Fossils on Friday – Sedimentary Rock

Hello and welcome back to Fossils on Friday on Blog About Britain!

 If you remember last term, we spent a long while discussing the principles of crossbedding and sideways deposition. This term, we’re going to be building on that idea, and discuss some of the different principles in geology, using examples from around Britain. This will give you a basis in geology, which will help you understand much better, as well as being able to refer to each subject as we go forward.

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Stunning sedimentary rock layers at Stair Hole, Lulworth Cove.

 First, let’s go through each major rock type, starting with sedimentary. Sedimentary rock is extremely common and is made up of sediments. Sediment refers to the small particles of rock, clay, or mud, which build up to create a rock. The names of the particles are a reference to their size, starting with the smallest, mud, moving up through silt, marl, sand, pebbles, cobbles, and boulders. A lot of time, the stone which the sediment forms is then named after the sediment type, i.e. sandstone, mudstone, etc.

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Red sandstone at Budleigh Salterton, Dorset.

 The old idea regarding sedimentary rock formation is that the sediment particles would be sitting suspended in water, slowly falling down, settling on the sea floor, building up the sedimentary rock. There are two big problems with this, however:

  1. How did the sediment get there in the first place? To get rock particles suspended in water, you need fast erosion, formed by water moving sideways at speed.
  2. Sediment deposition needs flowing water in order to carry and deposit it. Think of waves, rivers and tides. Everything we’ve observed shows that flowing water is required for sediment deposition.

 These problems are discussed more thoroughly in last term’s posts if you want to re-cap. But the point is simple, sedimentary rock was eroded, transported, and deposited by fast flowing water!

Next week, we look at a very explosive type of rock……