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| Geological photographs from around the world | ||||||||
Sedimentary structures are valuable indicators of the geographical conditions which prevailed at the time of deposition. They can also be used to show which whether the rocks in which they occur are still the correct way up, or whether they have been turned upside down by the immense forces of mountain building at a destructive plate margin when two tectonic plates move towards each other.
In each of the following photographs of sedimentary structures, identify the features which show the top and bottom of the structure, bearing in mind how each one was formed and what may have happened to it soon after its deposition.
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Current bedding How does the dip of each lamina change from top to bottom? What caused the horizontal boundary towards the top of the photograph? |
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Dune bedding How does the dip of each lamina change from top to bottom? What caused the horizontal boundaries between each set of dune bedding? |
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Ripple marks Is the crest of each ripple the same shape, in profile, as the trough? Can you account for this? |
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Mud cracks What is the shape of each crack, seen from the side? Why is it that shape? |
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Graded bedding This one is too easy - the coarser grains are at the bottom. But the coarse sediment forms the 'filling' in the middle of the sandwich of two layers of fine sediment. Can you tell the difference between
What differences would you expect to see? |
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Geopix Study Guide "Way-up Criteria"
- gives a full image of each of these and other types of way-up criteria;
- explains how the 'top' and 'bottom' of each criterion can be proved;
- explains other methods of relative dating - included fragments, cross-cutting relations, application to lava flows etc
- suggests how to answer exam questions on this topic.
The Way-up Criteria Study Guide contains the following pages:
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To order this CD go to the Geopix Online Store