Reba Carey, age 14, of Utica, N.Y., for her question:

WHAT EXACTLY IS SEDIMENTARY ROCK?

Sedimentary rock is rock formed when mineral matter settles out of water, or, less commonly, out of air or ice. Sedimentary rock covers about three fourth of the earth's land area. Some places, the sedimentary rock is over 40,000 feet thick.

Geologists tell us that sedimentary rock has been forming for at last 3.5 billion years. It is one of three major kinds of rock. The others are igneous rock and metamorphic rock.

The most common sedimentary rock is shale or mudstone. It is made of compressed silt which is fine particles of mineral matter. Other common sedimentary rocks include limestone, made of the mineral calcite, and sandstone, made of sand.

Most sedimentary rock starts forming when silt and grains of sand settle along river beds or on the bottoms of lakes and oceans. Year after year, these minerals collect and form broad, flat layers called beds or strata.