Kathryn Nabi, age 9, of Ottawa 6, Ontario, Canada, for her question:

What is a conglomerate rock?

Conglomerate means a wad of material, rolled up in a ball. A conglomerate rock may be a mass of pebbles wadded together with earth cement. It may look like a raisin biscuit made of stone. Some conglomerates are masses of hard gravel stuck together with sandy cement. The earthy cement may be reddish, or brown or pasty color. The hard stones may be hard, smooth, pale pebbles or rough, dark chunks. Most conglomerates are speckled and freckled with light and dark colors.

The earth uses water to make these rocky mixtures. Often they form in the gravelly bed of an old river. Fine fragments of dust and dirt sink to the bottom and nestle around the pebbles. Soon the hard pebbles are embedded in soft mud. If the river dries up, the soggy mud sets into a hard cement and sticks the hard stones together in a wad of conglomerate rock.