Welcome to You Ask Andy

Jim Tome, age 9, of Spokane, Wash., for his question:

How do shells get into rocks?

We expect to find shells on the seashore because the shellfish who make them live in the ocean. These shellfish are stodgy stay at homes who never leave the water if they can help it; n0 wonder we are surprised to find their shells in rocks on land hundreds of miles from the sea.

This very same question was asked by one of the world's greatest artists who happened also to be one of the world's greatest brains. He was Leonardo Da Vinci of Italy. He lived and created his master works 400 years ago. He painted pictures, earved statues and planned buildings and canals. When an ancient outdoor theater needed repairs, Leonardo was asked to direct the work. And some of the building stones used to make the benches were embedded with graceful seashells.

Leonardo collected samples and studied the shells in the rocks. He decided that the shells were very old. He thought they were the fossil remains of ancient shellfish, and he was correct. We know now that rocks embedded with shells were formed under the sea. Today we often find them on dry land hundreds or thousands of miles from the ocean, but millions of years ago this dry land was covered by the sea.

These ancient seas were populated with all kinds of shellfish which lived and died in countless numbers and left their crusty shells on the sea floors. Countless plants and animals lived, died and decayed in the ocean, and its salty waters filled with drifting fragments. The streams and rivers washed silt and muddy sand into the sea, and in time these solid fragments sifted down and settled on the ocean floors.

The crusty old seashells on the ocean floor became buried in layers of mud and sand. Through the long ages the face of the earth changed; sometimes the land and sea, changed places. A stretch of salty water drained away and the old sea became dry land. Its muddy carpet of silty sand became a hard layer of sandstone, and the shells of the ancient sea dwellers were embedded in the dry, solid rock.

Many of our most useful rocks were formed under the sea or were made from masses of ancient shells too small for our eyes to see. Chalk and limestone were made from these tiny shells. The little ocean dwellers who made them left layers of their shells on the sea floors. Later the sea water went away, and the oozy carpets of tiny shells became dry layers of rocky sandstone, chalk and limestone.

 

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