The World's Foundations; or, Geology for Beginners · Ch. XIV · Unit 2: Part II: Ancient seas and coal forests
15. The age of coal
Goal: Your child can explain, step by step, how the book says coal seams were made, and why there is a layer of clay under each seam.
You'll need
- The book, open to this chapter
- A tall clear cup or jar
- Leaves or grass clippings and some sand
- Paper and colored pencils
What's in the book
Chapter XIV takes the reader in imagination to a hill above the steamy swamp forests of the Coal Age, with giant club-mosses, horse-tails, tree-ferns and a large salamander-like creature. It explains how forests grew, sank under the sea, were covered with sand and rose again many times, why fire-clay lies under each coal seam, and why upright fossil trunks are dangerous in mines.
The lesson, step by step
A question to start
SayCoal is black and hard and burns. Would you believe it was once a forest?
Read to see how.
Read the chapter together
Read Chapter XIV (pages 125 to 134). Look at the picture of the Coal-Age forest. Sentences to stop on:
Only a gleam of light, not sunshine, for little or no sunshine can creep through the misty atmosphere.
the air is positively reeking with damp oppressive warmth, like the air of a hot-house.
Draw the scene
Re-read the paragraph about standing on the hill. Your child draws what the author describes: a slow stream and shallow lakes, swampy forests of giant club-mosses, horse-tails, tree-ferns and pines, misty air, and the large scaly creature creeping by the water.
Build coal layers
Make a model of the book's cycle in the cup: a layer of leaves (a forest grows and dies), then a layer of sand (the land sinks and the sea covers it), then leaves again (the land rises and a new forest grows), then sand. Do it three times and press down firmly.
SayEach leaf layer stands for one coal seam, with rock between.
The clay underneath
Read:
It is now believed that the clays were once the soils of the ground, upon which the forests grew,
The "Stigmaria" once thought to be a water-plant turned out to be the roots of the Sigillaria tree, still in its soil.
Seams thick and thin
Some are scarcely thicker than a piece of paper, while others are as much as thirty or forty feet in thickness.
Ask: If it takes a huge pile of plants to make one foot of coal, what does a forty-foot seam tell you?
Tell it back
Ask your child to explain the coal cycle in order using the cup: forest, sinking, sand, rising, new forest. Then: Why are upright fossil tree-trunks dangerous to miners?
What we know now
This age is now called the Carboniferous ("coal-bearing"), about 359 to 299 million years ago. The air then held more oxygen than today, and some insects grew huge: a dragonfly-like insect called Meganeura had wings over two feet across. The clays under coal seams are still understood as old forest soils.
Afterwards
Notice it this week: look for horse-tails (also called scouring rush) or club-mosses in damp places; they are small living relatives of the Coal-Age giants.
Answers to the book's problems
The forest grew in a swamp and its dead plants piled up; the land sank and the sea covered it with sand or mud; later the land rose and a new forest grew; this happened again and again, and the buried plants slowly hardened into coal. The trunks were filled with sandstone inside a thin coal bark, which cannot hold the weight when the mine takes away support, so they fall from the roof.
Worked out for this site from the scan. If a number in your copy differs, trust the book.