Reading a bed

Geologists can learn a lot from careful study of a rock face. For instance, parallel lines, similar to high tide marks on a beach, show that you are looking at a rock made from sediments deposited in shallow water, buried and lithified, and now exposed again by erosion.

However, the full story of a rock exposure can be a complex one. Take a look at a long, uniform stretch of sandstone. It was laid down at one time, right? Maybe. But what if the sea was moving inland at the time? One end could be a million years older than the other. This is called diachronism.

Or look at one layer of rock on top of another. Did it come straight after the lower layer, or did a few million years elapse between their being laid down? If so, the join between them is an unconformity and the layers are unconformable. In the past, only examining the fossils they contained would sort the issue out, unless there was another exposure somewhere else in which the same sediments were separated by some other layers. Often an unconformity was only apparent because of some tilting of the rocks between one deposition episode and the next. Nowadays it is sometimes possible to date the sediments directly, from the radioactive elements they contain.

Often, you will find that a rock exposure is bisected by a murky line, and that the rocks on either side of it do not line up. This is a fault, testament to ancient seismic activity. The crushed rock in it is called a fault breccia (an Italian word for a rock made up of cemented bits of other rocks). If you can find a small fault and spot the same bed on both sides, measure how far one has moved relative to the other. Now you have established the “throw” of the fault and may regard yourself as a true geologist.