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Tales of the Coast
America's Coasts

Glaciated Coast

During the Pleistocene epoch, massive continental glaciers, similar to the present Greenland and Antarctic ice caps, covered broad parts of the continents. The glaciers waxed and waned in cycles, probably because of climatic variations, causing vast changes to the morphology of coastal regions in the northern latitudes. As a result, glacially modified features dominate the northern coasts and continental shelves, although in many areas marine processes have reworked the shore and substantially modified the glacial imprint.

The most spectacular erosion forms are drowned glacial valleys known as fjords. The over-deepened valleys were invaded by the sea as sea level rose during the Holocene epoch. Today, fjords retain the typical U-shaped profile that is also seen in formerly glaciated mountain valleys. Fjords and other drowned glacial erosion features give Maine a spectacular, rugged coastline.

A fundamental division of coastal characteristics occurs along the Atlantic coast of North America due to the presence of glacial moraines. The Wisconsin terminal moraine formed a prominent series of islands (i.e., Long Island, Block Island, Nantucket, and Martha's Vineyard) and offshore banks (Georges and Nova Scotian Banks). South of the moraine, the topography is flatter and more regular, except for piedmont streams, which intersect the coastal plain.

Glaciated coasts typically display a greater variety of geomorphic forms than coasts in warmer latitudes. The forms include purely glacial, glacio-fluvial and marine types. Complexity is added by marine reworking of sediments, which can produce barriers, shoals, gravel shores and steep-cliffed shores. Because of the steep slopes of many glaciated coasts, slumping and turbidity flow are major erosive agents. In northern latitudes, the shallow seafloor is gouged by icebergs.

Because glacial ice is studded with rock fragments plucked from the underlying rock, a moving glacier performs like a giant rasp that scours the underlying land surfaces. This process, driven by the great size and weight of the ice sheets, caused enormous erosion and modification to thousands of square kilometers during the Pleistocene epoch.

As a glacier moves, huge amounts of sediment are incorporated into the moving mass. When the ice melts at the glacial front's farthest advance, the sediment load is dropped. Although the major part of the transported material is dumped in the form of a terminal moraine, some sediments are carried further downstream by meltwater streams. The result is a number of distinct geomorphic features such as drumlins, fjords, moraines and outwash plains that may appear along the coast or on the submerged continental shelf.

The Alaska Panhandle is a rugged, deep coast defined by glacial features as well as tectonic activity. It consists of deep fjords and many glaciers, which carry large amounts of gravel and rock to the coast. This material forms outwash fans and gravel beaches where the glaciers reach the ocean and deposit loads of sediment. Volcanic eruptions on the western side of this coast also provide sediment, and earthquakes cause changes in the elevation of the land.

The ocean along this area of the Alaskan coast generally does not freeze. The waves therefore constantly re-work glacial and volcanic sediments.

Glaciated coast
Source: National Oceanic & Atmospheric Adminstration (NOAA)

Glaciated Coasts

Coasts altered by glaciers usually have offshore regions that are highly dissected by relict drainage systems. These sinuous stream channels display highly irregular and varied topography. They are composed of sediment types ranging from outwash sand and gravels, to till, which consists of many types of intermingled sediments from clay to boulders.

Pleistocene Period: The geological period from about 1.8 million years ago to 8,000 years ago.

Holocene Period: The most recent geological period from about 8,000 years ago to the present.

Glaciated Coastal Variability

Slump: A movement along a curved surface in which the upper part moves vertically downward while the lower part moves outward.

Turbidity Flow: A flowing mass of sediment-laden water.

Gulf of Alaska

The Gulf of Alaska contains many volcanoes, which continue into the Aleutian Islands Chain.