The World's Foundations; or, Geology for Beginners · Ch. XXXI · Unit 5: Part III: Fire, coral and caves
36. Stalactite, and the end of the book
Goal: Your child can explain how stalactites and stalagmites form, why their growth cannot be used as a clock, and sum up what the book has taught.
You'll need
- The book, open to this chapter
- Two jars, a saucer and a piece of thick wool yarn about 18 inches long
- Epsom salts or baking soda, and warm tap water
- Your rock box and rock journal
What's in the book
Chapter XXXI touches on peat bogs and landslips (such as the Rossberg slide of 1806), then describes limestone caves dug out by water, with stalactites hanging from the roof and stalagmite covering the floor. It shows by examples that stalagmite grows at very different speeds, so it cannot tell us exactly how long bones under it have lain there, and ends the book with the author's closing words.
The lesson, step by step
A question to start
SayIn some caves, stone icicles hang from the roof. How could stone drip?
Read the chapter together
Read Chapter XXXI (pages 305 to 315). Look at the picture of the stalactite cavern. Two definitions:
The downward hanging forms are called Stalactites, and very beautiful they often are.
The same description of lime-formation, left by trickling water, often covers the whole cavern-floor, and it is then called Stalagmite.
A memory hook: stalaCtites hang from the Ceiling, stalaGmites rise from the Ground.
Grow a stalactite
Stir as much Epsom salt as will dissolve into two jars of warm tap water. Set them a few inches apart with the saucer between. Dip the yarn in, then lay one end in each jar so it sags over the saucer.
Over a week, the liquid creeps along the yarn, drips from the low point and leaves crystals behind: a small stalactite above and a stalagmite on the saucer. Check it each day.
How caves are made
In plain words: water with a little acid in it slowly dissolves limestone, digging out caves. Later, water dripping through the roof carries dissolved lime and leaves it behind as it drips. That is the same lime that fizzed in our vinegar tests.
Not a reliable clock
Read:
An iron nail was left in a forsaken lead-mine, where it caught a stalactite drip. In seventy-five years a quarter of an inch was formed upon it.
Compare: the book says a gas-pipe near Buxton got an eighth of an inch in just six months. Ask: If growth can be that different, can we tell how long a bone has lain under the stalagmite floor just by measuring it?
The author's last words
Read the closing paragraphs aloud. Her summary of geology:
We are but spelling out the broken sentences of the rock-volume, written, as it is, in a strange language, with many missing paragraphs. What marvel if we make some mistakes ?
Ask your child to connect this to the torn-up book from Lesson 1.
Look back over the whole book
Lay out the rock box and rock journal. Ask your child to give a short talk to the family: the three parts of the book, three kinds of rock, two ways fossils are made, and one thing that has changed since 1881.
What we know now
Scientists can now date many cave formations directly, by measuring tiny amounts of uranium in the stone, which slowly changes into another element at a known rate. So stalagmites, which Giberne rightly warned were unreliable clocks by their thickness alone, have become one of the best records of past climates.
Afterwards
Keep the stalactite jars going for a week and draw them each day. Finish your rock collection with a label for every rock: its name or best guess, where it was found, and what the vinegar and scratch tests showed.
Tip
If you can, visit a show cave on a guided tour. Never go into a wild cave or old mine without trained adults.
Answers to the book's problems
Water carrying dissolved lime drips from the cave roof and leaves a little lime behind each time, building stalactites downward and stalagmite on the floor. Because the speed varies so much (a quarter inch in seventy-five years on the nail, an eighth of an inch in six months on the gas-pipe), the thickness cannot tell us exactly how long a bone has lain there.
Worked out for this site from the scan. If a number in your copy differs, trust the book.