The World's Foundations; or, Geology for Beginners · Ch. XXV · Unit 4: Part III: Water at work today
28. Deltas
Goal: Your child can explain how a delta is built, name the three kinds the book describes, and explain why it is hard to say how old a delta is.
You'll need
- The book, open to this chapter
- The sand tray, with a shallow pan of water at the low end
- A cup of muddy water (from the sediment jar, shaken)
- A globe, atlas or free online map
- A notebook for a "rock journal" and a pencil
What's in the book
Chapter XXV shows water building land as well as wearing it away. It explains sand-bars, mud-banks and deltas, with three kinds: lake deltas (the Rhone in the Lake of Geneva), inland-sea deltas, and ocean deltas. It describes the Po, the Nile, the great delta of the Ganges and Brahmapootra, and the Mississippi, with two attempts to work out its age, and ends with rain-prints forming today on the mud-flats of the Bay of Fundy.
The lesson, step by step
A question to start
SayWhen the muddy river finally reaches the calm sea, what happens to the mud?
Think back to the sediment jar.
Read the chapter together
Read Chapter XXV (pages 236 to 245). Stop on:
The name Delta is given to the tract about the mouth of a large river, because of its likeness in shape to the Greek letter of that name Δ.
Draw the Greek letter Δ and a river splitting into channels inside it.
Build a delta
Set the sand tray on a gentle slope ending in the pan of water (the sea). Pour water slowly from the watering can at the top, or pour the muddy water down a channel. Watch where the water slows down at the "sea": sand drops in a fan, and the channel splits.
The Rhone and the lake
Read:
and flows out at the other end, pure and clear, having left a load of mud behind.
Ask: Why is the river clear when it leaves the lake? (The still lake let the mud settle, like the sediment jar.) Find Lake Geneva on a map.
Great deltas
Find the deltas of the Nile, the Ganges and Brahmapootra, and the Mississippi on a map. The book says each Nile flood laid a new thin film of earth on Egypt, making its soil.
How old is the Mississippi delta?
The book says a first calculation gave about sixty-seven thousand years, and a second, after more mud was found to arrive each year, gave thirty-three thousand five hundred. Ask: What is half of 67,000? (33,500.) And the author asks whether it will be halved again: what would that give? (16,750.)
That the time was long, very long, appears highly probable. But how long it lasted man cannot say.
Tell it back
Ask your child to explain how a river builds a delta. Then: Why did the second calculation give a smaller age than the first?
What we know now
The part of the Mississippi delta we see today was built within roughly the last 7,000 years, after the sea stopped rising at the end of the Ice Age. Today Louisiana is losing land along its coast, partly because dams and levees keep the river's mud from reaching the delta, the opposite of the Po example in the book.
Afterwards
Notice it this week: look at a map or satellite view of a big river's mouth with an adult. Does it split into channels?
Answers to the book's problems
When the river meets the still lake or sea it slows down, can no longer carry its load, and drops sand and mud in layers, which build up into new land. The three kinds are lake deltas, deltas in almost tideless inland seas, and ocean deltas with full tides. The second calculation was smaller because the river was found to carry much more mud each year than first thought, so the delta could have been built faster.
Worked out for this site from the scan. If a number in your copy differs, trust the book.