color standard from the International Commission on Stratigraphy that is somewhat similar, though it differs for the Mesozoic (blue) and Paleozoic (green) eras. However, even given those standards, choosing the right colors for geologic map units can be difficult, especially on maps that contain a large number of different map units. The general rule is that younger map units are assigned light colors and older map units are assigned dark colors. This kind of color scheme can help to illustrate the evolutionary construction of an area by allowing the viewer to see how the age of rocks changes through the landscape at a single glance. The Washington Department of Natural Resources Division of Geology and Earth Resources uses the following general guidelines for rendering map units by age (Figure 6.105). These colors mostly follow the colors found on the U.S. Geological Society’s Rocks of Ages legend, though the Quaternary period colors differ.10 It is important to note that these colors apply mostly to sedimentary rocks. Igneous and metamorphic rocks — the other rock classes — are traditionally red and orange and brown and olive, respectively. It is also standard for geologic map unit legends to be organized with the youngest map unit at the top of the legend and the oldest map unit at the bottom of the legend. Though there are definitely traditions and standards as discussed and illustrated above, these are not applied as consistently as those of us who want a simple key would hope. For example, some sources say that plutonic rocks are pink and volca‑ nic rocks are red, while others say that plutonic rocks are red and volcanic rocks are orange. Even the concept of colors getting darker as the age of the rock gets older is not equally applied everywhere. The best thing to do when handling new geologic data then is to first find out if the originating agency has a predefined color scheme. If so, take a look at it and decide if it is adequate for your particular map. Pay par‑ ticular attention to the differentiation of the colors between adjacent polygons. You may have to tweak colors here and there depending on the geology of the area you are mapping so that differences between map units are easier to detect. This issue also comes up when you are designing your own color scheme. While you might want to assign a different color for every different type of rock, of which there could be fifty or more types, you may not know until you have assigned all the colors whether two very similar colors will wind up adjacent to one another. When this occurs, you may need to consider an alternate color scheme or utilize patterning for one or more of the rock types. Assigning a pattern fill to rock types is a common practice for maps when there are simply too many map units for color alone to allow adequate differentiation.