Geology of the New York City Region

11. Mianus River Gorge

The Mianus River Gorge Wildlife Refuge and Botanical Reserve is one of the first sites owned and managed by The Nature Conservancy. It consists of 615 acres of reforested farmlands and hillsides along the Mianus River downstream from the S. J. Bargh Reservoir (managed by the American Water Company). The park is just north of the Connecticut border and south of the town of Bedford in Westchester County, New York. A small parking area for this public preserve is on Mianus River Road about a half mile south of the intersection of Miller Mill Road and Stamford Road (which continues as Long Ridge Road, CT Route 104). The preserve is open only between April 1 and November 30.

The park consists of a system of trails totaling about five miles along the Mianus River and along the hillsides and hilltops on the west side of the river. The bedrock is schist and gneiss with pegmatite intrusives. The Bedford Augen Gneiss is an intrusive body that invaded older bedrock during the Late Devonian. The rock along southern end of the preserve is Hartland Schist (east of Cameron's Line), whereas the bedrock north of the preserve the rock consists of Precambrian and Cambrian gneiss and quartzite (west of Cameron's line)(Figure 32).

Geologic map of western Long Island Sound region of New York
Figure 32. Generalized geologic map showing the location of parks near the north and south shores of Long Island Sound (map modified after Rogers, 1985 and Fisher et al., 1995).

The Mianus River gorge is a periglacial feature created by blockage and rerouting of streams below the ice front. The stream carved the gorge along a zone of weakness in the rock, possibly as a retreating series of rapids and waterfalls that carved steadily upstream to their present locations.

The trails through the park follow and cross numerous rock walls made by upland farmers attempting to farm the thin rocky soils of this hilly glaciated terrain. The building of the stone walls must have been back-breaking work with little reward. Eventually the farmers abandoned the land, allowing the hemlock and hardwood forests to reclaim the landscape. A park brochure claims that the preserve is host to over 100 species of trees, shrubs and vines, more than 150 species of birds, and over 250 species of wildflowers. One hemlock is reported to be over 375 years old.

Approximately one mile north of the parking area is an abandoned pegmatite quarry. The pegmatite, consisting of large crystals of quartz, mica, and feldspar, was mined in the 18th century. Some of the platy crystals of muscovite (mica) range up to several inches in diameter. This site is closed to collecting.

Approximately two miles north of the parking area is a spur trail to Havenmeyer Falls, a small, but very beautiful cascade (Figure 33). Not far to the north the trail ends on the shore of the reservoir. The scenic trail out to the reservoir and back is about five miles of moderately strenuous walking.

Havenmeyer Falls in the Mianus River Gorge
Figure 33. Havenmeyer Falls in the Mianus River Gorge.

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Last modified: 3/11/2019