DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY
A vertical bed of quartzite at Mitcham, Australia, part of the Adelaide Geosycline. Quartzite is a metamorphic rock derived from sedimentary sand- stone. The undulating structures visible on the rock are ripple marks, a relic of this sedimentary origin; they were shaped, before rock formation took place, by the movement of fluid over the unconsolidated sediments. The rocks here were formed in a shallow water environment around 750 million years ago and converted to quartzites and slates around 450-500 million years ago. Following a period of mountain building which occurred about 50 million years ago, the beds are presently around 300 metres above sea-level and vertical.
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