Glaciated Coasts

Glaciated and recently deglaciated coastlines offer a variety of environments that are significantly different from other coastal features so far discussed. some coastlines, such as many in Antarctica, Greenland, and Alaska, have active glaciers that reach the sea, whereas other coasts, such as from New England northward into Canada, scandinavia, and parts of the Far East have recently been deglaciated (within the past 18,000 years).

The primary effects of glaciers on coastlines include the carving out of wide u-shaped glacial valleys and erosion of loose material overlying bedrock, the deposition of huge quantities of sediment especially near the termini of glaciers, and lowering of global sea levels during periods of widespread glaciation. In addition, many coastal areas that had thick ice sheets on them were depressed by the weight of the glaciers, and have been slowly rebounding upward since the weight of the glaciers was removed. This glacial rebound causes coastal features to move seaward and former beaches and coastlines to be uplifted.

When glaciers move across the land surface, they can erode bedrock by a combination of grinding and abrasion, plucking material away from the bedrock, and ice wedging where water penetrates cracks, expands as it freezes, and pushes pieces of bedrock away from its base. The material removed from the bedrock and overburden is then transported with the glacier to its end point, often at the coast, where it may be deposited as a pile of gravel, sand, and boulders known as a glacial moraine. some glacial moraines are relatively small and outline places where individual glaciers flowed out of valleys and ended at the sea. These form where the glaciers were relatively small and were confined to valleys. other glacial moraines are huge, and mark places where continental ice sheets made their farthest movement southward, depositing vast piles of sand and gravel at their terminus. on the eastern seaboard of the united states, New York's Long island and Massachusetts's Cape Cod, martha's Vineyard, and Nantucket island represent the complex terminal moraine from the Pleistocene ice sheets. in places like New England that were covered by large continental ice sheets, the glaciers tended to scour the surface to the bedrock, leaving behind irregular and rocky coasts characterized by promontories and embayments, islands, but only rare sandy beaches.

Depositional features on deglaciated coasts are varied. Glacial drift is a general term for all sediment deposited directly by glaciers, or by glacial meltwa-ter in streams, lakes, and the sea. Till is glacial drift that was deposited directly by the ice. it is a non-sorted random mixture of rock fragments. Glacial marine drift is sediment deposited on the seafloor from floating ice shelves or bergs, and may include many isolated pebbles or boulders that were initially trapped in glaciers on land, then floated in icebergs that calved off from tidewater glaciers. These rocks melted out while over open water, and fell into the sediment on the sea bottom. These isolated dropstones are often one of the hallmarks of ancient glaciation in rock layers that geologists find in the rock record. stratified drift is deposited by meltwater and may include a range of sizes, deposited in different fluvial or lacustrine environments.

Terminal or end moraines are ridgelike accumulations of drift deposited at the farthest point of travel of a glacier's terminus. Terminal moraines may be found as depositional landforms at the bases of mountain or valley glaciers marking the locations of the farthest advance of that particular glacier, or may be more regional in extent, marking the farthest advance of a continental ice sheet. There are several different categories of terminal moraines, some related to the farthest advance during a particular glacial stage, and others referring to the farthest advance of a group of or all glacial stages in a region. Continental terminal moraines are typically succeeded poleward by a series of recessional moraines marking temporary stops in the glacial retreat or even short advances during the retreat. They may also mark the boundary between a glacial outwash terrain and a knob and kettle or hummocky terrain toward more poleward latitudes from the moraine. The knob and kettle terrain is characterized by knobs of outwash gravels and sand separated by depressions filled with finer material. many of these kettle holes were formed when large blocks of ice were left by the retreating glacier, and the ice blocks melted later, leaving large pits where the ice once was. Kettle holes are typically filled with lakes; many regions characterized by many small lakes have a recessional kettle hole origin.

Glacial erratics are glacially deposited rock fragments with compositions different from underlying rocks. in many cases the erratics are composed of rock types that do not occur in the area they are resting in, but are found only hundreds or even thousands of miles away. many glacial erratics in the northern part of the united states can be shown to have come from parts of Canada. sediment deposited by streams washing out of glacial moraines, known as outwash, is typically deposited by braided streams. many of these glacial outwash braided streams form on broad plains known as outwash plains. When glaciers retreat, the load is diminished, and a series of outwash terraces may form.

Drumlins are teardrop-shaped accumulations of till that are up to about 150 feet (50 m) in height, and tend to occur in groups of many drumlins. These have a steep side that faces in the direction that the glacier advanced from and a back side with a more gentle slope. Drumlins are thought to form beneath ice sheets and record the direction of movement of the glacier. Drumlin coasts are found on the eastern side of Nova scotia and in Massachusetts Bay, including many in Boston Harbor. A final common depositional landform of glaciers found on many coasts are eskers, elongate ridges of sands and gravel that may extend many miles but be only a few tens of feet (several m) wide. These represent the paths of meltwater streams that flowed inside and underneath the glaciers, depositing the sand and gravel in the stream bed, which got left behind as the glacier retreated.

Coastlines that were mountainous when the glaciers advanced had their valleys deepened by the glaciers carving out their floors and sides, creating fjords. Fjords are steep-sided glacial valleys that open to the sea. southern Alaska has numerous fjords that have active tidewater glaciers in them, which are now experiencing a phase of rapid retreat. The Hudson River valley and Palisades just north of New York City comprise a fjord formed in the Pleistocene, and many fjords are found in scandinavia, New Zealand, Greenland, Chile, and Antarctica.

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The Basic Survival Guide

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