Acadia National Park – Paleozoic rocks

While the Pleistocene ice sheets caused all kinds of dramatic modifications to Acadia National Park, the actual rocks there are lower Paleozoic in age. Because it’s late, and there are a lot of these rocks, tonight I’m only going to go up to the Silurian; I’ll save the Devonian for tomorrow.

The oldest rocks in the park are the Ellsworth Schist (above), which are highly metamorphosed late Cambrian-early Ordovician marine sediments. And I do mean highly metamorphosed; these rocks are messed up!

The next younger unit is the Bar Harbor Formation, which are early Silurian siltstones and sandstones, exposed all around the town of Bar Harbor:

Up close the Bar Harbor also shows some evidence of deformation, but not like the Ellsworth:

At least some of the sandstones still seem to have recognizable cross-bedding, as well as possible load structures (soft sediment deformation caused by compression of water-saturated sediments):

At this point things start to get tectonically exciting. Starting in the Ordovician and continuing into the Pennsylvanian, the Iapetus Ocean was subducting under what is now North America. A subduction zone causes lots of volcanoes, which typically produce high-viscosity felsic and intermediate lavas that result in explosive eruptions (think Mt. St. Helens). Rocks from these eruptions produce tuffs and porphyries, and are exposed on the south side of the park as the Silurian Cranberry Island Series:

Tomorrow I’ll post photos of Devonian igneous rocks, that are very different from these volcanic deposits.

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