Thursday, November 1, 2007

Bits and Pieces

When the team of sedimentologists looks at the core, they describe one layer of rock at a time - one episode of deposition. One of the main characteristics that sedimentologists describe in a rock layer is its texture.

Texture in the wider world usually refers to how something feels when you touch it and describing texture uses words like rough, smooth, bumpy, fuzzy, soft, and scratchy. In geological terms, texture refers to the sizes and roundness of the particles or grains that make up rock. It does often translate into differences you can feel when you touch the rock, but the terms used describe the grains.

Some common terms used to describe rock texture are:
sand (smallest grain size you can still see with your naked eye, feels gritty);
silt (smallest grain size you can see with a magnifying lens, still feels gritty);
clay (no grains visible without the use of a microscope, it feels smooth);
angular (sharp edges to grains);
rounded (curved edges to grains); and
spherical (how closely the 3D grain shape conforms to a globe shape).

While rock texture is purely descriptive observation of the rock and includes no interpretation of the processes that created the rock, the texture is a strong indicator of the environment at the time of deposition and what happened to the rock particles before deposition.

Look at the rock in the two pictures. How are their grain size and shape similar? How are they different? What other comparisons can you make?

How are the grains arranged or sorted?

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