General Study: Strike and Dip + Rock ages and types

When it comes to geologic maps specifically, a legend or key is often provided to explain what rocks are present and how old each one is. Simpler maps will often just state the specific rocks and their ages, but scientific geologic maps possess more depth. Geologic maps will often use patterns, colors, or both to indicate whether a rock is metamorphic, sedimentary, or igneous. Special alphanumeric symbols are utilized to present a rock's ID and age alongside the previous indications to allow no confusion for readers. Familiarity with the geologic timescale is needed to make sense of the different ages that rocks may have been formed in such as the Cenozoic, Triassic, or Pleistocene (Teacher Background Sheet: How to Read A Geologic Map). While they may not be present on every map, fossil records of a specific layer of rock can also help to determine its age, as organisms of the past only existed during certain time periods. If the information on a geologic map is lacking the above, the best simple conclusion you can make about its structure relating to age is that lower layers are older than higher layers. If the structure is inclined (such as in the above image), you can see which layer is the oldest by finding the layer furthest down and in the direction the angle is pointing. Using the image as an example, the oldest layer is likely the grey in the bottom left corner. This is because it takes the least amount of movement to put it on the bottom of a horizontal stack compared to the black in the top right ("Teacher Background Sheet: How to Read A Geologic Map").