Mount Desert Island - Geology

Geology

Mount Desert Island is rich in geological history dating back to approximately 550 million years ago. The earliest formation on the island is the Ellsworth Schist Formation, which was a sea-floor mud deposit created during the Cambrian period by volcanic ash. During the Ordovician period the collision of Laurentia, Gondwanaland, and Avalonia, referred to as the Acadian orogeny, caused the formation to fold, thrust, and uplift above sea level, where later layers were eroded away and the schist was exposed. The Bar Harbor Formation, which is made up predominantly of sands and silts, and Cranberry Island Formation, made up from volcanic ash and magmatic debris, occurred under similar circumstances in the Silurian and Devonian periods, and were deposited on top of the Ellsworth Schist. However, due to less tectonic activity at that time, their deformation was less severe.

As mentioned above, quarrying of granite was historically an important industry. Due to orogenic activity during the Devonian, Mount Desert Island has three granite units, the Cadillac Mountain granite, the fine grained Somesville granite, and the medium grained Somesville granite. Surrounding DCg, is a zone of brecciated material, known as DSz (Devonian Shatter Zone).

Most recently, Mount Desert Island was host to the Laurentide Ice Sheet as it extended and receded during the Pleistocene epoch. The glacier left a number of visible marks upon the landscape, such as Bubble Rock, a glacial erratic carried 19 miles by the ice sheet from a Lucerne granite outcrop and deposited precariously on the side of South Bubble Mountain in Acadia National Park. Other such examples are the moraines deposited at the southern ends of many of the glacier-carved valleys on the Island such as the Jordan Pond valley, indicating the extent of the glacier; and the beach sediments located in a regressional sequence beneath and around Jordan Pond, indicating the rebound of the continent after the glacier's recession approximately 25,000 years ago.