High School Earth Science/Glacial Erosion and Deposition
Today glaciers cover about 10% of the land surface on Earth, but there have been times in Earth's recent history when glaciers have covered as much as 30% of the land surface. Around eight to six hundred million years ago, geologists believe that almost all of the Earth was covered in snow and ice. So today, scientists do a kind of detective work to figure out where the ice once was. We can figure this out by observing the ways the land has been eroded and by looking at the deposits that have been left behind. It is possible that there once was ice on the land where you are living right now. How can you find out? Let's talk about some of the features that scientists look for.
- Discuss the different erosional features formed by alpine glaciers.
- Describe the processes by which glaciers change the underlying rocks.
- Discuss the sorting and types of particles deposited by glaciers as they advance and recede.
- Describe the landforms created by glacial deposits.
Formation and Movement of Glaciers
Today, we have glaciers near Earth's poles and at high altitudes in mountainous regions. The ice in a glacier erodes away the underlying rocks, just as rivers and streams shape the land they flow over. Like rivers and streams, glaciers tend to flow along existing valleys, but while the thick ice of glaciers is slowly moving over the land, it scours away the rocks below somewhat like a very slow and steady bulldozer. Especially up in the mountains, rivers cut 'V' shaped valleys as running water cuts deep into the rock. As a glacier flows through this same valley, it widens the valley and forms steeper sides to the valley walls, making a 'U' shape valley instead (Figure 10.34).
In mountainous areas, often many smaller glaciers flow from higher elevations joining the main glacier as they move to lower places. Generally, these smaller glaciers carve shallower 'U' shaped valleys than the main glacier. A beautiful erosional feature, called a hanging valley, forms where the smaller 'U' shaped valley meets the deeper one of the main glacier. River water cascades down the steep valley walls forming breathtaking waterfalls (Figure 10.35).
The two main ways that glaciers erode the underlying rock are abrasion and plucking. As the thick layer of ice pushes against the underlying rock, it scrapes and polishes the rock surface. As glaciers flow, they scratch the underlying bedrock with all the rocky material they are carrying. These scratches make long, parallel grooves in the bedrock, called glacial striations, which show the direction the glacier moved. Also as the glacier slowly moves over the rock, glacial meltwater seeps into cracks and fractures of the underlying rock. As the water freezes, it pushes pieces of rock out of the underlying rock surface. These pieces of rock get plucked out and carried away by the flowing ice of the moving glacier (Figure 10.36).
There are several erosional features that form as a glacier both scours the rock and pulls pieces away. As rocks are pulled away from valley walls, a steep sided, bowl shaped depression forms at the top of a mountain, called a cirque. The word comes from the French word for circle. Once the ice melts away, a high altitude lake, called a tarn often forms from meltwater trapped in the cirque. If several glaciers flow down in different directions from a central mountain peak, these steep walled depressions can leave behind an angular, sharp sided peak called a horn. The Matterhorn in Switzerland is the most famous example of this type of erosion (Figure 10.37).
When two glaciers move down opposite sides of the mountain, the erosional landform that is created where they meet is a sharp edged, steep sided ridge, called an arête. Sometimes hiking trails follow along these narrow ridges, providing dramatic views in all directions (Figure 10.38).
As glaciers flow down a mountainside, the ice may also sculpt and shape the underlying bedrock as it flows. When a knob of bedrock is carved into an asymmetrical hill, it is called a roche moutonnée. In French, it means 'sheep rock'. Perhaps the villagers below the mountain thought these hills looked like sheep grazing in the valley. A roche moutonnée has a gently sloping side in the uphill direction of ice flow, with a steep side facing the downslope direction (Figure 10.39).
Depositional Features of Glaciers
As glaciers flow over many years, all sorts of debris falls onto the glacier through mechanical weathering of the valley walls. Glaciers are solid ice, so unlike water, they can carry pieces of rock of any size. Glaciers move boulders as large as a house as easily as the smallest particles of sand and silt. These pieces of rock are carried by the glacier for many kilometers and are only deposited as the ice melts. When you think of a glacier, you may think of white ice and snow, but actually glaciers have lots of rocky bits all over them. Each of these different deposits has its own name based on where it forms, but as a group they are called moraines. A long pile of rocky material at the edge of a glacier is called a lateral moraine and one in the middle of the glacier is called a medial moraine. Lateral moraines form at the edges of the glacier as material drops onto the glacier from erosion of the valley walls. Medial moraines form where two glaciers join together. In this case, the lateral moraines from the edges of each glacier meet in the middle to form the medial moraine (Figure 10.40).
Wherever a glacier is located, it is always slowly flowing downhill. Sometimes the rate at which it is flowing downhill is faster than the rate at which it melts. In this situation, you will see the glacier advancing down the valley, with more and more ice with each successive year. More likely what you will see today if you get the chance to visit a glacier, is that the glacier is retreating. This means that is there is less ice in the glacier this year than there was the year before (Figure 10.41).
When glaciers melt more than they flow forward, they deposit all the big and small bits of rocky material they have been carrying. In general, all these unsorted deposits of rock, formed directly by the ice, are called glacial till. If you live in an area where glaciers once were, you may have seen large boulders in the woods or even in the middle of a field. If these large rocks are a different type of rock than the bedrock in that area, they are called glacial erratics (Figure 10.42). Scientists know only ice could carry these large boulders great distances. The largest glacial erratic, called Big Rock found in Alberta, Canada weighs thousands of tons!
Sometimes long ridges of rock are deposited at the furthest point that the glacier reached. These are called terminal and end moraines. Just as the conveyor belt at the grocery store moves your groceries to the end of the counter, a glacier transports rock and sediment while it flows. If you couldn't stop the conveyor belt at the grocery store, you would end up with a big jumbled pile of food at the end of the counter. An end moraine is a little bit like that. Whatever the glacier has been carrying, all gets left behind in a pile as the ice melts away. Geologists study these materials to figure out how far glaciers once extended and they can also figure out how long it took them to melt away. Long Island in New York was formed by two glacial end moraines. The end moraine that formed this pile of rock and stone deposited by glaciers extends all the way out to Cape Cod, Massachusetts.
Even while a glacier is flowing slowly downhill, it deposits a layer of sediment underneath the glacier, which scientists call ground moraine. This layer of sediment makes a thick layer of unsorted sediment under the glacier that fills in low spots and evens out higher areas. Ground moraine is an important contribution to the fertile transported soils in many regions. Scientists knew that all these rocks and thick soils came from somewhere else but for a while they did not know that they came from glacial ice. This is easy to understand because the ice is not there today. Many scientists thought the big rocks looked like they had been dropped there and they thought maybe icebergs carried by a huge flood had brought them there. Because of this early hypothesis, lots of glacial deposits are called 'drift', because they were thought to have drifted in on icebergs. They correctly understood that only ice could have brought these materials, but not that there were thick 'rivers' of ice moving over the Earth in places where no ice exists today.
Another glacial depositional landform which forms under a glacier by water melting from the ice is an esker (Figure 10.43). These curving ridges of sand are deposited by streams that run within the ice along the base of the glacier. A normal stream carves its channel into the ground, forming a 'V' shaped channel, with the wide part of the 'V' at ground level. Because the water in this stream moves through the ice, not on the ground, only the deposits mark where these streams flowed. When the ice melts, the sediments form an upside down 'V' on the ground.
A drumlin is another type of asymmetrical hill that glaciers form but this one is made of sediments. A drumlin is an upside down teaspoon shaped hill which lines up with the direction the ice moved. The sediments dropped by the glacier are thought to be formed into a long narrow hill by the flowing glacier with the gentle sloped end pointing in the direction of ice flow. Usually drumlins are found in groups called drumlin fields.
Once material has been deposited by the glacier, water melting from the glacier can sort and retransport these sediments. An important difference between glacial deposits formed directly from the ice and those that form from glacial meltwater is their degree of sorting. Ice is capable of carrying a tremendous range of particle sizes, but solid ice does not sort any of these particles. So when material is dropped as ice melts, you will find very large pieces jumbled together in an unsorted deposit along with all the other size particles it carried. A very different situation occurs when running water moves particles. Liquid water cannot carry the large particles that ice carries. So as water moves through these unsorted deposits, it will select out only the smaller bits of sand and silt that it can carry. This produces a sorted deposit of just the sand and smaller particles transported by liquid water. Often these deposits form layer on layer and show the direction that rivers flowed. These deposits are called stratified drift. Often a broad area of stratified drift blankets the region just beyond the furthest reach of the glacier, as meltwater streams spread material out forming a broad plain called an outwash plain (Figure 10.44).
If an isolated block of ice remains behind as the glacier retreats, it may be surrounded and eventually covered over by these layers of sediment. In many years time, as the ice melts, it fills the depression with water, forming small circular lakes called kettle lakes (Figure 10.45). These small lakes are common in the areas where glaciers made their farthest advances.
Several types of stratified deposits form in glacial regions but are not formed directly by the ice. In glaciated areas, lakes are covered by ice in the winter. During the winter months, darker, fine grained clays sink to the bottom of the quiet waters in the lake. In the spring, with glaciers producing lots of melting water, lighter colored sands are deposited on top of the darker layers at the bottom of the lakes. These distinctive layers, called varves have paired dark/light layers, with each layer representing one year of deposits.
Loess is a very fine grained, wind transported deposit which forms in areas of stratified drift glacial deposits. It is common in the middle of North America as well as the eastern central portion of Europe. This fine sediment is produced as glaciers grind the underlying rock producing a fine powder called rock flour.
- The movement of ice in the form of glaciers has transformed our mountainous land surfaces with its tremendous power of erosion.
- U-shaped valleys, hanging valleys, cirques, horns, and arêtes are just a few of the features sculpted by ice.
- The eroded material is later deposited as large glacial erratics, in moraines, stratified drift, outwash plains, and drumlins.
- Varves are a very useful yearly deposit that forms in glacial lakes.
- How much of the Earth's land surface is covered by glaciers today? Was the Earth ever covered by more ice than it is today?
- What is the shape of a valley that has been eroded by glaciers? How did it get that shape?
- What are two different features that can form as smaller side glaciers join the central main glacier?
- Name and describe the two processes by which glaciers erode the surrounding rocks.
- Name the erosional feature that would form as several glaciers in a mountainous region move downslope in different directions from a central peak.
- Describe the different types of moraines formed by glaciers.
- Describe the difference between glacial till and stratified drift. Give an example of how each type of deposit forms.
- Name and describe the two asymmetrical hill shaped landforms created by glaciers.
- Scraping of the underlying bedrock, produced as ice flows against it.
- Steep-sided, sharp-edged ridge that forms as two glaciers erode in opposite directions.
- Steep-sided, bowl-shaped depression formed as a glacier plucks and erodes underlying bedrock.
- An asymmetrical hill formed from sediments under the flowing glacier.
- end moraine
- Unsorted pile of glacial till that marks the furthest reach of a glacier's advance.
- Long, curving, upside-down 'V' shaped ridge of sediment deposited under a glacier by meltwater.
- glacial erratic
- Large boulder with a different rock type or origin from the surrounding bedrock.
- glacial striations
- Long, parallel scratches carved into underlying bedrock by moving glaciers.
- glacial till
- Any unsorted deposit of sediment deposited by glacial ice.
- ground moraine
- Thick layer of sediment deposited under a flowing glacier.
- Sharp-sided, angular peak formed as glaciers move away from a central peak.
- kettle lake
- Often circular lake formed in the outwash plain by stranded ice.
- Deposit of unsorted, rocky material on, under or left behind by glacial ice.
- Removal of blocks of underlying bedrock by the glacier as meltwater seeps into cracks and freezes.
- roche moutonnée
- Asymmetrical hill of bedrock formed by abrasion and plucking of the moving glacier.
- rock flour
- Fine sediments produced by abrasion of glaciers scrape over bedrock.
- Mountain lake formed by glacial meltwater and precipitation.
- Paired deposit of light colored, coarser sediments and darker, fine grained sediments.
Points to Consider
- What features would you look for around where you live, to determine if glaciers had ever been present there?
- If glaciers had never formed on Earth, how would that affect the type of soil in the middle of North America?
- Can the process of erosion produce landforms that are beautiful?