Subscribe to our newsletter. The map explanation is so elaborate that it's hard to boil down to something simple. If you live in the area, this map is well worth owning. 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. 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.