Harold Jackson, age 9, of Eureka, Kansas, for his question:
How was sandstone formed?
Sandstone is a gritty rock which comes in many shades of brown, grey, bone white and chestnut red. Compared with, say, granite, it is a very soft stone. It is porous and able to drink up to a quarter of its weight in water. Sometimes it gets soaked with rain water and along comes the frost. The rain water swells as it becomes ice. This cracks and chips the sandstone. Because of its softness, wind and weather are able to wear it away. Sometimes a river wears a deep gorge through layers of sandstone. We see these colorful canyons of sandstone in the Painted Desert of Arizona, in Grand Canyon and other showy gorges of the western states.
These beds of sandstone started a life of adventure countless millions of years ago. Like all the rocks, they are made from minerals which became solid when the earth was young. For a long time, the bare dry land heaved and bubbled with volcanoes. Melted rock from below boiled up and poured over the face of the earth. But at last things became quieter. Even then the solid rocks were not left to rest peacefully.
Wind and rain, the pounding ocean and icy glaciers started warfare on the solid rocks and this warfare has been going on ever since. Boulders were cracked open by heat and frost. Big stones were pounded into bits of gravel.
When gravel collected on the sea shore or on the beds of running rivers, the water bashed and broke it into small grains of sand. Such beds of sand, are still forming beside rushing rivers, on beaches and on windy deserts.
As a rule, only hard minerals such as quartz and calcium live long enough to become grains of sand. The softer minerals are usually dissolved.
In times past, the sea has often washed over the dry land to make shallow swamps. Sometimes the beds of the swamps were covered with a soupy mixture of sand, gravel and muddy water. When the seas went back, these old swamp beds were left to become dry land. The soupy mixture dried out and became layers of sandstone.
Sandstone is a mixture of gritty grains of sand and gravel, all cemented together. The cement was formed from the minerals which were dissolved in the muddy water. Many sandstones are cemented with lime or iron oxide. The hardest are cemented with silica, the mineral from which most sand is made.
The colors of sandstone come from impurities in the mixture. Iron oxide makes the rock brown or chestnut red. Flecks of mica make it glitter with a snowy frosting. We may find a shell embedded in the rock, which tells us that the mixture was once the floor of an ancient sea. There may be a dinosaur footprint, which shows that the mixture was only partly dry when these giant reptiles walked the earth.