The World's Foundations; or, Geology for Beginners · Ch. VIII, part 2 · Unit 1: Part I: How to read the record
9. Ice-work: glaciers
Goal: Your child can explain how snow becomes a glacier, what moraines are, and why the book thinks glaciers once covered much of Britain and North America.
You'll need
- The book, open to this chapter
- A plastic tub or yogurt cup, water, sand and small gravel (freeze it the night before)
- A lump of modeling clay or a bar of soap
- An old towel
- A notebook for a "rock journal" and a pencil
What's in the book
The second half of Chapter VIII (pages 76 to 82) explains how mountain snow is pressed into ice that creeps downhill as a glacier, with crevasses and moraines, scratching the rocks beneath. It argues that great glaciers once covered Scotland, northern Europe and North America, and says no one yet knows how or when the Glacial Age came about.
The lesson, step by step
Get ready the night before
Put a layer of sand and small gravel in the bottom of the tub, add water, and freeze it overnight. This will be your glacier.
Read together
Read from the paragraph that begins "In the higher mountains of earth" (page 76) to the end of the chapter (page 82). The key sentences:
It is, in fact, a frozen stream — a river of ice.
Yet the real wonder is, not that these great masses of solid ice should move slowly, but that they should move at all.
From snow to ice
In plain words: on high mountains more snow falls each year than melts. Its weight squeezes the snow at the bottom into ice, and the ice is slowly pushed out and down the valley. If you have snow, squeeze a handful very hard: it turns icy.
How fast? The book says only a few inches a day:
One mile in fifteen years is a very good pace for glacier ice.
Make glacier scratches
Pop the ice block out of the tub. Flatten the clay (or use the soap). Press the gritty side of the ice down and push it slowly across the clay a few times. Look: scratches and grooves, all running the same way.
SayThis is just what the book says glaciers do to the stones and rocks under them.
Moraines
Rocks fall from the cliffs onto the edges of the glacier and ride along on it in long lines called moraines. Where the ice melts, it drops them in a heap: the terminal moraine. Look at the picture of a glacier in the book if your copy shows it.
Solving the puzzle
Go back to last lesson's guess. The book's answer: long ago, huge glaciers covered Scotland, much of Europe and North America, carrying giant blocks and scratching stones. Giberne adds that she cannot say how or when this cold time came:
For in truth we simply do not and cannot yet know, either how or when there came about this great period of extreme cold
Tell it back
Ask your child to explain, using the ice block, how a glacier can scratch rocks and carry boulders. Then: What is a moraine? Why did some people first think icebergs did the work?
What we know now
We now know there was not just one Ice Age. Over roughly the last two and a half million years, great ice sheets have grown and melted back many times. The last ice age ended about 12,000 years ago, and the great ice sheets over North America and northern Europe had melted away within a few thousand years after that. The main pacemaker is small, regular changes in the earth's orbit and tilt. Some glaciers also move much faster than the book knew: a few in Greenland move more than 100 feet a day.
Afterwards
Notice it this week: if you live in the northern United States, Canada or northern Europe, the ground around you may hold glacial till and erratic boulders. Look for large lone boulders in fields and parks.
Tip
Melting ice is slippery and cold; do the scratching on a towel and let your child wear a mitten to hold the ice.
Answers to the book's problems
Heavy snow presses its lower layers into ice, and the weight pushes the ice slowly downhill as a glacier. A moraine is a line or heap of rocks carried by a glacier and dropped where it melts. Icebergs do carry and drop rocks, but they could not finely scratch thousands of small stones, so the book prefers the glacier explanation.
Worked out for this site from the scan. If a number in your copy differs, trust the book.