Tony Grenga, age 11, of Youngstown, Ohio, for his question:
How is sandstone formed?
Most of it was formed cohere shallow waters washed over the shores. The restless waves stirred grains of sand with clay and lime, silica and whatever other ingredients were handy. No two mixtures were exactly alike. When the waters were calm the solid stuff sank to the bottom. Later these thick layers of sediment were lifted high and dry above the water and became deposits of sandstone.
The earth's recipe for making sandstone calls for gritty grains of hard minerals and a moist paste of fine softer particles, plus lots of help from water, wind or melting ice. The gritty grains are mixed with the paste and then dry. When the muddy paste sets, it cements the gritty grains in solid layers of sandstone rock.
A vast variety of different minerals can be used to build sandstone, which explains why no two deposits are exactly alike. Usually the gritty material is grains of sand. Though it may be small grains of hard granite or felspar, such as orthoclose or microcline. The gritty particles in sand¬stone are smaller than peas and larger than one 500th of an inch. A few grains may be rich brown garnet or shiny mica.
The cement may be lime or some other form of muddy clay. The grit and the cement may be mixed under shallow waters near the shore or on a sandy, windblown desert. Sometimes it is mixed by melting ice as it cracks hard rocks to fragments and washes them with soggy clay. The mixture of sedi¬ments may be lifted above the water or crushed under new layers of lava.
The roughness of sandstone depends on the size of its grains and how tightly they are packed together. Some deposits are so porous that they can hold enough water to equal 25 per cent of their bulk. When such porous deposits are buried, they hold groundwater or deep reservoirs of petroleum.
On the surface, sandstones add great glory to the scenery because they come in such an array of colors. Different deposits range through white and dove greys, bluish mauves and greens, yellows, browns and rich rough reds. In most cases, the colors are added by the cements used to stick the grains together. But green sandstone is colored by its gritty grains of the hard, greenish blue mineral, glauconite.
Sandstones display their artist's colors in the famous Painted Desert of Arizona. The layers of rainbow colored rocks in Grand Canyon also are sandstones. The white and grey layers are cemented with limy materials. Cements with iron oxides add the candy stripes of brown, pinks and rich reds. The colored layers were formed at different times, when different cement materials were available.
The hardest common mineral is quartz, or silica. In some sandstone, silica was used to cement grains of silica sand. Sometimes this extra hard sandstone is crushed under heat and pressure. In time it is recrystallized and becomes a deposit of a metamorphic rock called quartzite.