GEO 101 Environmental Geology

Dec. 10, 2007

Glaciers

 Glaciers form when snow does not melt completely during summer.

Each year more snow accumulates.

This occurs where it is cold

• high latitudes

• high altitudes

 With increasing depth snow is compressed.

• Air is squeezed.

• Ice crystals grow
              snow eventually changes to ice.

 As the  ice thickens, it begins to flow away from the center of accumulation.

Glaciers are dynamic features.

• They need excess snow.

• They flow away from area of thickest accumulation.

• They actively
                          erode,
                          transport and
                          deposit rock and sediment

 Two main types of glaciers

• Continental

• Alpine

The alpine or mountain glaciers are melting rapidly as a result of global warming. Due to their small size many of them may disappear in the next few decades.

 There have been at least seven multimillion year long glacial periods in Earth's History

 During glacial periods glaciers systematically advance and retreat

• Glacial about 100,000 year duration

• Interglacial about 10,000 year duration

 Onset, advance and retreat of glaciers may be due to minor changes in earth’s rotation and revolution around the sun.

 For onset of Glaciation colder summers may be most important

• Sea Level falls with onset of glaciation

• Sea Level rises with melting of glaciers

• During the Pleistocene the continental glaciers in North America and Europe have systematically advanced and retreated. 
  • The cooler or glacial phase lasts about 100,000 years during which time the glaciers in North America wax and wane but do not disappear completely
  • During the warmer or interglacial phase which lasts about 10,000 years continental glaciers are severely restricted in size or do not exist in North America
  The earth is presently in an interglacial phase.

  If interglacial phases last about 10,000 years and this interglacial period began bout 10,000 years ago, when will the next period of glaciation begin?

   

• So far, the present interglacial period, that began about 10,000 years ago, has had relatively steady temperatures. Ideal for agriculture!

• The cycling from glacial to interglacial periods may be due to minor changes in the orientation of the Earth's axis of rotation and in the ellipticity of the earth's orbit around the sun. These changes are referred to as Milankovich cycles.
• Oxygen isotope studies of deep-sea cores which indicate sea water temperatures show a more complete record of the cycling between glacial and interglacial periods than does the record on land. This because each new glacier as it travels across the countryside erases or seriously disturbs the deposits left by earlier glaciers.
• During a glacial period there are relatively large changes in the global temperatures and the continental glaciers advance and retreat many times. During the last glacial period, the Wisconsin, maximum advance of the continental glaciers in North America occurred near the end of the glacial period about 20,000 years ago. At that time the glacier reached the position of Long Island and left the Ronkonkoma and Harbor Hills moraines and their associated outwash plains.

Glacial Features

As the ice in a glacier moves away from the area of accumulating snow the front of the glacier eventually reaches a position where extensive melting takes place. If melting is more rapid than the advance of the ice the front of the glacier retreats. The ice, however always flows away from the center of accumulation. The ice does not flow backward. If the ice at the front of the glacier melts at the same rate as the ice advances, the front of the glacier remains in one position for an extended period depositing the boulders, cobbles, sand, silt and clay that the glacier picked up on its travels.

• The unsorted material deposited without reworking by the glacial water under and near the front of the glacier is called till, which is a heterogeneous mixture of cobbles, sand, silt and clay.
• The many streams on and under the glacier transport the gravel, sand, silt and clay in the till and the underlying sediments and deposit them as stream and lake deposits in front of the glacier. This material is called outwash. The relatively flat surface underlain by outwash is an outwash plain.
• Commonly ice from the glacier is partially or completely buried by till or outwash. When the ice melts it leaves a depression in the topography which is called a kettle. If the base of the depression is below the groundwater table, the kettle fills with water and forms a kettle lake. Lake Ronkonkoma is an example of a large kettle lakes.
• Large boulders left by the glacier which were transported far from where they were picked up are known as erratics. There are many erratics around campus that were picked up by the glaciers and carried here.
• If the glacier leaves a significant ridge at the front of the glacier after it leaves, this is an end moraine. If it forms a ridge at the most extreme distance that the front of the glacier reached, this is a terminal moraine. If it forms a ridge as the front of the glacier retreats, it is a recessional moraine.
• The Ronkonkoma Moraine is considered a terminal moraine.
• The Harbor Hill Moraine is most likely a recessional moraine.
• As a glacier travels across a terrane it often will scour former river valleys, changing their typical V-shape to a U-shape which is typical of glaciers. The Hudson River valley near Bear Mountain is a typical U-shaped valley. The Finger Lakes are glacier scoured U-shaped valleys that are now filled with ground water because the ends of the valleys are dammed.
• As the glaciers scour the bedrock boulders at the base of the glacier cut grooves parallel to the motion of the base of the glacier. The orientation of these grooves allow geologists to determine the direction of the movement of a glacier. There are excellent grooves in the bedrock exposed in Central Park. Erratics can also be used to tell the direction of the movement of a glacier, if they are made up of distinctive rock types exposed only in a limited area.

22, 000 years ago

The surface of Long Island consisting of older Pleistocene and Cretaceous sediments was about 100 feet below present sea level, but sea level at that time was about 400 feet below present sea level.

Long Island was a tundra.

A glacier kilometers thick was approaching.
The shoreline was about 70 miles to the southeast.

Features Produced by Glaciers on Long Island
and in the Metropolitan New York area

This link to the Ice Age in Connecticut is pertinent to Long Island glacial geology

This link gives the glacial history of Long Island Sound

End Moraines

Terminal Moraine  - Ronkonkoma Moraine
Recessional Moraine - Harbor Hill Moraine
Push Moraine (Ice Shoved Moraine) both the Harbor Hill and Ronkonkoma Moraines have ice shove features.

Outwash Plains

To the south of both the Harbor Hill and Ronkonkoma Moraines are outwash plains

Kettle Holes

There are numerous kettle holes in both moraines. Lake Ronkonkoma is considered to be a kettle hole lake.

Grooves

Grooves are common on the bedrock throughout the metropolitan area north of the terminal moraine.

Loess

Loess or wind blown silt is found on top of till or outwash in deposits up to six feet thick. The areas with loess deposits make good farmland on Long Island.