Rosemary Paliani, age 11, of Rochester, New York, for her question:

   How are sedimentary rocks formed?

Solids mixed in liquids tend to settle to the bottom. In time, the tiny fragments arrange themselves in layers of sediment. These fallen dregs become sediments. On a grand scale, this operation goes on in the seas. It has been going on for millions of years and vast beds of sediment have had time to accumulate on the floors of the world oceans. There are oozy sediments of muddy clay and silty sands under lakes and seas. Many of them contain calcium ,deposits from generations of discarded seashells. The bones and scaly remains of long gone fishes also are mixed with sediments.

The ocean bed is as restless as the land and from time to time large areas have pushed .up above the waves. The ancient seabeds then become dry land, topped with layers of sediment. In time, the oozy sediments dry out and compress into solid rocks. These are the so called sedimentary rocks of the earth's crust. Many of them are sandstones, others are limestones full of the calcium remains of ancient sea dwellers.