Bedrock Geologic Map of Rhode Island

The smallest state, Rhode Island has been lovingly mapped at 1:100,000 scale. The map explanation is so elaborate that it's hard to boil down to something simple. If you live in the area, this inexpensive map is well worth buying from the Rhode Island Geological Survey.

Like the rest of New England, Rhode Island is largely covered by sand and gravel dating from the latest ice age. Bedrock crops out in small scattered occurrences, or in roadcuts and building foundations and mines. This map ignores the surface coating for the living rock beneath, except for a small area on the coast and on Block Island to the south, in Long Island Sound.

The whole state lies in the Avalon terrane, a block of crustal rocks that once lay off the North American continent in Late Proterozoic time more than 550 million years ago. Two chunks of that terrane are separated by a major shear zone running down the west edge of the state. The Hope Valley subterrane is on the west (in light brown) and the Esmond-Dedham subterrane is on the right covering the rest of the state. It in turn is broken in two by the light-toned Narragansett basin.

These subterranes have been intruded by granites and other igneous rocks in two main orogenies, or mountain-building episodes. The first was the Avalonian orogeny in the Late Proterozoic, and the second includes the Alleghenian orogeny, from the Devonian through the Permian Periods (about 400 to 290 million years ago). The heat and forces of those orogenies left most of the state's rocks metamorphosed. The colored lines in the Narragansett basin are contours of metamorphic grade where this can be mapped.

The Narragansett basin formed during this second orogeny and is filled with largely sedimentary rocks, now metamorphosed. Here is where Rhode Island's few fossils and coal beds are found. The green strip on the south shore represents a later Permian intrusion of granites near the end of the Alleghenian orogeny. The next 250 million years are years of erosion and uplift, exposing the deeply buried layers that now lie on the surface.

The 2000x2900 pixel version of this map (2 MB) shows much more detail along with an explanation of geologic symbols. The chart of rock units (1600x1750, 600 KB) will help you identify each color zone. The giant-sized 3200x4600 pixel version (3 MB) of this map has all the detail of my original scan.

More about Rhode Island Geology

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