrees toward the southeast and strike

in a northeast direction. Note that the shale is thin-bedded and

contains numerous cleavage planes parallel to the layering. The

handle of the geologic pick is about 1 foot long.]

The fact that these two units are not parallel could mean that the Stony

Point Shale was deposited, hardened into rock, uplifted, folded and

eroded, all prior to the deposition of the Monkton Quartzite. But,

first, what is the age of the underlying Stony Point Shale? If the story

is as listed above, the Stony Point Shale _must be older_[12] than the

overlying Monkton Quartzite. From the fossil animal remains found in the

Stony Point Shale, geologists have dated the Stony Point Shale as upper

middle Ordovician (see Standard Geologic Time Scale, Fig. 4). And so,

here we have older rocks (Lower Cambrian) resting on younger (upper

middle Ordovician).

[Illustration: Fig. 11. View of Mt. Philo, looking toward the

northeast. The black line approximates the position of the thrust

fault. The Monkton Quartzite, which is above the line, was thrust

westward over the Stony Point Shale (note the arrow). Line A-B, Fig.

9, approximates the section.]

_Structural Geology_

How can we explain this inverted order of rock units? The geologic

evidence presented in the Park does not indicate that folding of the

rocks was responsible. From the surface distribution of the two rock

types (see Geologic map, Fig. 9) and the nature of their contact with

each other, a fault relation is envisioned, in which older rocks were

thrust westward over the younger and thus to rest upon them (see Fig. 11

We know the ages of both rock units involved in this thrust fault but

what is the geologic age of the actual thrust movement? Both the Stony

Point Shale and the Monkton Quartzite were hard rock when this thrusting

took place, therefore, the thrusting would have occurred later than

upper middle Ordovician time, but before late Silurian time. Two other

fault systems are recognized in or near the Park (see Geologic map, Fig.

9). They are _high angle_[13] faults which formed later than the thrust

fault, but still preceding late Silurian time.

The Iberville Shale (this is not described in the section on “The

Rocks,” but is seen on the Geologic map, Fig. 9), which is questionably

exposed on the south side of Mt. Philo, would be the youngest rock found

in the Park. This shale is about 390 million years old. The most recent

faulting took place no later than about 340 million years ago. There are

no rocks in the Park which give us any positive geological clues to the

Park’s history from the last episode of faulting to the Pleistocene

glaciers less than 1 million years ago. However, the fact that rocks

representing this interval of time are not present does indicate that

the area was above water during most of these 339 million years (this

number of years is very approximate). If any rocks were deposited during

this “rock-gap” period, they have since been washed away.

_The Pleistocene Deposits_

Beginning between 60,000 and 70,000 years ago two glacial advances and

retreats took place in the Champlain Valley. This was during the most

recent or the Wisconsin Stage of the Pleistocene Epoch. Scratches or

striations were cut into the overridden rock by rock debris carried

along at the base of the ice as it advanced (note the arrow in (“br”)

area at overlook in Fig. 12; this shows striation orientation,

therefore, the direction in which the glacier advanced). The glacial

sediments found on Mt. Philo were deposited during the final retreat of

glacial ice, which took place from 11,000 to 12,000 years ago. Most of

the Park is covered with these glacial deposits and by more recent

soils.

Most of the glacial deposits found on Mt. Philo are classified as

glacial _till_[14] (see Map of Glacial Deposits, Fig. 12), but other

glacial deposits are also mapped. A _kame_[15] (designated “K” in Fig.

12) is a glacial feature found in the southern part of the Park.

[Illustration: Fig. 12. Map of the Pleistocene deposits of Mt. Philo

State Forest Park (after D. P. Stewart, 1961).]

LEGEND

bc Boulder strewn lake sediments

bgm Marine beach gravel

bg Beach gravel

ps Pebbly sand

ls Lake sand

t Till

k Kame

br Bedrock

Park roads

Other roads

With the slow retreat of the glacial ice front from the Mt. Philo

region, deposits were left which indicate that a series of lakes formed

in front of the wasting ice mass. There is also evidence just west of

Mt. Philo (see “bgm” in Fig. 12) which indicates that just prior to the

formation of present-day Lake Champlain, an arm of the Atlantic Ocean

reached into the Champlain Valley from the St. Lawrence River region.

Lake-beach gravels (designated “bg” in Fig. 12) are found on both the

east and west slopes of Mt. Philo. The interesting fact about these

beach gravels is that they occur almost 500 feet above the present-day

level of Lake Champlain. Lake sand (designated “ls” in Fig. 12) is found

some 450 feet above Lake Champlain. This means that during a good

portion of its recent geologic history, Mt. Philo was an island

surrounded by lake water. From the distribution of marine beach gravel

(designated “bgm” in Fig. 12), it appears that the invasion of sea water

from the St. Lawrence region did not isolate Mt. Philo as an island.

The complete story of the lake series is still not known, but, for the

most up-to-date treatment of this subject see D. P. Stewart’s paper

entitled “The glacial geology of Vermont”: Vermont Geological Survey

Bulletin 19 (1961). Suggested also is C. H. Chapman’s article entitled

“Late glacial and postglacial history of the Champlain valley” in the

American Journal of Science, 5th series, volume 34, pages 89-124 (1937).

Looking out over the Champlain lowlands from the summit of Mt. Philo

leaves little doubt in the visitor’s mind as to the prior existence of

lakes which surrounded Mt. Philo in the not too distant past (see Cover

picture).

_Summary of the Geologic History_

During lower Cambrian time, the Monkton Quartzite and dolostone followed

by the Winooski Dolostone (not seen in the Park) were deposited east of

Mt. Philo State Forest Park. During late Cambrian and early Ordovician,

thick dolostones were deposited from the sea water which covered the Mt.

Philo area (not seen at the surface in the Park).

During middle Ordovician time, a series of shale, calcareous shale and

limestone was deposited from the sea water. Then, sometime between the

beginning of late Ordovician and late Silurian time, the eastern lower

Cambrian sequence was thrust westward over the middle Ordovician rocks.

This low-angle thrusting was succeeded by high angle faulting.

The Park rocks were subjected to weathering and erosion for over 300

million years or until glaciers advanced over the area less than 60,000

to 70,000 years ago. Advancing glaciers scoured the rock; retreating or

wasting glacial ice left deposits of clay, sand and gravel in the Park.

A series of lakes formed south of the northward wasting glacial ice and

deposits of beach-gravel and lake-sand formed along the slopes of Mt.

Philo, which was then an island. An arm of the sea next advanced

southward into the Champlain Valley leaving marine beach-gravels just

west of Mt. Philo. The marine waters retreated and present-day Lake

Champlain came into existence. The formation of the present soil cover

and the deposition of recent alluvium from presently flowing rivers and

streams concludes this brief summary of the Park’s geologic history.