The part of the earth’s history known as the coal age began more than 250 million years ago and lasted millions of years. During that time, in the area that is now the United States, coal beds formed from Texas to the north Atlantic coast.
At that time thick forests swamps（沼泽） covered much of the earth’s surface. Great trees died and fell into the swamps. Then they rotted （腐烂）and new trees and plants grew on top of them. The process repeated itself for thousands of years. The rotted plants turned into a substance called peat（泥炭）, which is still forming today in many swamps.
