So, let’s start with a question.
How does this giant angular boulder end up jammed in among these tightly packed pine trees in Yellowstone National Park?
To find an answer, we need to learn about some of the characteristic landforms left behind as a result of glacial erosion and deposition.
A glacier is a long-lasting mass of slow-moving ice and snow.
See our Classifying Glaciers video for more about the different types of glaciers.
Just like streams, moving ice can be an agent for erosion and deposition.
And just like streams, glaciers erode material from near their source, transport rock and sediment downslope, and deposit much of that material where they end.
In areas with present day glaciers, erosion forms steep-walled, bowl-shaped depressions known as cirques at the head of valley glaciers.
Adjacent glacial valleys are typically separated by sharp, knife-edge ridges known as arêtes.
Glacial erosion can occur where ice is in contact with rocks
Who among us hasn’t frozen a large rock into a block of ice to test how tightly the ice binds to the rock.
When this happens in nature it results in one type of glacial erosion known a plucking.
Plucking happens when water invades cracks in the rock. As the water turns to ice, it binds onto the blocks of rock.
The motion of the glacier plucks the blocks of rock out of the cliff face,
embedding them in the moving mass of ice and transporting them downslope.
Rocks frozen into the base and sides of the glacier act like sandpaper,
scraping against the surrounding valley walls and floor in a second process of glacial erosion known as abrasion.
Abrasion leaves behind tell-tale striations in the bedrock that identify the direction of motion of ancient glaciers.
Some striations, like these grooves worn in to this limestone bedrock can be very deep,
indicating that they formed below thick, powerful ice sheets.
The combination of plucking and abrasion deepens and widens V-shaped stream valleys to produce a U-shaped profile characterized by a flat-valley bottom and steep sides
Like this example from Scotland,
and this U-shaped valley from the Beartooth Mountains in Montana.
If rising sea levels were to flood one of these things, we would have to call it a fjord.
Up to now we’ve been focusing on processes that cause erosion.
Now we want to turn our attention to the features formed when glaciers deposit material near their terminus.
Most deposits left behind by glaciers are composed of a generally unsorted mixture of clay, silt sand boulders known as glacial till.
Ice at the terminus of the glacier will melt and dump piles of till.
If the position of the terminus remains stationary for some time, it can leave behind a ridge of unsorted till known as a moraine.
Moraines essentially define where the glacier paused and the position of the moraine that is farthest down the valley, the terminal moraine, indicates how far the glacier extended during its history.
Ice sheets leave behind moraines that can be traced for hundreds of kilometers across multiple states.
For example, this shaded relief map of Ohio shows the positions of several moraines stretching across the northwest corner of the state and into neighboring Indiana.
Piles of unsorted glacial till can be trapped by an advancing glacier and shaped into oblong mounds known as drumlins that narrow in the direction of glacial motion.
Behind the moraines we sometimes find hundreds of small lakes known as kettle lakes.
Kettle lakes form when blocks of ice break off a retreating glacier and are subsequently buried.
The ice gradually melts, forming a depression in the glacial sediment known as a kettle.
These low areas may then be partially filled to form a lake.
Just like these kettle lakes in North Dakota.
A feature known as an esker may be formed when till is sorted by streams flowing at the base of the melting glacier.
The sediment deposited within these sub-glacial streams leaves behind a skinny sinuous landform.
Likewise, as the glacier melts and its ice turns to water, meltwater streams transport sediment away, depositing some of it on the adjacent outwash plain.
Thick ice sheets can pick up very large rocks, known as erratics, from one location and deposit them elsewhere,
sometimes, hundreds of kilometers from their source.
By matching the geology of the erratic with their source area, we can unravel the direction and distance of motion of the ice that moved these wandering boulders.
That brings us back to our 500-ton friend among the trees in Yellowstone National Park.
This boulder of metamorphic gneiss sits on top of volcanic rocks hundreds of millions of years younger in age
and must have been carried from at least 25 kilometers (or 15 miles) away.
Let’s finish off with a matching exercise. We will show you pictures of four glacial features.
Match each one with a term from the provided list, we’ll check your answers when you are done.
So, how did you do?
You had two learning objectives for todays lesson,
how confident are you that you can do these things?