Turbidites!
These rock layers may not look that distinctive, but to geologists these tell a really cool story. These are turbidites, the remnants of debris flows off the shore of an ancient ocean.
Turbidites form when sediment piles up just off of a shoreline, often carried to the area by a river. Eventually, even underwater, big enough piles of sediment will collapse and avalanche downslope. Sometimes they do so under their own weight, sometimes an earthquake will set them off.
The avalanche of debris produces a recognizable pattern to geologists. The heaviest particles, the biggest grains, settle out at the bottom of the debris flow, and the sequence “fines upward”, meaning the grain sizes get smaller.
A typical turbidite will start at the bottom with sandy grains, maybe even larger stuff, and the grain size will decrease going upward as progressively finer grains settle out. Finally, each turbidite is topped by a layer of very fine grained clay particles that can even be a different color from the stuff below it. This sequence even has a name – the “Bouma” sequence.
Turbidites show up throughout the geologic record because they’re easily preserved. They form in areas in the ocean that aren’t likely to be eroded and they form in areas with lots of sediment that can bury and protect them afterwards. This sequence photographed here shows several turbidites - the big units are the coarse-grained sand, while the thinner layers are silt and clay rich.
-JBB
Image credit: https://flic.kr/p/UgSoRead more: https://courses.washington.edu/sicilia/pdf/JBturbidites_fans.pdf http://trg.leeds.ac.uk/