Many kinds of rocks form in broad, flat layers, called beds, that stack up like the layers of a cake. In areas like northern Arizona, thick stacks of rock beds that have built up over millions of years remain in their original flat orientation (where they can be viewed as multicolored horizontal layers of rock that make up the spectacular walls of the Grand Canyon). In places like California near active plate boundaries, however, the forces that make earthquakes don’t leave the beds flat for long, but bend and tilt them.

Tilted beds are shown on a geological map with a strike and dip symbol (location 10). The symbol consists of three parts: a long line, a short line, and a number. The long line is called the strike line, and shows the direction in the bed that is still horizontal. Any tilted surface has a direction that is horizontal (think about walking on the side of a hill, there is always a way to go that is neither up nor down, but is level).

The strike line shows that horizontal direction in the beds. The short line is called the dip line, and shows which way the bed is tilted. The number is called the dip, and shows how much the bed is tilted, in degrees, from flat. The higher the number, the steeper the tilting of the bed, all the way up to 90 degrees if the bed is tilted all the way onto its side. Strike and dip symbols can be modified to give more information about the tilted beds just like lines can be, and these modifications are also explained in the MAP KEY.

Information provided by: http://geology.wr.usgs.gov