10.4 Glacial Erosion and Deposition
Lesson Objectives
- 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.
Introduction
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.
Formation and Movement of Glaciers – Continental and Val-
ley 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 (Figure10.34).
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 ahanging
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).