Earth Time the rocks

Most people do not realise that the vast majority of the rock record of sedimentary strata (otherwise known as the stratigraphic record) is made up from deposits (inorganic sediments and organic remains of dead organisms) that were originally laid down on the seabed - sands, muds and carbonates (lime-rich sediments that subsequently become limestones). So enormous has been the public 'puffing' of dinosaurs, which were solely land-living reptiles, that it is often assumed that there must be a good rock record of terrestrial deposits. Dinosaur fossils are not nearly as common as you might think. The fossil remains of our extinct human relatives are even rarer, partly because our ancestors lived on land but also because there are far fewer human-related species than there are dinosaur species, and ancient human-related populations were small and mostly confined to Africa. The existence of the fossil record of our immediate ancestors is largely thanks to the geological nature of the Great East Africa Rift Valley, in which great thicknesses of relatively recent deposits have accumulated and been preserved.

Overall, land-based sedimentary strata are uncommon in the rock record, although there were times, such as parts of the Devonian, late Carboniferous (known as the Pennsylvanian in North America), Permian and Triassic periods and regions of the continents (especially the flanks of major mountain belts), when significant thicknesses of land-based sediments accumulated.

Why are they not so common? In a word - gravity. Land is for the most part above sea level and subject to all the processes of weathering and erosion by wind, rain, ice, running water and biochemical attack that tend to reduce land surfaces. Loosened material is carried away from uplands and temporarily dumped in lowlands before eventually being removed to the sea.

Preservation of land-based sediment is certainly possible, witness the plant-derived coal deposits that fuelled the Industrial Revolution. But certain geological conditions have to be met. First, there has to be some largescale 'trap' or 'sink' in which sediments can accumulate in significant thicknesses, such as a deep lake basin or down-faulted valley. Secondly, the deposits have to be covered over by younger sediments and protected from subsequent erosion by being buried to a considerable depth (in the order of kilometres). And finally, the deposits have to be lithified (turned into rock), a process that normally results from burial, which also compacts and chemically transforms sediments into tougher and more long-lasting sedimentary rocks. When brought back to the surface through earth movements, such rocks are more resistant to weathering and erosion than unconsolidated sediments.

So the stratigraphic rock record is highly biased towards deposits that were laid down in seas that developed on and around the continents, as opposed to the much deeper oceans that lie beyond the edge of the continents. At present global sea levels are relatively low and not much of the continental surface is extensively flooded, except for regions like Hudson's Bay in North America, the Black Sea and the Caspian Sea in southwest Asia.

Of course, it is also true that some two-thirds of the Earth's surface is covered with sea, but most of this is deep ocean, with an average depth of some 3000 m (9000 ft). While sediment and organic remains do accumulate on the ocean floor, very little of these deposits are recruited into the rock record in the long term because of the movements of the Earth's crustal plates. The oldest ocean floor is no more than about 180 million years old, which might seem old enough but is not when compared with the 4.5-billion-year age of the Earth.

We will return to this topic, but in general as much ocean floor is destroyed in subduction zones (marked by deep ocean trenches) as is created in ocean-spreading ridges, otherwise the Earth would expand, which it is observably not doing. Consequently, ocean floor sediments and their organic remains tend to be destroyed by being dragged down into the Earth's interior rather than being added to the stratigraphic rock record on the continents. Nevertheless, as we shall see, there are interesting and important exceptions.

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