Photograph by Martin Richard
Written by Martin Richard
Last week, we looked at scratch marks in rocks. They showed us that a glacier was once there, and scratched the rocks by dragging other rocks across them.
In this picture we see something much larger made by a glacier: an entire valley!
All of this valley was carved by a glacier!
Most of what you see here was once covered in ice. Only the tops of the highest mountains were above the ice.
Rivers carve valleys too. How do we know this was carved by a glacier? There are two clues.
First, this valley is shaped like a U! Rivers carve valleys that are V-shaped. Glaciers carve valleys that are U-shaped.
Second, you can see a long way down this valley. Rivers wander. They have lots of curves and you usually cannot see a long way down their valleys.
Why is this?
The water at the bottom of a river moves more slowly than the water at the top. Rivers erode the land where they move the fastest, which is at the top of the river. So they erode away their edges and get wider. That is how they carve a V.
Rivers go around things in their way, but they have to go downhill, so they wander. As they become curvy, the river moves faster at the outsides of the curves. So the river eats away the land at the outside of the curve, and makes larger curves, shaped like an S.
When glaciers form, they start in the high mountains, where it is coldest. As they grow they move down into the valleys already carved by rivers. BUT! Glaciers are not rivers! Rivers, remember, erode the land where the move the fastest, at their tops. Glaciers erode the land where they are deepest!
As a glacier moves down a valley that was carved by the river, it erodes the bottom of the valley, the bottom of the V, and makes the sides steeper and the bottom flatter. That is why glacier valleys are shaped like a U.
Glaciers pile up and run right over hills that are in its way. The glacier then erodes the hill away. So glaciers straighten out the S-curves left by the river, and you can see a long way down the valley.
A big glacier can plow a straight valley with a view that is many miles long.
I took the picture above in Glacier National Park. I was standing on a very steep cliff at the head of this valley. (If I had taken five steps forward, I would have fallen hundreds of feet straight down.) You can see the U-shape of the valley, and the cliff I was standing on was even steeper than the sides you can see in the picture.
The glacier that carved this valley was not the biggest glacier in the park! It fed ice into a much larger glacier that carved a valley more than 20 miles long!
So the next time you are in the mountains, and you notice that you can see a long way down a valley, look to see if the valley has steep sides and is shaped like a U. if it does, you will know that the valley was carved by a glacier!
You can read more about this picture here: Going to the Sun Road and the Garden Wall That page was not written for kids, but you might like reading it anyway.