In addition, because sediment deposition is not continuous and much rock material has been removed by erosion, the fossil record from many localities has to be integrated before a complete picture of the evolution of life on Earth can be assembled.
Using this established record, geologists have been able to piece together events over the past 635 million years, or about one-eighth of Earth history, during which time useful fossils have been abundant.
By mid-century the fossiliferous strata of Europe had been grouped into systems arrayed in chronological order.
To avoid confusion later on, let us say at once that the "absolute" in "absolute dating" is not short for "absolutely correct".
In this context, the word "absolute" is not the opposite of "approximate" but just of the word "relative".
Local relationships on a single outcrop or archaeological site can often be interpreted to deduce the sequence in which the materials were assembled.
This then can be used to deduce the sequence of events and processes that took place or the history of that brief period of time as recorded in the rocks or soil.
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.A method of dating which was accurate to within a billion years either way would still technically speaking be a method of absolute dating, it would just be a very bad one.Suppose that we wanted to find out how long it has been since an hourglass was set running by measuring the amount of sand in the lower bulb.To do so successfully, we would need to assure ourselves of the following conditions: Given these conditions, we can find out how long the hourglass has been running.If we wish to use a geological process as the geological equivalent of an hourglass, we would want to have similar conditions: we would like to find some quantity which we can measure reliably (corresponding to the condition that we can measure the amount of sand in the lower bulb of the hourglass); which increases or decreases from a known quantity (corresponding to the lower bulb of the hourglass being empty when it starts running) at a known rate (corresponding to knowing the rate of flow of sand); and so forth.Coastal Winds and Clouds Greenhouse Effect Ray Tracing X-ray Imaging Ultrasound: How Does it Work?