THE GEOLOGY OF THE 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]).

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.

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).

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.

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.