Stop #3 - Beavertail


The southern tip of eight-mile long Conanicut Island had a watch house marking the entrance to busy Narragansett Bay as early as 1705. Starting around 1712 a pitch fire was maintained by local Indians, under orders from the Jamestown Town Council. The lighthouse built at Beavertail in 1749 was the third in the American colonies, after Boston Light (1716) and Brant Point Light (1746) on Nantucket.

The tower was burned by the British, who removed some of the lighting apparatus, in 1779. Repairs authorized by President Washington were completed in 1790, and the lighthouse was ceded to the federal government in 1793.

In 1817 Beavertail Light became the first American lighthouse to be lit by gas. Beavertail has probably seen more types of fog signals than any other New England lighthouse station. A fog bell and tower installed in 1829 lasted only four years. In 1851 an experiment was tried with an air whistle and an air trumpet, both operated by compressed air. The compressor was powered by a horse. A new steam whistle, first in the nation, was installed in 1857, then a fog signal driven by a hot air engine in 1868. The hot air engine was designed by John Ericsson, the builder of the famous iron-clad vessel Monitor. A new compressed air siren went into use in 1900, replaced by diaphragm horns and finally by electronic fog signals. The present tower was built in 1856, and the 45-foot square granite lighthouse still stands. In the 1880s kerosene replaced whale oil as the fuel for the light. Beavertail Light was electrified in 1931.

During World War II, the lighthouse shared Beavertail with Fort Burnside, a Harbor Entrance Control Point monitoring the movement of ships in and out of Narragansett Bay.


At this stop, we see multiply deformed Cambrian metasedimentary rocks of the late Precambrian Avalon terrain that underlies all of RI, southeastern MA and CT.

The Beavertail Point Member of the Jamestown Formation and the Dutch Island Harbor Formation are well displayed here in a folded tectonic slide contact, in turn cut by the Beavertail fault zone. The tectonic slide contact between the Dutch Island Harbor Formation and the Beavertail Point Member is deformed by F2-folds just northwest of the Beavertail fault. This contact and the units on either side are offset by the adjacent Beavertail fault, a late post-F2-fault.

Beavertail Point Member - this unit consists of 80-90% green phyllite comprising the assemblage of quartz + chlorite + muscovite + feldspar +/- siderite +/- paragonite. Buff and white siltstone make up about 10-15% and is typically characterized by brown weathering; the buff siltstone in beds 2 mm to 30 cm thick is micaceous (approximately 10 percent muscovite) and may contain 40 percent dolomite or a ferroan carbonate. Where siltstone beds are present, the bedding is recognized as cyclical. The black phyllite (5 percent) has the same mineral assemblages as the green. Dolomite concretions (2.5 to 16 in; 6 to 40 cm) are less abundant than in the Dutch Island Harbor Formation.

Dutch Island Harbor Formation - this formation is a black rhythmically bedded phyllite whose beds are 1-4 cm thick, commonly featuring 1-cm-deep scour channels, and brown-weathering carbonate beds containing concretions 4-12 in (10-30 cm) long. The Dutch Island Harbor Formation is about 300 ft (100 m) thick (Skehan and others, 1981). Although this formation has certain sedimentologic similarities to the underlying formations, it may range upward in age beyond Middle Cambrian.

Geologic features of interest here include:

(source: Skehan, Webster, and Logue. Cambrian Stratigraphy, Structural Geology, Narragansett Bay.)

Figure 3: Evolution of folds at Beavertail. White arrows indicate fold produced in first two deformation events while red arrows indicate refolding during third deformation event (as per Skehan and Murray [1979]).