For an explanation, see the main splats page
SPLATS about sedimentary rocks
The principles of sedimentary rocks
- Sediments may be compressed and heated to form sedimentary rocks, while sedimentary rocks are eroded in turn and weathered to form yet more sediments.
- When you look at sedimentary rocks, the ones on the bottom are the oldest, because the rocks are laid down in order as sediments that later harden into rock.
- Sedimentary rocks are formed when sediments are covered, compressed and heated to some extent, so the grains of sediment become cemented together.
- The oldest sediments contain the oldest fossils, so lower sedimentary rocks contain older fossils. This provides some of the evidence for evolution.
- In 1669 Steno, in his 'Prodromus', suggested that tilted strata of geology were originally laid down horizontally, and were later lifted up by some force.
- When sediment is carried to the front of an advancing bank of sediment and pushed over the edge, it forms a characteristic slope at the angle of repose or rest.
- Sediments are laid down in strata, comparatively horizontal layers, except in cross-bedding or current bedding, when they are laid at the angle of rest.
- When sediment forms a slope at the angle of repose, this angle is determined mainly by the shapes of the particles and the medium (air or water) of deposition.
- When sediment is pushed down a slope at the angle of repose, it forms a bed laid down at that angle, rather than horizontal. This is current bedding.
- Wind transports sediment, but generally over short distances only, unless the particles are fine. Dust particles may be carried from one continent to another.
- Fine sediments can be carried long distances by wind: from Australia across to New Zealand, from China to the US or from the Sahara to the US in proven cases.
- The sediment particle size depends on the speed of the wind or water flow, and coarser sediment settles as the speed drops away, producing graded deposits.
- Glaciers transport sediment, picking up rocks and grinding them across the countryside, producing 'rock flour' along the way, and washing it out in meltwater.
- Glaciers may move and create sediment throughout the year, but more sediment is released in summer when the ice melts more and the meltwater flows increase.
- The seasonal variation each year in the flow of meltwater from the head of a glacier produces varved deposits, which later may be compressed to varved shales.
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