The Geology of Mount Desert Island
A Visitor's Guide to the Geology of Acadia National Park
Shaping of the Landscape by Glacial Erosion
A glacier is a mass of snow and ice that has accumulated on land and is large enough to move under its own weight. Glaciers form where there is a net buildup of snow over many years, and the more deeply buried snow layers become compacted into ice. Glaciers develop in various sizes and shapes. The rivers of ice that occur in high mountain valleys are called alpine glaciers, and the huge pancake-like masses covering nearly all of Greenland and Antarctica are continental glaciers.
Glacial ice expands outward like a pool of cold molasses (but much more slowly), to eventually dissipate because of melting or the breaking away of icebergs into lakes or the ocean. The ice in a continental glacier constantly flows from the center toward its outer margin. The location of the ice margin may advance or retreat depending on the balance between ice accumulation in the glacier's source area and the rate at which it melts away near the margin. It is interesting to note that even when the edge of a glacier is "retreating," the ice may continue to flow forward from the source area toward the glacier margin. At its maximum extent, the margin of the Wisconsin glacier was in the Gulf of Maine, but later ice margin positions indicate retreat of the glacier as ice wastage exceeded ice accumulation during deglaciation.
Erosion by glacial ice flowing in a southerly direction was responsible for the major landforms that we see today in Acadia National Park. The Wisconsin ice sheet flowed over Mount Desert Island for at least 10,000 years, eroding bedrock by two processes: abrasion and plucking.
Let's consider abrasion first. Rock fragments carried along under the base of the glacier acted like the grit on sandpaper, abrading and scratching the underlying bedrock. This abrasion was stronger on the north sides of hills, which faced against the southward-flowing ice. We cannot say just how much rock was removed by abrasion during the last glaciation, but the average bedrock surface was probably lowered one or two yards by this process.
Today if you visit the north-facing bedrock slopes you'll find the effects of glacial abrasion. Close examination of ledges where soil has recently been removed is likely to reveal a polished and scratched bedrock surface. These scratches are called striations, and plotting their orientations on a map shows us the direction of glacial movement over Mount Desert Island. A large bedrock ledge located across Maine Route 3 from the north end of The Tarn (a pond south of Bar Harbor) provides an excellent place to examine glacially quarried, polished, and striated bedrock (Figure 17 and Figure 18). On ledges where soil was never present, or has been removed for a long time, the rock surface is usually weathered to the extent that striations are not preserved. This is especially true for areas of granite.
Because of weathering, none of the other striations plotted on the surficial geology map are as pronounced as those near The Tarn. Most of the striations occur in very small patches, especially where the drift has just been removed. One might notice striations along hiking trails such as the one leading to South Bubble.
The second type of glacial erosion, plucking, removed large amounts of rock and produced many of the spectacular cliffs that we see in the park today. This process occurred because the ice at the base of the glacier at times required only a slight increase in pressure to cause it to melt. The movement of the ice sheet over the hills on Mount Desert Island produced a slightly higher pressure on the north side of the hills which melted the ice at the bottom of the glacier and allowed water to seep into fractures in the bedrock. This meltwater would freeze again as it moved into zones of lower pressure on the south sides of hills where the ice was no longer riding up over the slope. As the water refroze in rock fractures, it expanded with tremendous pressure and forced the rock apart. After blocks of bedrock were pried loose in this manner, they were then frozen into the glacier and carried away. The southeast side of The Beehive (Figure 19) shows prominent cliffs that were produced by the plucking process.
The combination of glacial abrasion on the north slopes and plucking on the south slopes caused many of the hills and mountains in Acadia National Park to have a distinctively asymmetric profile: gentle and smooth on the north side facing into the ice flow, and steep and rugged on the south side. These hills also tend to be elongated and streamlined in a north-south direction as a consequence of glacial flow and erosion. Such glacially sculpted bedrock hills are called whalebacks or roches moutonnées. The Bubbles, located north of Jordan Pond, are a fine example of these features (Figure 20).
The amount and rate of erosion of any landscape vary depending on the eroding agent (e.g., glaciers, rivers, or waves), the resistance of the bedrock, and time. In the case of Mount Desert Island, the Cadillac Mountain granite has proven to be more resistant than the surrounding rock types and consequently forms the highest terrain on the island. Within this granite there are zones of relative weakness (for example, areas where jointing is especially strong) that have been preferentially eroded by glacial ice. The glacier removed larger volumes of rock from these zones, producing deep valleys with a distinctive U-shape when viewed along their length. Like the whalebacks described above, the U-shaped valleys have a preferred north-south trend, parallel to the direction of former ice flow. They may have developed where preglacial streams had already eroded channels into the weaker parts of the bedrock. These valleys, modified and deepened by concentrated glacial flow, are called dorrs after George B. Dorr, who was instrumental in the founding of Acadia National Park. G. H. Chadwick named these features and suggested that Somes Sound was an ideal example of a dorr. A number of lakes on Mount Desert Island are located in such valleys (Figure 21). The profile of the island's mountains and valleys is especially well seen on any of the boat trips along the south side of the island.
Most of the rock fragments that are eroded, incorporated into the ice, and transported at the base of a glacier are deposited less than two miles from their source. However, fragments eroded from hillsides or hilltops in areas of high relief, such as Cadillac Mountain, are often incorporated high in the ice as the glacier moves away from the side or top of the hill. This material may travel for tens or even hundreds of miles before being deposited. Many of the glacially plucked rocks from Mount Desert Island can still be found on the island today, although they have been displaced to the south from their original positions. Others were carried out into the Gulf of Maine. By the same token, we find foreign rock fragments on the island, carried there by ice from the mainland north of Mount Desert. For example, numerous gray granite boulders which contain large rectangular white crystals of feldspar are found on top of Cadillac Mountain. These boulders were transported to that and other locations on the island from outcrops of Lucerne granite, located 19 miles to the north. The famous balanced boulder called Bubble Rock on South Bubble, just north of Jordan Pond, is the best known of these far-traveled erratics (Figure 22).
Glacially transported boulders are commonly mixed with silty to sandy rock debris released from the melting ice, forming a deposit called till. Most of the rock debris carried by the glacier was concentrated near the base of the ice, where it was subjected to intense grinding and crushing. Some of this pulverized rock material was deposited beneath the moving ice sheet. The resulting basal till is generally a compact, fine-grained sediment containing many striated stones. Elsewhere, the material carried by the glacier was released into nearby water (see the description of deglaciation) where it flowed as a slurry. Glacial sediment deposited in this manner may appear very similar to basal till, so both types are combined into a single "till" unit on the map. Appreciable thicknesses of till occur only in a few low areas of Mount Desert Island (Wt on the surficial geologic map - pdf format).
Last updated on January 11, 2008