The westernmost part is the least disturbed. It contains limestone and mudstone from the seas near the ancient Taconic mountain-building episode (orogeny), crumpled and uplifted by later events but not appreciably metamorphosed. Its eastern edge is a major fault called Cameron's Line.
The middle of the state is the Iapetus terrane, oceanic volcanic rocks that erupted during the opening of a pre-Atlantic ocean in the early Paleozoic. The rest, to the east of a line running from roughly the west corner of Rhode Island to the northeastern coast, is the Avalonian terrane. It is a former chunk of Gondwanaland. Both the Taconian and Iapetus terranes are shown with dotted patterns that signify significant "overprints" of later metamorphism.
Both terranes were sutured to North America during a collision with Baltica, which closed the Iapetus ocean during the Devonian. Large bodies of granite (random pattern) represent magmas that once fed great volcano chains. At that time Massachusetts probably resembled southern Europe, which is undergoing a similar collision with Africa. Today we are looking at rocks that were once deeply buried, and most traces of their original nature, including any fossils, have been wiped out by metamorphism.
During the Triassic the ocean we know today as the Atlantic opened up. One of the initial cracks ran through Massachusetts and Connecticut, filling with lava flows and redbeds (dark green). Dinosaur tracks occur in these rocks. Another Triassic rift zone is in New Jersey.
For more than 200 million years after that, little happened here. During the Pleistocene ice ages, the state was scrubbed by a continental ice sheet. The sand and gravel created and carried off by the glaciers formed Cap Cod and the islands Nantucket and Martha's Vineyard. See a gallery of Massachusetts geological attractions.
Many local geologic maps in Massachusetts are available for free download from the Office of the Massachusetts State Geologist.
More Massachusetts resources on About.com:
Massachusetts Geography, State Symbols & Facts
Massachusetts National Parks
Massachusetts State Parks
Massachusetts Bed & Breakfasts