Stories in rock

Most people notice the different layers, thicknesses, and colors of rocks. They are the ubiquitous backbone of our scenery and when uplifted and eroded into shapes like mountains, arches, mesas, buttes, caves, canyons, and cliffs they become the focal points of parks and scenic overlooks. Other than that, rocks usually don't get much attention except from geologists. Not only do we admire them from afar, we look at them with microscopes, x-rays, and MRI. We crush them, dissolve them, sort them by size, and dye them. We see which parts float in what liquids. The things geologists haven't done to rocks to try and tease out a bit more information probably just haven't been thought of yet.

The rocks that contain fossils are sedimentary rocks. As part of the rock cycle, erosion breaks rocks into particles. These particles are moved around by wind, water, or ice and eventually settle into layers. Each process, blowing wind, still or flowing water, and creeping ice leave traces in the rocks. Wind blown dust is small. The particles are often sharp, angular, and frosted like ground glass. Swiftly flowing water rapidly rounds and polishes particles and sorts them by size. As flowing water slows down, bigger particles settle first followed by smaller ones.

Each environment places its own characteristic mark on rocks. Geologists study many modern environments: beach, estuary, swamp, marsh, reefs, barrier islands, sabkha, tidal channel, river, delta, pond, lake, desert, and more. The conditions that prevail in each environment leave clues in rocks that can be used as analogs to determine how ancient layers were deposited; this is Hutton's "uniformitarianism". The type of rock deposited is often limited by the environment. Most limestones form in relatively shallow, warm tropical seas; they don't indicate arctic conditions. Coal forms in swamps. The rocks on an ocean beach pile up differently than the rocks forming in a delta where a river meets that same ocean.

Fossils also are characteristic of the environments where rocks form. A mixture of fish, shells, and star fish indicate ocean conditions. The tree trunks, branches, and leaves (as in coal) indicate swamps. Some environments don't have many fossils. Creatures that die in open, temperate grass lands are scavenged, scattered, and crushed. Bugs and microbes finish the job breaking down the bones into the soil. In other places, the relentless action of waves grinds any fossils to tiny particles you wouldn't recognize as a fossil, but geologists with a microscope would.

It is the type of rock (limestone, sandstone, shale, and others), the mixture of particle sizes and shapes, the abundance and types of fossils, the marks left by wind, water, and ice (ripples, cross bedding, striae, and more) that all tell stories about how a rock layer came to be. The rock layers are stacked one upon another in somewhat predictable patterns. Rock types formed side-by-side over large distances can often be stacked as well. For example, along the Gulf Coast from New Orleans to Key West several different rock types are being deposited. As these environments shift over time, the places where mud (Mississippi River delta), quartz sand (Destin), and lime sand (Key West) will shift and stack.

The processes responsible for depositing rocks occur at different rates. Floods, storms, landslides, slumps, and earthquakes are all very fast and each can change the rocks being deposited in the course of seconds to weeks. Glaciers, lakes, and rivers are slow, the processes often persisting for thousands of years. Regular daily tides move mud around broad flats (like the Bahamas) but show variations related to spring and neap tides; the deposition of a single layer may be fast (a day), but the environmental conditions may persist for millennia. All of these processes are dynamic, a continuous cycle of eroding and depositing, shifting environments, and drifting continents.

It is the stacking of rock layers that demonstrate the many layers weren't deposited by a single catastrophic flood. The variations of rock layers over time show rocks were deposited by many different processes. In geology, the studies of the environments and processes whereby rocks are deposited and then relating that information to understanding ancient layered rocks are stratigraphy and sedimentology.