Sea-Level Change on Mt. Desert Island
Figure 1 |
The level of the ocean changes in response to many factors and on a variety of time scales. In Maine, evidence for significant, long-term changes in the level of the sea have been recognized since at least 1836 when Charles Jackson noted seashells high above the ocean in what is now called glacial-marine mud (the Presumpscot Formation). These were attributed to Noah's Flood by all observers until the Glacial Theory was advanced by Louis Agassiz in the middle of the 19th century. This theory explained many surficial geological features in high-latitude locations as a consequence of a great Ice Age. It was recognized by many scientists that the water for the great glaciers must have come from evaporation of seawater and caused a lowering of sea level. It was further observed that the weight of the ice near coastal areas was probably sufficient to depress the land and permit an invasion by the sea as the glaciers melted. The first place where abundant higher-than-present sea-level indicators were observed in Maine was on Mt. Desert Island (Figure 1). |
The motto of geology is "The present is the key to the past." Before delving into ancient sea level locations, it is worth noting some coastal features which characterize the contemporary shoreline. At Monument Cove, directly below the Park Loop Road from the Gorham Mountain parking lot, there is a magnificent boulder beach (Figure 2). The boulders are worked out of the cliffs by waves and tumbled into near-spherical shapes during storms (Figure 3). At the distant (eastern) end of the cove a sea stack exists (Figure 3). This column of granite was formerly attached to the mainland, but wave erosion, by focusing on the vertical fractures in the rocks, has separated it (Figure 4). At the back of the cove stands a vertical sea cliff that has retreated landward as blocks of granite were quarried away by waves (Figure 2). Another erosional feature, not seen in Monument Cove, but common in Acadia National Park, is a sea cave (Figure 1, Figure 5). Sea caves, like sea stacks, form where fractures are eroded by waves near the high-tide line (Figure 4). The shape of the fractures and their spacing determines whether sea caves, sea arches, or sea stacks will form. None of these features are long-lived, and the waves that create them soon lead to their destruction. Most of the rocks on the Maine coast have too many fractures to allow these features to form or exist for very long, but the granite of Mt. Desert Island appears ideal for their formation.
All of this was known to late 19th-early 20th century geologists, and "Chimney Rock" was recognized as solid evidence for a higher-than-present sea-level position in the late 19th century (Shaler, 1889) (Figure 6). The raised sea stack is about 60 m above present sea level and remarkably similar in appearance to the Monument Cove sea stack (Figure 3). Chimney Rock is such striking testimony to a raised sea level that wealthy Mt. Desert Island summer resident, John Rockefeller, was interested in it, and Chimney Rock was described in local newspapers. Most impressive was the uppermost stone, which must have been last turned by a great storm perhaps 13,000 years ago. Unfortunately vandals read of Chimney Rock and managed to topple the uppermost stone. As testimony to his fondness for science, Mr. Rockefeller had a crane brought up the nearby Carriage Path and replaced the fallen stone.
Behind Chimney Rock and stretching for more than a kilometer is a pronounced sea cliff upon which the Carriage Path is built. Below Chimney Rock, the uneven ground is composed of innumerable rounded granite cobbles (Figure 7). These are strikingly reminiscent of the boulder beach at Monument Cove.
Figure 8 |
On Cadillac Cliffs, directly up Gorham Mountain from Monument Cove (Figure 1), similar features exist. Here one may also find a large sea cave cut into the raised granite sea cliff (Figure 8). In the back of the cave is lodged a rounded granite boulder, that was presumably hurled into the cave by a storm 13,000 years ago. The sea cliff surrounding the cave is 10 m high and seaward of the cliff are rounded boulders as at Monument Cove. All of these raised coastal features exist between 60-70 m above present sea level, the highest elevation the sea reached in this area based on other observations in the region. |
Originally published on the web as the March 2002 Site of the Month.
Last updated on October 6, 2005
