THE GEOLOGY OF THE MOUNT INDEPENDENCE STATE HISTORIC SITE, ORWELL, VERMONT

The Mount Independence Revolutionary War site in Orwell, west-central Vermont, occupies a peninsula in Lake Champlain that is 2.2 km (1.37 mi) long and 1 km (0.62 mi) wide. The stratigraphy includes the Cambrian Potsdam and Ticonderoga Formations, and the lower Ordovician Whitehall and Cutting Formations. The lower three units dip northward at a low angle (about 5°). The Potsdam Formation, a reddish, coarse-grained, thin- to medium-bedded sandstone/quartzite, crops out at the southern end of the peninsula, and is well exposed for about 24 m (80 ft) of thickness. Conformably overlying the Potsdam is the Ticonderoga Dolomite, a siliceous, thick- to massively-bedded dolomite. This formation constitutes the bulk of Mount Independence, with a thickness of approximately 58 m (190 ft). Quartz occurs as scattered grains, in layers, and in clastic dikes. Black chert occurs in nodules, layers and breccias. Conformably overlying the Ticonderoga is the lowermost part of the Whitehall Formation, another siliceous dolomite. The contact is at best ephemeral; the two formations contain similar lithologies and published descriptions of the contact vary. However, an earthy matrix and fetid odor is characteristic of the lowermost Whitehall, and is observed in several scattered outcrops at the highest elevations of Mount Independence. Thin section analysis and the determination of insoluble fractions may help constrain the position of the contact. A section of the lowermost member of the Cutting Formation, a distinctive cross-bedded sandstone and breccia, occurs toward the north end of the peninsula, where it lies unconformably on Ticonderoga rocks. This contact is most likely a fault, as the beds above dip at a distinctively different angle than the layers below, and because of the lack of Whitehall Formation in the immediate vicinity. The information gained through this mapping project will be the basis of an interpretive trail guide for visitors to the State Historic Site, and will also appear on the Vermont Geological Survey's website.