The World's Foundations; or, Geology for Beginners · Ch. VII · Unit 1: Part I: How to read the record
7. Rock-bendings
Goal: Your child can explain the difference between a layer, a stratum and a formation, and show with cloth or paper how rock layers fold and slip into faults.
You'll need
- The book, open to this chapter
- Colored paper (three or four colors, a few sheets of each)
- Three or four bath towels or pieces of cloth, different colors if possible
- Modeling clay in two or three colors and a butter knife (optional)
- A notebook for a "rock journal" and a pencil
What's in the book
Chapter VII says the stratified rocks, if all piled up at their thickest, would be about twenty miles deep, and that the record is a chain with many missing links. It explains layer, stratum and formation with a pile of colored paper, describes bent, crumpled and folded rocks, and ends with faults that make coal-seams suddenly stop.
The lesson, step by step
A question to start
SayRock is hard. Can rock bend?
Hear the answer, then read.
Read the chapter together
Read Chapter VII (pages 61 to 69). Look at the Chart of Strata near the start of the chapter: the drawing shows the layers in order.
Layer and stratum
Read the book's own example:
Suppose you had a great pile of paper, first a number of blue sheets, over them a number of white sheets, then a few pink sheets,
Each sheet would picture a ' layer,' but each supply of one colour lying close together would picture a ' stratum.'
Build the pile with your colored paper. Point to a layer, then a stratum. A formation is the set of strata from one age.
Fold the layers
Again from the book:
Take half-a-dozen pieces of thick cloth, and fold them backwards and forwards into several massive plaits, to make the matter more clear.
Lay the towels flat on top of each other. Push slowly from both ends. The humps are like mountain folds; each towel still runs up and down through them, as a rock layer does.
Faults
Read:
These slips are called faults, both by geologists and by miners.
If you have clay, press three thin colored slabs into a block, cut it in two with the butter knife at a slant, and slide one half down. The layers no longer line up. Ask: If you were a miner following the coal layer, what would happen at the fault?
Rocks that bend
Giberne says that under huge pressure even hard rock yields like clay, and gives an example of crumpled cliffs on the Lake of Lucerne in Switzerland. Find Lucerne on a map if you like.
Tell it back
Ask your child to show and tell, with the paper or towels: what is a layer, a stratum, a fold and a fault?
What we know now
Giberne could not say what squeezed the rocks. We now know the force comes from the earth's moving plates. The Alps, including the folded rocks near Lucerne, were crumpled as the African plate pushed slowly north against Europe. Earthquakes happen when rocks suddenly slip along faults.
Afterwards
Notice it this week: look at photos of folded rock layers (search for "folded rock strata" with an adult), or look for slanting layers in road cuts as you travel.
Answers to the book's problems
A layer is one sheet of rock; a stratum is a whole band of one kind of rock, made of many layers; a formation is all the strata from one age. A fault is a place where rock has cracked and one side has slipped up or down, so the layers no longer line up; a miner following a coal seam would find it suddenly stop and would have to look higher or lower for it.
Worked out for this site from the scan. If a number in your copy differs, trust the book.