Rock Strata

Reading the stratigraphical notebook is not just a question of opening the pages.  A geological map of the world today is in effect a patch work of different environmental conditions, and the same was true at each stage in the past. The limestone was forming in one place a hundred million years ago does not imply that limestone was forming everywhere at that time. While rocks are deposited in one place, another place may be uplifted into mountains and eroded.

 There are however, gaps in the record, and things are not always what they seem. Although younger rocks are deposited on top of older ones, folding and faulting can be so intense that the younger rocks end up underneath the old. Layers can be folded up to steep angles, or the original layers may be sloping such as current bedding at a river delta or a continental slope. Thick shale beds may have take a few hundred thousand years to form and be tens of meters thick; in the same strata there may be another layer only a few centimeters thick that was five million years in the making. So thickness is no certain clue to age; neither is apparent depth of water. Sea level can change by tens of meters, land level by hundreds. Nor is present latitude much help, when continents have skated across the globe. Britain was once on the equator and there were glaciers in what is now the Sahara desert. But there are plenty of clues for geologists and with their knowledge of processes at work can compute the stratigraphical evidence from around the world to confirm their theories on geological maps.