Exactly how a rock can record the passage of time is one of the main themes of this book. It has long been recognised that much of the rock material of the Earth is preserved in layers that were originally laid down by natural processes, one upon the other. Look at any ancient sandstones or limestones used as building stone. It is not too difficult to see layered grains of sand or fossil shells that give clues as to how they were originally formed.
And it does not require any great imaginative feat to realise that this successive layering could be interpreted as some measure of time passing - if only we knew how quickly or slowly the layers were laid down. By comparison with everyday depositional events, it was apparent that earlier deposited layers were to be found at the bottom of the pile, younger ones were to be found higher up and the most recent would be on top - pedantically known by geologists as the 'law of superposition of strata'. Observation of how mud and sand become layered in riverbanks and on beaches, how snow accumulates in glaciers and ice sheets, how volcanic ash blankets landscapes or even how trash accumulates in a waste bin, all illustrate this very simple law and allow us to understand how layered deposits build up on the Earth's surface under the prevailing influence of gravity.
My aim is to tell the story of how the existence of the Earth's deep prehistoric geological past was first discovered, recovered from the rock record and then reconstructed into our present understanding of Earth history - what I am calling Earth Time.
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