In this photo we are looking at rock beds, tilted till they are nearly vertical, and exposed in three levels in a quarry near Kirby, Arkansas, Ouachita Mountains. Like a humongous 3-step staircase, each ascending level of the outcrop provides a deeper view into the rock formation. An outcrop like this one illuminates a couple of basic but important concepts of geology: key beds, and strike and dip.
The key beds or beds that can be traced across the outcrop, such as the one marked with red dots above, appear to shift to the right as your eyes ascend the steps. These are not faults! It’s an optical illusion. If our view were aligned parallel with the sides of the beds, they would appear aligned, but our view is actually diagonal to the bedding. To illustrate this, here is the same picture with a drawing of the key bed as if it were jutting out of the outcrop.
This is why geologists measure the orientation of rock beds – known as the bed’s strike and dip. Knowing how a bed is oriented in one place can help you to predict where it will be in another, perhaps inaccessible, place such as deep in the subsurface. If that bed is full of oil, gas, or other precious commodity, predicting where it is becomes very important.