The Geology of Mount Desert Island
A Visitor's Guide to the Geology of Acadia National Park
Isle au Haut
Isle au Haut is composed for the most part of rocks similar to those on Mount Desert Island: gabbro-diorite, fine-grained granite, and volcanic rocks similar to the Cranberry Island Series. Since all of these rock types have been discussed earlier, we will simply point out some of the best places for you to see them (Figure 13).
As a summertime day visitor to Acadia National Park on Isle au Haut, the part of the island you will mostly likely explore will be the southern end, south of the park landing at Duck Harbor. From Duck Harbor, follow the Western Head Road (used for hiking, not driving) south to Deep Cove. This trail crosses several areas of ledges, but the rock is difficult to study because of the lichen and moss cover. But the exposures at the shore are excellent. From the head of Deep Cove you have a choice of two shore trails to follow, either south toward Western Head and then back to Duck Harbor, or easterly along the Goat Trail toward Squeaker Cove. In either case you will find exposures of rock believed to be related to the Cranberry Island Series.
Following the trail to Western Head, you will see almost continuous exposure of light gray volcanic tuff containing angular fragments, similar to rock seen at the parking area at Seawall picnic area on Mount Desert Island. This rock represents the settling of ash and rock fragments that were blown into the air at the time of an explosive volcanic eruption. In some cases you will find that the rocks are bedded, some beds having more fragments than others, and that these are consistently inclined toward the west at a fairly steep angle (see Figure 14).
If you take the Goat Trail toward Squeaker Cove, you will also see excellent exposures of volcanic tuff. Just before you reach Barred Harbor you will cross the contact of tuff with the fine-grained granite. Near this contact you will be able to find several large pieces of the volcanic rock enclosed in the granite, proving that the granite is younger than the tuff. The trail then leads north from Barred Harbor where it joins the main road. Taking this road to the left brings you back to Duck Harbor, whereas a turn to the right will take you to Eastern Head. This central part of the island is underlain by fine-grained granite, and a hike along the trail leading from the main road to Moore Harbor crosses excellent exposures of this rock.
Gabbro-diorite is best seen along the eastern shore of the island, particularly at Eastern Head. Here the ledges offer excellent views of gabbro-diorite which in some instances show lighter colored layers gently inclined toward the west. These layers have led geologists to the conclusion that the gabbro-diorite, and probably the fine-grained granite as well, form a large sheet of intrusive rock that was injected into the volcanic rocks seen on the west side of the island (Figure 14).
A mixed or transitional type of rock is found in a narrow zone between the gabbro-diorite and the granite. Geologists interpret this to mean that the gabbro-diorite and granite on Isle au Haut are related to the same episode of magma intrusion.
These rock types can be traced over the remainder of the island. Birch Point, at the island's extreme northern end, consists of a metamorphic rock that is of uncertain relationship to other rocks in the area; it may be related to the Ellsworth Schist seen on Mount Desert Island. Burnt Island, just north of Birch Point, consists of the same metamorphic rock and gabbro-diorite, as well as granite that is part of a very large body believed to underlie most of the bay between Isle au Haut and Stonington.
Last updated on January 11, 2008