Chapter 7.1

 At one time nearly 30% of the earth’s surface was covered in ice. This time period was known as the ice age.  Today, only 10% of the earth’s surface is covered in ice. Most of that is found on either Greenland or Antarctica.  Many of the features that we see today on the surface of the earth were formed in the past by the powerful forces of glaciers.

An Ice Age Short-Faced Bear compared with a Kodiak Brown Bear and a Black Bear.

An Irish Elk from the Ice Age would make our modern day moose look tiny. The antler spread on an Irish Elk could get up to 12 ft. wide! A large set of moose antlers is only 4-6 feet wide.

Long-Horned Bison of the Ice Age were twice the size of the Bison found today in North America.

Similar to the modern day wolf, only 25% bigger. The Dire Wolf may have weighed up to 175 lbs. A large wolf today rarely weighs over 125 lbs.

7-8 ft. long, lbs., and 7 inch long canine teeth. The Saber- Toothed Tiger was a powerful and formidable predator.

Wooly Mammoths and Mastodons were similar in size to modern day elephants only they had much more hair and were well adapted to living in cold climates.

 Whenever or wherever you have more snow accumulate than snow melting year round, you can form a glacier.  There are two zones found on a glacier. The area where snow and ice accumulates is called the zone of accumulation.  The area where snow and ice melt and evaporate is called the zone of wastage or zone of ablation.  Where the two zones meet is called the snowline or equilibrium line.

 There are two main types of glaciers.  Valley Glaciers are glaciers that occupy old stream valleys. As they follow the valleys they carve out old V-shaped stream valleys to form U-shaped valleys.  Continental Ice Sheets are gigantic sheets of ice that cover continents. Because of their enormous size, they do not travel down valleys but rather radiate outwardly from the center of the continents.

The only two continental ice sheets still in existence today are Greenland and Antarctica.

 Because glaciers are made of ice, we don’t typically think of them as moving objects.  However, glaciers are actually constantly moving due to the force of gravity.  Valley glaciers move similar to a river.  Continental glaciers move from their center outwardly in all directions.

Two ways that glaciers move is by plastic flow and basal slip. Plastic flow occurs about 50 meters (150 ft.) below the surface of the glacier. The high pressures cause the ice to be more fluid; a property known as plasticity. Basal slip occurs at the base of the glacier where melt water lubricates the base of the glacier and the glacier slides.

The surface of the glacier does not experience plastic flow and instead remains brittle. Because the surface of the ice is brittle, the ice fractures into deep crevasses.

 Glaciers are constantly changing.  Every winter, new snow falls and adds to the overall mass of a glacier.  Every summer, glacial ice melts and calves off at the glaciers terminus.  Glaciologists keep track of how much mass a glacier gains and loses every year due to accumulation and melting.  The data they keep track of is known as a glacial budget.

If a glacier’s budget is losing ice, the glacier will retreat from the terminus or end of the glacier. If a glacier’s budget is gaining ice, the glacier will advance from the terminus or end of the glacier.

The South Cascade Glacier in Washington State. It is hard to tell from the picture what occurred from 1928 to Perhaps there was a glacial advance. But from 1979 until 2003 there is a drastic change in the glacial mass due to glacial retreat.

 Glaciers can erode in two ways; Plucking and Abrasion.  Plucking is when the glacier actually picks up rocks and boulders and carries them over great distances.  Sometimes glaciers pick up gigantic boulders the size of houses and carry them for miles.

When these large boulders are eventually deposited in strange places we call them glacial erratics.

When glaciers scour and abrade the bedrock underneath them they leave behind deep scratch marks or grooves called striations.

 Once valley glaciers retreat from a previously occupied valley, they leave behind many strange and unusual features.  One major change is that the stream valleys are no longer V-shaped.  They instead take on a new U-shaped profile.  These U-shaped valleys are called glacial troughs.  They resemble large water troughs that animals drink out of.

 When these U-shaped valleys are formed they also cut off side tributaries to produce steep- sided hanging valleys.  Often times you can find waterfalls along the edges of hanging valleys.  In between two U-shaped valleys or two hanging valleys you can often find sharp knife-like ridges or spines called aretes.

 Wherever a glacier originally started, it carves out a deep steep-sided depression.  When the glacier eventually melts away it leaves behind an amphitheater-like bowl or depression where the glacier started.  That bowl or depression is called a cirque.  When several cirques surround a mountain, it forms a steep-sided pyramid-shaped mountain peak called a horn.

Horn in Switzerland

 There are two main types of glacial sediments and debris left as deposits.  Glacial Till – Glacial till is sediment and debris that is deposited directly by the glacier itself. Glacial till is not sorted like deposits from streams. Ice can not sort the till like running water from streams do.  As a result, glacial till is usually a mixture of all sorts of sediment sizes varying from sediment the size of boulders to silt.

 The other main type of glacial deposit is called Stratified Drift.  Stratified Drift – Stratified drift is the sediment and debris that is deposited from the melt water produced from a glacier.  Unlike the sediments from glacial till, the melt water from a glacier produces sediments that are well sorted into layers.  Because stratified drift is sorted it resembles sediments deposited by streams.

 Layers, piles, and ridges of glacial till left behind by glaciers are called moraines.  As glaciers move, some of the till is pushed off to the side, producing lateral moraines.  When two lateral moraines converge, like when two glacial troughs converge, the two lateral moraines form a medial moraine.

 At the terminus or farthest end of a glacier, end moraines will form.  There are two types of end moraines.  A terminal end moraine forms at the farthest point that a glacier advances to.  Recessional end moraines form at places in between where a glacier may have stopped for a long period of time.

 Ground moraines are deposits that form in between terminal and recessional moraines.  Outwash plains are made from the deposits of stratified drift that form from the melt water of the glaciers.  Kettle lakes form when large blocks of ice from the glacier melt and leave behind a large depression that forms a lake.

 Drumlins are streamlined hills made of till that are steeper on one end because the glacier plowed it over.  The tapering end of a drumlin points to the direction the glacier travelled.  Eskers are snake-like ridges made of stratified drift deposited by streams flowing in tunnels underneath the glacier.