I want to get back to the questions in my mind about radiometric dating methods which scientists use to date fossils. I'm not satisfied that anyone has adequately answered those questions.
Just for gits and shiggles, let me give it a try.
First, you need to understand what radiometric dating is actually measuring. What it measures is how long a mineral has been closed. That is, how long the isotopes in question have been locked in place. For example, in liquid rock (i.e. magma) gasses, like argon, escape quite easily. When the rock goes from liquid to solid this isn't the case anymore. With respect to argon, the rock becomes closed when it solidifies. Any argon produced by the decay of potassium-40 is now trapped in the rock. Therefore, by measuring the amount of potassium and argon in the rock you can tell how long ago it went from liquid to solid.
So what happens if that solid rock gets weathered and turned into a sediment further down the stream? Well, dating the chunks from that weathered rock can not tell you how old that sediment is. It can only tell you how long ago the source rock formed. This is why sediments are not generally dated. Instead, igneous rocks that solidified in their current position above and below the sediments are used to give a date range for the formation of that sedimentary layer.
Clear as mud (pun intended)?