Part 4

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 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).

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 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 (see Map of Glacial Deposits, Fig. 12), but other glacial deposits are also mapped. A kame (designated “K” in Fig. 12) is a glacial feature found in the southern part of the Park.

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.

SUGGESTED READING (in addition to the general references listed for D.A.R. State Park)

Cady, W. M., 1945, Stratigraphy and structure of west-central Vermont, Geological Society of America Bulletin, volume 56, pages 515-588.

Chapman, C. H., 1937, The glacial and postglacial history of the Champlain valley, American Journal of Science, 5th series, volume 34, pages 89-124.

Stewart, David P., 1961, The glacial geology of Vermont, Vermont Geological Survey Bull. 19.

Stewart, David P. and Paul MacClintock, 1969, The Surficial Geology and Pleistocene History of Vermont, Vermont Geological Survey Bull. 31 (in press).

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

THE GEOLOGY OF SAND BAR STATE PARK

INTRODUCTION

Sand Bar State Park is located in northwestern Vermont on U.S. Route 2, approximately 14 miles north of Burlington and near the east approach to Sand Bar Bridge which leads to South Hero Island in Lake Champlain (see map, Fig. 1). Tenting, picnicking and swimming are the Parks main attractions (Fig. 12a). The swimming beach is on the north side of U.S. Route 2 and fronts on Lake Champlain. Its shallowness makes the beach safe for children (Fig. 13). The tenting facilities are located on the south side of U.S. Route 2 on a south-facing shoreline.

THE GEOLOGY OF THE PARK

The geologic history of Sand Bar State Park is recent, geologically speaking, especially when compared with that of the other Parks treated in this pamphlet. The sediments of the park are blue and brown clay which were deposited throughout the Champlain Valley less than 10,000 years ago. This clay, which can be seen in many places along the bathing beach, was deposited from marine waters which flooded the Champlain Valley just prior to the formation of present-day Lake Champlain. No bedrock crops out in Sand Bar State Park.

The blue clay is covered with deposits brought downstream by the Lamoille River during very recent times and deposited as a delta into Lake Champlain. This delta has shifted its distributary channels frequently and continues to grow southwestwardly into Lake Champlain. Much of the finer material (sand) brought into Lake Champlain by the Lamoille River has been shifted and concentrated by lake currents into ridges or bars; one sand bar stretches to South Hero Island and forms the foundation for the causeway named Sand Bar Bridge. Prior to the building of Sand Bar Bridge (causeway was started in 1849, opened to travel on December 5, 1850), this sand bar was fordable and was used as a link between South Hero Island and the mainland.

Most of the sand now found north of the Park bathing beach and which is responsible for the extensive “shallows” in the swimming area, was supplied by the now abandoned northern channel of the Lamoille River. It is interesting to note that most of the sand now seen on the bathing beach has been imported from nearby areas of Vermont. Since the northern distributary channel of the Lamoille River is no longer supplying sand, and sand from the active southern channel cannot work its way northward because of the Sand Bar Bridge causeway, there is a lack of sand for the beach.