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).