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Scientists base absolute dating on measurable physical or chemical changes or on written records of events.
In the field of archeology, the term "absolute" is somewhat misleading.
Here is an example: All you then need is one formation somewhere with a particular suite of fossils and a convenient igneous layer to give you an absolute date.
Then any other sedimentary rock with the same fossils is going to be the same basic age. The most problematic are sedimentary rocks that have been metamorphosed.
The geological processes that we'll be discussing work rather like that.
We have no reason to believe that nature will be so obliging as to always provide us with the ideal conditions that would provide us with ideal clocks; but by analysis of the geological conditions as they actually are, we can often set limits to how imperfect the geological clocks can be.
The sedimentary rocks can then be dated based on superposition.Note that the conditions we have given for the hourglass are ideal conditions which we would require to know exactly how long it is since the hourglass started running.If conditions are less than ideal, we may still be in a position to come up with an approximate figure which is better than nothing.If, for example, we cannot measure precisely the amount of sand in the lower bulb, but we have good reason to think that our estimate of it must be within 10% of the true value, then we also have a good reason to think that we can give the time it's been running to within 10%.Or again, if there may have been sand in the lower bulb when it started running, but we have a good reason to think that there can't have been very much, then we also have a good reason to think that the figure which we get for the time can't be very wrong; and so on.