Figure 15. Schematic maps and cross sections of a possible plate tectonic interpretation of the geologic history of Mount Desert Island. Cross section (left-hand diagrams) are drawn from left to right through the map views (right-hand diagrams).

A. Cambrian-Ordovician. Sediments of the Ellsworth Schist (COe), mud and ash, were deposited in the Iapetus Ocean along the leading edge of the Avalonian plate. At this time the crust of the Iapetus Ocean began sliding beneath the Avalonian plate. The Ellsworth Schist was then metamorphosed, folded, and uplifted.

B. Late-Silurian-Early Devonian. The crust of the Iapetus Ocean continued to be destroyed beneath the Avalonian plate. The Cranberry Island volcanic rocks (SDci) resulted from partial melting of the oceanic crust; sediments of the Bar Harbor Formation (SDbh) were deposited unconformably on top of the Ellsworth Schist.

C. Middle-Late Devonian. The North American and Avalonian plates collided as the Iapetus Ocean was completely eliminated. Heat generated by this process caused some rocks in the collision zone to melt, resulting in large igneous intrusions at depth.

D. Early Mesozoic. The two plates separated along a fracture which was southeast of the line where the plates joined during collision, leaving part of the Avalonian plate attached to North America. This separation began the development of the present-day Atlantic Ocean. The mountains formed during the collision were subsequently eroded to the level of the Ellsworth Schist and granites.

Note: Details of the earlier history of the North American and Avalonian plates have been omitted.

Last updated on February 22, 2006.