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The Ovens, Bar Harbor
Since the days of horse and buggy, visitors to the northeastern coast of Mt. Desert Island have been intrigued by the curious craggy cavities called The Ovens. Located on the headland between Salsbury Cove and Hulls Cove, The Ovens are best approached by kayak (Figure 1) at low to mid-tide. They are largely submerged at high tide, and there is no public land access.
From their location, it is clear that The Ovens have been carved out by the sea, and yet there must be something about the geology here that is different from other places around the island, to produce these curious cavities (Figure 2). Sure enough, three geologic features come together in these rocks in just the right way: horizontal rock layers, vertical fractures, and small inclined faults.
Horizontal Layering in the Bar Harbor Formation
The bedrock at The Ovens is part of the Bar Harbor Formation. It is a series of layered rocks, primarily siltstone and shale, that have been baked by intrusion of the Cadillac Mountain granite which underlies most of the interior of Mount Desert Island. The Bar Harbor Formation is named for exposures along the shore in the town of Bar Harbor, and outliers of the formation are found in other small areas around the edge of the island. In most places, the many layers that comprise the Bar Harbor Formation are each a few inches thick. This sort of layering is found in the rocks just east of The Ovens (Figure 3). At The Ovens, however, the rock is more massive and uniform, so the layers are difficult to distinguish. It appears that there is a layer a few feet thick that is more easily eroded than the overlying rock. It is this more easily eroded layer that has been excavated by the sea to produce The Ovens (Figure 4).
While the layering is approximately horizontal, the rock is cut by a set of vertical fractures. These fractures have several orientations, best seen by looking down on a horizontal surface (Figure 5). In places along the cliffs, one orientation is prominent, causing the rock to break into flat, vertical faces (Figure 6). It is the massive nature of the rock here that allows the fractures to be so long and straight. Where layering is more prominent, the vertical fractures are shorter and the rock breaks into smaller blocks. The intersection of various vertical fractures gives the rock a natural column-like structure that has lent the name "The Cathedral" (Figure 7) to one prominent rock headland.
Intersection of Layering and Fractures
Behind "The Cathedral" is a tunnel through the rock over 10 feet high (Figure 8). This narrow tunnel is at the intersection of the weak horizontal layer and a prominent vertical fracture. The rock above the hole is strong enough to remain intact. A close look at the weaker layer shows white streaks of volcanic debris that may explain why this layer is more easily eroded (Figure 9). Sighting along this tunnel, it is clear that it is on a major vertical fracture, because it lines up with other steep vertical faces farther down the shore.
A third feature important in the formation of The Ovens is faulting. In places where the layering is well developed, it is clear that the layers have been offset along small faults (Figure 10). Where the layers are all about the same strength, the faults do not make a notable difference in the erosion pattern (Figure 11). But at The Ovens, where there is a massive, stronger rock layer above a thinner, weaker one, the faults break up the weaker layer enough that it can be excavated by the sea (Figure 12).
Putting it all Together
The Ovens are a fascinating example of differential erosion, where certain rocks are more easily eroded than others. In this case, the mechanical breaking and erosion by the ocean is much more effective where there is a combination of a weaker rock layer overlain by a more resistant one, vertical fractures to break the weak rock into smaller pieces, and minor inclined faults that effectively undercut the roofs of The Ovens.
The Geology of Mount Desert Island - A Visitor's Guide to the Geology of Acadia National Park, by R.A. Gilman, C.A. Chapman, T.V. Lowell, and H.W. Borns, Jr., Jr., 1988, 50 p., 28 figs., 2 color maps (1:50,000). Popular guide to park geology. Includes color bedrock map of Mount Desert Island and black-and-white maps of Isle au Haut and Schoodic Point.
Text and photos by Henry Berry.
Archival post card courtesy of Wally Gray at Emery's Cottages, Bar Harbor.
Originally published on the web as the July 2003 Site of the Month.
Last updated on October 6, 2005
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