Glacier National Park in Montana is an excellent example of a glacial landscape.
While very few glaciers exist today, scientific evidence indicates that much of the Earth was covered by glaciers in the past. Much of the landscape in the northern and western parts of North America has been shaped by these monolithic sheets of ice. The last glacial age ended about 10,000 years ago. Does this Spark an idea?
Types of Glaciers
A glacier on Mt. Kilimanjaro has shrunk 26 percent since the year 2000.
A glacier is a large, persistent body of ice that flows very slowly due to its weight and the forces of gravity. Glaciers are formed by recrystallized snow and sleet over millions of years. Due to their immense size, glaciers erode soil and rock, transport sediment and also deposit new sediment and glacial debris.
Alpine, or mountain glaciers are relatively small glaciers that form at high altitudes. Continental glaciers, or ice sheets, cover large swaths of land. Today, continental glaciers are found in Greenland and Antarctica. The glacial ice in these two areas comprise 95 percent of the Earth’s modern glaciers. Evidence from past glaciers, however, is easily recognizable in many landscapes.
Small-Scale Glacial Erosion
The Margerie Glacier in Alaska is still shaping the mountains around it.
Glacial striations are one of the most recognizable glacial landscape features. Striations are formed when the glacier picks up rock and sediment as is flows. These debris are incorporated into the glacier. As the large mass of ice flows over rock, the rocks and sediment scratch long, deep grooves into the rock. This is called scouring. Sometimes the underlying bedrock is polished smooth when small pieces of sediment act as sandpaper. This is called “glacial polish.”
The other erosional process that occurs is called “plucking.” Ice wedges itself into rock cracks and crevasses. As the glacier moves, it plucks off pieces of rock from these joints and they’re incorporated into the ice. A rocky outcropping affected by plucking is smooth on one side and jagged on the other side. This asymmetrical rock formation is called “roche moutonn e.”
Glaciers shaped the famous Matterhorn over many centuries.
A variety of glacial landforms exist. A cirque is a bowl-shaped depression on the side of a mountain created by glacial abrasion and plucking. Glacial valleys are identified by the signature “U” shape. This differs from a valley created by a river, which has a “V” shape.
Arêtes occur when two U-shape valleys are next to each other. The sharp ridge in between the valleys is called an “arête.” Several small cirques in close proximity may form a sharp, pyramidal point called a “horn,” the most famous example being the Matterhorn in the Swiss Alps.
Hanging valleys are are created when a smaller glacier meets a larger glacier. The small glacier cannot erode all the way to the floor of the main valley. Waterfalls, such as Yosemite Falls in California, are a hallmark of hanging valleys. Fjords are another water feature influenced by glaciers. These narrow inlets that occupy a space where a valley glacier once met the ocean.
The large formation in this photo is a glacial moraine.
Glaciers also leave evidence in the form of deposits. Glacial till, which is made of of small rock fragments, is deposited by the moving ice. A glacial moraine is a landform made up of accumulated till. Boulders transported over a large distance by the glacier and deposited on different bedrock are called “erratics.” Scientists examine the location of erratics to determine the flow of ancient glaciers.
Glaciofluvial deposits are those carried away by the meltwater. An outwash deposit or outwash plain forms in front of the glacier. Small, braided streams are often present in the outwash landscape. An esker is a stream bed formed below a glacier.
Kames and kame terraces are formed by sediment that settled in lakes and streams on top of the glacier. When the glacier melted, this sediment was deposited on the valley floor. On the other hand, depressions that form underneath a glacier and then become lakes when glacier melts are called kettle lakes. Minnesota’s many lakes are kettle lakes.