1-Starting on the Ground - Topography on Maps
The map in your car's glove compartment doesn't have much on it beyond highways, towns, shorelines, and borders. And yet if you look at it closely, you can see how hard it is to fit all that detail on paper so it's useful. Now imagine that you want to also include useful information about the geology of that same area.
What's important to geologists? For one thing, geology is about the shape of the land—where the hills and valleys lie, the pattern of streams and angle of slopes, and so on. For that kind of detail about the land itself, you want a topographic or contour map, like those published by the government.
Here's the classic illustration from the U.S. Geological Survey of how a real landscape on the top translates to the contour map beneath it. The shapes of the hills and dales are depicted on the map by fine lines that are contours—lines of equal elevation. If you imagine the sea rising, those lines show where the shoreline would be after every 20 feet of depth. (They could equally well represent meters, of course.)
2- Contour Maps
Notice that even though the map is a flat sheet, you can still figure out accurate numbers for hill slopes and gradients from the data encoded in the image: you can measure horizontal distance right off the paper, and the vertical distance is in the contours. That's simple arithmetic, suitable for computers. And indeed the USGS has taken all its maps and created a "3D" digital map for the 48 states that reconstitutes the shape of the land that way. The map is shaded through another calculation to model how the sun would illuminate it.
3-Topographic Map Symbols
4-Symbolizing Geology on Geologic Maps
Contours and topography are just the first part of a geologic map. The map also puts rock types, geologic structures and more onto the printed page through colors, patterns and symbols.
Here's a small sample of a real geologic map. You can see the basic things discussed earlier—the shorelines, roads, towns, buildings and borders—in gray. The contours are there too, in brown, plus the symbols for various water features in blue. All of that is on the map's base. The geologic part consists of the black lines, symbols and labels, plus the areas of color. The lines and the symbols condense a great deal of information that geologists have gathered through years of fieldwork.
5-Contacts, Faults, Strikes and Dips on Geologic Maps
Lines on the map outline various rock units, or formations. Geologists prefer to say that the lines show the contacts between different rock units. Contacts are shown by a fine line unless the contact is determined to be a fault, a discontinuity so sharp that it's clear something has moved there. (see more about the types of faults)
The short lines with numbers next to them are strike-and-dip symbols. These give us the third dimension of the rock layers—the direction they extend into the ground. Geologists measure the orientation of rocks wherever they can find a suitable outcrop, using a compass and transit. In sedimentary rocks they look for the bedding planes, the layers of sediment. In other rocks the signs of bedding may be wiped out, so the direction of foliation, or layers of minerals, is measured instead.
In either case the orientation is recorded as a strike and a dip. The strike of the rock's bedding or foliation is the direction of a level line across its surface—the direction you would walk without going uphill or downhill. Dip is how steeply the bed or foliation slopes downhill. If you picture a street running straight down a hillside, the painted center line on the road is the dip direction and a painted crosswalk is the strike. Those two numbers are all you need to characterize the rock's orientation. On the map, each symbol usually represents the average of many measurements.
These symbols may also show the direction of lineation with an extra arrow. Lineation might be a set of folds, or a slickenside, or stretched-out mineral grains or similar feature. If you imagine a random sheet of newspaper lying on that street, lineation is the printing on it, and the arrow shows the direction it reads. The number represents the plunge, or the dip angle in that direction.