Concerning "Uranium-series dating", also known as "Uranium-thorium dating". Uranium is present in deposits, "typically at levels of between a few parts per billion and few parts per million by weight", according to the wikipedia article on the subject.
As an example, with carbon dating, the amount of carbon-14 originally present in any given sample is consistent with the amount of carbon-14 created by the activity of sunlight in the atmosphere.
How do we know how much uranium-234 was in any given sample of rock when it was created/deposited?