In the uranium-thorium dating method (for geochronology), the amount of 238-U, 236-U, and 230-Th in a rock are measured. All three isotopes are part of the 238-U decay chain, but thorium is not water-soluable under normal conditions so rocks that precipitate in water (e.g. cave formation calcite) should have no initial thorium. Any thorium that exists in the rock should come from the decay of uranium.
Sometimes particles of dirt are incorporated in the cave formation. This dirt can contain thorium. So 232-Th is measured to gauge how much, if any, 230-Th was initially in the sample.
229-Th, 233-U, and 236-U are not found in nature. They are added to samples to gauge the reliability of the machine analyzing the sample.
238-U has a half-life of 4.5 billion years, which by coincidence (or is it??) is just about as long as the Earth has been around. Pretty handy for remembering the half-life!
When conditions are favorable, the U-Th dating method is one of the best available for the late Pleistocene. It is good for the past 700,000 years.
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