Giant boulders seemingly out of place in their environment, were thought to be the “putting stones” of the gods in medieval times. Large potholes were the devil’s punch bowls. There were other demonic interpretations for what we now know, are glacial phenomena (Menzies, 1995).
Geologists have studied modern glacial environments to understand and make inferences regarding the past. Core sampling of glacial ice in Greenland and Antarctica tells us volumes about the Earth’s glacial history. The more we understand about the past, the better we can understand the present and predict the future.
Early in the 19th century, Louis Agassiz was the first scientist to compile persuasive evidence that glacial ice at one time covered large parts of the northern continents. He argued that polished and scratched bedrock and piles of sand and rocks in the Alps were evidence of a glacial history. He believed that huge boulders much too large for flowing streams to have carried could only have been left by retreating glaciers.
Agassiz’s theories explain such strange phenomena as the gravel deposits, polished and striated rocks found in the northeast (Erickson, 1996). They explain the landscape found on the areas of Long Island to Cape Cod.
The shape of Long Island is continuously being sculpted by ocean and atmospheric conditions. The dramatic destruction of barrier beaches and erosions of cliffs and sea- walls however, are almost inconsequential when compared to the changes over geologic time.
The Laurentide Ice Sheet developed the island’s fish like shape over 20,000 years ago. Spreading over eastern North America, this vast continental glacier’s source was eastern Canada (Sirkin, 1996).
During the Paleozoic era, 500 million years ago, two major continental blocks were developed. Laurasia in the northern hemisphere was positioned along the equator. Gondwana sat in the southern hemisphere. Collisions between the components of Laurasia created the mountain ranges found between Europe and Asia as well as those between Europe and North America. The continental margin where western Long Island would form will not emerge for another 300 million years.
Laurasia and Baltica gradually collide in the Devonian period, 380million years ago. The process ends after 130 million years, with the formation of the supercontinent Pangea in the Permian period 300 million years ago. Rifting began again 100million years later at the end of the Triassic period.
The Atlantic Ocean formed as Pangea breaks apart. Sediments eroding from Paleozoic mountains were deposited on the margins of North America. By the end of the Cretaceous period 65 million years ago, a wedge of sediments including stratified sand, silt and clay up to 20,000 feet thick has been deposited on top of the Paleozoic basement (Sirkin, 1996).
Bedrock on top of which Long Island has formed is at depths of 100-600 feet in northwestern sections, and at 1400-2000feet in central sections of the island. These data derived from well logs show a 1o dip to the south (Sirkin, 1991).
A Cretaceous conglomerate and sandy unit known as the Lloyd aquifer overlies the bedrock. Above the Lloyd aquifer is the clay of Raritan confining unit. Cretaceous sands and clay above the Raritan clay form the Magothy aquifer (fig. 10).
During the subsequent Pleistocene, glaciers deposited an assortment of materials. Deposition ended with the advance of the late Wisconsin glacier 20,000 years ago. The ice remained here for about 1500 years forming end, interlobate and recessional moraines at the front of each of three lobes. The Hudson, Connecticut, and Connecticut/Rhode Island lobes formed Long Island, the outer islands and Cape Cod (Sirkin, 1991).
Glacial periods have been common
during Earth’s 4.5 billion year history. Glacial erosion and deposition have an
important effect on us. These deposits over lowland areas have provided rich
farmland. They have also given us vast reserves of sand and gravel, vital to
the construction industry in addition to local concentrations of valuable
metals, and of course our beautiful beaches (Hambrey, 1992).
Glaciers are often thought of as “rivers of ice”. This is true in the sense that they do flow in response to gravity. For a glacier to form, climate conditions must allow for successive layers of snow to accumulate without melting through the summer. Eventually the lower layers of snow which is about 90% air, will compress first to firn, (about 30% air) and finally to ice, (about 1%air). Under the pressure of its own weight and the force of gravity, the glacier will begin moving downhill.
You can think of glaciers in terms of profit and loss. The
northern part receives new snow, which does not melt through the summer, and is
called the area of accumulation, (profit).
Continuing downward we find the wet snow zone. Here most of all the winter snow is wet and slushy. In polar areas this slush will refreeze in winter to form superimposed ice. The boundary between the superimposed ice and the wet snow zone is called the firn line. The lower limit of superimposed ice marks the equilibrium line. In temperate regions the firn line and the equilibrium line are in about the same location. Here profit equals loss.
Below the equilibrium line, all of the previous winter’s snowfall has melted as well as an increasing amount of ice. Here in the ablation area, we have a net loss of ice and snow (Hambrey, 1992).
It is important to realize that the glacier is always moving forward while accumulations add to its mass above. If the rate of melting in the ablation area exceeds the rate of movement downhill, the glacier is said to recede. The glacier ice never actually moved backward or toward the north.
As the face of the glacier melts, unsorted sediments called glacial drift are deposited along a curved front forming a moraine. The deposition representing the farthest advance of the glacier is the terminal moraine. When the glacier pauses in its retreat long enough to deposit substantial sediment along another front, it is called a recessional moraine.
As a glacier moves sliding on its bed, geothermal gradients and friction cause basal ice to melt. Water is released at the base of the face of the glacier. In addition, large quantities of melt water released during warmer summer seasons travels down cracks in the surface ice, runs through channels within the glacier and is released at the glacial face as well. In temperate glaciers most water reaches the bed well before the snout.
Melting is due primarily to air temperature, although even
in sub-zero temperatures melting occurs due to intense radiation. Areas covered
with darker material will melt faster as this material absorbs radiation more
efficiently. The speed of the water draining from a glacier can range from 0.8
– 1.8m/sec. This depends on the rate of melting and the interconnectedness of
englacial channels. Sub glacial streams can cut deep channels into bedrock. It
is apparent that hydrology is a critical factor in our understanding of glacial
processes. (Menzies, 1995)
Melted water carries sediments incorporated into the body of the glacier from the surface and from erosion occurring at the base. Valley glaciers will also collect sediments carved from and falling from the walls of the valley through which they travel.
Debris or drift traveling through the glacier is modified by
contact with the ice and with other debris. It will move toward the base of the
glacier and then slightly upward toward the snout. Upon reaching the glacial
bed, the debris becomes the tool by which the glacier carves its bed. The
grinding of drift at the glacial bed generates much fine material including
clay and silt. This “rock flour” gives a characteristic milky appearance to
glacial streams.
Depositional landforms are created by sediments delivered by meltwater as well as those deposited by ice melting at the margins and interfaces of glaciers forming the moraines.