The Fairy-Land of Science · Lecture V, part 3 · Part 5: Lecture V: The two great sculptors, water and ice
15. Caves, cliffs, frost and glaciers
Goal: Your child can explain how stalactites grow, how waves wear away cliffs, and how glaciers carry stones and scratch the rocks beneath them.
You'll need
- An ice cube made with sand and small pebbles frozen into it (freeze it the night before)
- A lump of play dough or modeling clay
- Science notebook and pencils
What's in the book
Pages 115 to 123. Water dripping in caves builds stalactites and stalagmites; storm waves hurl stones at cliffs and hollow out caves; frost breaks up soil for the farmer; glaciers bend, carry lines of stones (moraines), and scratch and round the rocks with stones frozen into their bottoms; and huge "erratic blocks" left all over northern Europe by the ice of long ago.
The lesson, step by step
Hook question
SayIn some caves there are stone icicles hanging from the roof. How could stone drip?
Let your child guess, then read page 115.
These rods are called stalactites
The idea in plain words
Water trickling through the rock carries a little dissolved lime. Each drip leaves a tiny bit behind, and a stone icicle slowly grows down from the roof. Where the drops land, a pillar (a stalagmite) grows up. Sometimes they meet.
Then read or tell pages 116 to 118: storm waves throw stones against cliffs and hollow out caves.
Frost and ice
Read page 118.
The farmers know this, and always plough after a frost, because the moisture, freezing in the ground, has broken up the clods, and done half their work for them.
Look at the frontispiece, the picture at the very front of the book: a glacier carrying long lines of stones down the valley.
Try it: a glacier in your hand
Flatten the play dough into a smooth slab. Rub the plain top of your sandy ice cube across it: hardly a mark. Now turn it over and rub the side with stones frozen in it. Scratches!
SayGlaciers carry stones frozen into their bottoms, like the blade of a carpenter's plane. As the ice creeps along, the stones scratch and grind the rock.
Clues the ice left behind
Read page 121.
These rounded rocks are called "roches moutonnees," because at the distance they look like sheep lying down.
Read or tell page 122: huge blocks "as big as cottages" were carried hundreds of miles by ice and left behind when it melted. In 1840 the scientist Agassiz showed that ice had carried them.
Tell it back and check
Your child tells back how a stalactite grows, or how a glacier scratches rock. Then ask:
1. What does a stalactite grow from?
2. Why do farmers like a frost before ploughing?
3. What made the scratches in your play dough?
What we know now
The last Ice Age ended about 12,000 years ago. Thick ice then covered much of northern Europe and North America, and its scratched rocks and big boulders can still be found in many places, including the northern United States.
Afterwards
Notice it this week: icicles growing drip by drip, cracked sidewalks where water froze in the cracks, or big lone boulders in a field or park.
Tip
Make the sandy ice cube by putting a spoonful of sand and a few tiny pebbles in an ice-cube tray or paper cup of water. Most will sink to the bottom, which is just where a glacier carries its stones.
Answers to the book's problems
1. From drips of water that leave behind a tiny bit of the lime they carried through the rock. 2. The water in the ground freezes, swells and breaks up the clods for them. 3. The sand and pebbles frozen into the ice, just like the stones in the bottom of a glacier.
Worked out for this site from the scan. If a number in your copy differs, trust the book.