Miocene To Pleistocene

In the immediate Canyon area, the rock record stops in Permian times over 250 million years ago, although Earth Time has continued ever since. It is Permian-age rock strata that mostly form the present bedrock of the plateau landscape immediately surrounding the Canyon. We have climbed through about a mile's (1.6 km) thickness of Precambrian and Palaeozoic strata from the bottom of the chasm to the rim with its Permian-age strata. And although we come to an abrupt halt in our ascent through Earth Time, there is evidence to show that another mile's worth of Mesozoic and Cenozoic strata were originally laid down in the region, but have since been stripped away by the very processes that created the Colorado Plateau and reactivated an ancient river to cut the Grand Canyon.

The bad news for the aficionados of dinosaurs is that, as a result, the 'terrible lizards' and their relatives have been cut out of the story. Not only that, but the two major extinction events that frame the beginning and end of the Mesozoic Era have also been wiped off the record here. We cannot completely ignore these major episodes in Earth Time, but they really require another book or two to do them justice.

Mind the gap - the small matter of the Permo-Triassic extinction...

Neither Murchison nor any other mid-nineteenth-century geologists spotted any sign that the end of Permian times was marked by the biggest extinction event in the history of life. Perhaps as few as 10 per cent of species survived beyond the end of the Permian, so there really was a huge turnover in life forms. It is not as if Murchison and his contemporaries were not concerned about the changes in fossils from one group of strata to the next; they were. It was because it was the presence of such changes that they were using to justify distinct 'systems' of strata.

Part of the problem was that although the concept of catastrophic revolutions in the history of life was not new, such 'revolutions' had generally been dismissed as a reality by the 1830s. Catastrophism was associated with the outmoded ideas of the Diluvialists and the very idea of 'revolution' was too closely associated with the all too real and recent horrors of French Revolution for comfort. Since the dénouement of the Flood as a significant event in the history of life, gradualism, as propounded by Lyell, was the generally accepted historical process underpinning Earth processes and the rock record. Extinction of species and even whole groups of organisms such as the trilobites, graptolites or ammonites was accepted, but their loss was not connected to any specific large-scale catastrophic events.

Indeed, the most innovative idea about the history of life, the Darwin/Wallace theory of evolution, required slow, gradual progression and change rather than any dramatic collapses and subsequent 'explosions' of renewed life. Detailed records and descriptions of fossils and their distribution in time and space was only just beginning in the mid-nineteenth century. Nobody was in a position to review the overall state of the record yet, but it was not long before such overviews were possible.

Curiously, it was Darwin's attack, in The Origin of Species, on the imperfection of the fossil record that in 1860 prompted one of the first and most informative of the early overviews of the record by John Phillips. In i860 Phillips was President of the Geological Society and had been invited to give the prestigious Rede

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