Individual places and topics of interest
Gibraltar, "upside down"
One of the many remarkable things about the rock of Gibraltar is that it is almost upside down !
The Rock is composed of Jurassic limestone, originally deposited in warm shallow seas (Chapter 2), subsequently buried, and then uplifted during the Miocene as part of the collision between Africa and Eurasia (Chapter 3). Sometimes when layers of rocks are compressed and folded, they can get to the extreme where the folds themselves fold over - a bit like they would if you pushed a tablecloth from each end. The diagram opposite illustrates the concept. The cartoon shows the progressive stages of rock layers being folded over; the centre photo shows a real-life example of an overfold (not Gibraltar); and the right hand photo shows how the real Gibraltar relates to such a fold - with previously surrounding material now eroded away ! The result is that the upper layers of limestone on the Rock are actually older than the ones below them - normally younger sediments would be on top of older sediments. Overturned, or "recumbent" folds are just one example of the dramatic deformation that rocks can suffer during the plate tectonic processes of mountain building. These are summarised in the Background section on 'Deformation'. Some more local examples are shown opposite. |
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