Stalagmites form on the floors of some caves. Like tree rings, the bands in a stalagmite represent periods of growth. But unlike tree rings, these bands don't form every year.
Stalagmites grow only when the weather is warm and there is ample rain.
To date a section of a stalagmite, scientists compare the relative amounts of certain uranium and thorium atoms. The older the section, the greater proportion of thorium it will contain, because the uranium atoms decay into thorium. After determining when the stalagmite grew, scientists can study it further to get an idea of what past climate was like. The bottom of this stalagmite grew for some time and then stopped. When the stalagmite began growing again a few thousand years later, the water source on the roof had shifted.