The Geology of D.A.R. State Park, Mt. Philo State Forest Park, Sand Bar State Park

by Harry W. Dodge

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.