The geology of Maine as a whole has remained practically unknown until the present time in spite of the fact that study of it was begun nearly a century ago. Several major difficulties have caused this result; first is the omnipresent cover of glacial drift which leaves exposed less than 1 per cent of the bedrock; second, the enormous tracts of lake and swamp; third, the dense forest cover which over half the State has remained in private hands; finally, the huge areas of slate and phyllite standing on edge for many thousands of square miles.

The map shows the sedimentary rocks divided into pre-Cambrian, Cambrian-Ordovician, Silurian, Devonian, and Mississippian. The igneous rocks are divided into pre-Cambrian granite, Silurian metadiabase, Devonian granite, rhyolite, diabase, trachyte, and tuff, and Carboniferous granite.

These rocks are distributed so as to form a general belt of pre-Cambrian gneiss cut by late Paleozoic granites near the coast, and a central belt of similar granites cutting the Silurian and Devonian and dividing the State nearly in half. These two granite belts tend to coalesce at the southwest. Around the northeast end of the central granite belt are distributed many separated bodies of Paleozoic effusive igneous rocks, forming the most interesting and varied exhibition of the sort in the eastern United States.

Sedimentary rocks cover most of the State and are intensely deformed, mainly in closed vertical folds. Devonian rocks appear in two main central belts closely overlapping one another and almost crossing the State. At the northeast the synclinoria deepen, the Devonian areas widen, and small outliers appear of the Mississippian rocks that are so common just across the border in New Brunswick.