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Geology of the Monkton Area, Vermont

Bedrock geologic map of the Monkton area. Click on map for a larger version (2.95 mb pdf).

USDA color orthophoto of Cedar Lake (Monkton Pond). Click on map for a larger version (2.01 mb pdf).

Surficial geologic map of Cedar Lake (Monkton Pond) from D. Stewart (north end) and G. Connally (south end).

AL - alluvium; P - peat and muck; BG - beach gravel; PS - pebbly sand; LS - well-sorted sand; VC - varved clay; BC - silty clay and/or clay containing ice-rafted boulders; T- till; K - kame.

The Cambrian (543– 490 Ma) formations in Monkton are, from oldest to youngest: Cheshire Quartzite, Dunham Dolomite, Monkton Quartzite and Winooski Dolomite. The depositional sequence consists of alternating quartz-rich clastic sediments and carbonates. All the units record deposition in shallow water along a continental margin in the Iapetus Ocean, a warm shallow sea. Carbonates (Dunham and Winooski) precipitated directly from sea water and sea level fluctuated so that land-derived sands (Monkton) were carried out over the carbonates. Mud cracks and ripple marks in the Monkton Quartzite attest to the shallow, tidal flat environment. The oldest unit in the area, the Cheshire Quartzite, was beach sand and represents the transition from rift sedimentation to sedimentation on a more passive, subsiding margin.

Although the rock types record sedimentation on the continental margin, younger structures in the rocks record Taconian, Acadian and Mesozoic deformation in response to plate collisions and eventual rifting to form the current Atlantic Ocean. Thrust faults in the Monkton area most likely occurred during the Taconian Orogeny, an island arc-continent collision, around 450 million years ago. The thrusts were re-activated during the Acadian Orogeny around 350 million years ago when the Iapetus Ocean closed and the eastern portion of New England collided with Laurentia. The resulting supercontinent of Pangea began to break up around 200 million years ago and the high angle faults in the Monkton area are considered coincident with that event. The north-south trending St. George fault has a maximum estimated down-to-the-east offset of 500 meters (Stanley, 1980).

Cedar Lake, previously named Monkton Pond, occupies 114 acres in Monkton, VT. The lake is 11 - 12 feet deep (click here for a depth chart). The lake, at an elevation of 518 feet, is above the level of the Champlain Sea and below the high level of glacial Lake Vermont. In 1845, State Geologist C.B. Adams explored the bottom of the pond and found peat overlying shell marl overlying clay. The complete description from Adams is given below, although his time estimates are likely not accurate.

Annual Report on the Geology of the State of Vermont, 1845, C.B. Adams, State Geologist (p 148-149):

"The history of these beds of marl may be learned from the great deposit of Monkton pond, which is now in a stage of progress most favorable for illustrating their origin. This pond contains about three hundred acres, most of it, except the east portion which has a steeper shore, has the bottom covered with marl, which was probed to the depth of eight or ten feet, without finding the bottom. About one-third of the pond has less than five feet of water. The marl consists of shells more or less broken, and slimy and reddish matter. The shells are of several species, which still exist in the pond. The accompanying section represents the several deposits, beginning at the north end of the pond with the hill a, and extending about seventy rods into the pond: cc, a muck bed; nn, the pond; ee, shell marl; oo, blue clay. On this section, one rod from the pond, the peat is two and a half feet thick; beneath it we find a few inches of marl; and then blue clay, which was bored seven feet without finding the bottom. Four rods from the pond, the marl is but an inch or two in thickness.

It is obvious that we have here a type of the usual process. First the blue clay of the older pleistocene was deposited over drift; for although in this case we did not penetrate through it to the subadjacent drift, the known examples of its immediate superposition on the drift are so numerous, that there can be little or no doubt; then commenced the growth of the mollusca, which, although for the most part less than one quarter of an inch in diameter and occupying much less space after comminution, have accumulated to the amount probably of 300,00 cords or more than 6,000,000,000,000 of shells.

Meanwhile the vegetable deposit commenced not far from the margin of the pond and is now advancing into it over the marl, which, however, is still in progress, thus showing us how of two deposits superimposed the one on the other, a part of the oldest portion of the upper one may be more ancient than the newest part of the lower bed.

Since some of these beds are yet in progress, and the others are entirely covered with several feet of peat and that with a heavy growth of timber, their ages must be various. That they are subsequent to the blue clays of the older pleistocene appear from their very general superposition of these clays. In many cases, the process therefore must have commenced with the newer pleistocene period or possibly somewhat earlier, as it is not known they overlie brown clay or sands. The process of filling up the ponds in many cases also has been completed, and our views of the antiquity of this period must depend on those which we adopt respecting the age of the overlying peat beds.

The length of this period is strikingly illustrated in the Monkton marl bed. A long series of years is required to furnish shells sufficient for a single layer, and yet they have accumulated to more than ten feet in depth. 20,000 years is a very moderate estimate for the time required at the present rate of accumulation, and it is more likely to have exceeded this many fold. It is obvious to remark, that, since this is but a part of the time subsequent to the drift agency, the impossibility of identifying that agency with any historical deluge in manifest to the unscientific reader, as it has long been to geologists."


To see
Water quality data for Cedar Lake:
Depth chart:
Geologic Time Scale:

Generalized Geologic 
	Map of Vermont - 1970 - click for larger map image

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