Geologic units in Grand Isle county, Vermont

Stony Point Formation (Upper Ordovician) at surface, covers 69 % of this area

Dark-gray calcareous shale with beds of bluish-gray limestone. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts). Also part of the Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Glens Falls Limestone (Upper Ordovician) at surface, covers 9 % of this area

Dark-bluish-gray-weathering, thinly bedded dark-gray to black granular limestone; locally grades upward into sooty- weathering shaly limestone beds rich in fragments of the trilobite Cryptolithus. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts). Also part of the Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Iberville Formation (Upper Ordovician) at surface, covers 8 % of this area

Dark-gray shale with thin discontinuous beds of crossbedded and graded dolomitic siltstone. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts).

Cumberland Head Formation (Upper Ordovician) at surface, covers 5 % of this area

Interbedded graded bioclastic limestone, bluish-gray calcareous shale, and laminated shale. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts).

Crown Point Formation (Middle Ordovician) at surface, covers 3 % of this area

Gray, massively bedded fossiliferous limestone. Part of the Chazy Group. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts). Also part of the Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Valcour Formation (Upper Ordovician) at surface, covers 2 % of this area

Dark- to light-gray fossiliferous limestone. Part of the Chazy Group. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts). Also part of the Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Day Point Formation (Middle Ordovician) at surface, covers 2 % of this area

Gray, thin-bedded to massive sandstone and fossiliferous limestone. Part of the Chazy Group. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts). Also part of the Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Hathaway Formation (Upper Ordovician) at surface, covers 1 % of this area

Clasts of limestone, dolostone, sandstone, and chert within beds of black shale and chert. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts).

Black River Group - undivided (Upper Ordovician) at surface, covers 0.9 % of this area

Thin-bedded to massive dolostone, sandy dolostone, and light-gray limestone; vertical burrows in basal beds; upper beds fossiliferous with black chert. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts).

Chipman Formation - Bascom Formation (Middle and Lower Ordovician) at surface, covers 0.4 % of this area

Interbedded orangey-tan- to buff-weathering dolostone and bluish-gray to gray mottled dolomitic limestone or calcite marble and calcareous sandstone. In southern Vermont east of the Taconic Range, rocks mapped as Bascom Formation may include unmapped members of the Chipman Formation. Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Providence Island Dolostone (Middle and Lower Ordovician) at surface, covers 0.3 % of this area

Light-buff-tan- weathering, massive to thick-bedded, fine-grained dark-gray dolostone; has “beeswax-scored” surfaces; contains thin layers of fossiliferous bluish-gray limestone (largely equivalent to the Bridport Member of the Chipman Formation, north of Orwell). Part of the Beekmantown Group. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts).

Beekmantown Group, undivided (Middle Ordovician to Upper Cambrian) at surface, covers < 0.1 % of this area

Gray limestones, fossiliferous limestones, and dolostones. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts).