mysterious rock to figure out First of all, most readers of these words probably live atop trucked-in dirt, or dirt that has been scalped and leveled by large machines. Therefore, here we're using he term "backyard" in a figurative sense, meaning "in the general neighborhood, or at least in the region," where surely someplace something like the mysterious outcropping at the right must exist. Having understood that, here are some insights into "backyard geology."

Using Geology Maps

Geology maps can tell us what kind of geology to expect in our areas. Take a look at our generalized geology map of the US. That map is gorgeous but it doesn't tell us much without the color-coded key that goes with it. I'm not providing it here because on a computer screen it's almost impossible to match the colors. The map is presented just to show you an example of a geology map, and so you can get a little practice using one.

For example, on that map I grew up on a farm in the very center of the gray area shown in the central part of western Kentucky. (Kentucky is that state in the middle of the eastern US, shaped like a club, with the Ohio river forming its meandering northern border). The map shows that the same geological unit on which I grew up extends way into Illinois. The color key provided with the map says that the gray area denotes areas of outcropping rocks of Pennsylvanian age -- rocks 290-323 million years old. And books tell me that Pennsylvanian rocks in that part of the world are mostly coal-bearing sandstone, with a little shale and a tiny bit of limestone.

Well, my wanderings there confirm that all that is perfectly true. There's a lot of coal mining in my home area, as it is in southern Illinois, and the fossils I found there as a kid are Pennsylvanian-age ones.

The more local your map is, the more specific the information can be for your local area. Below is a typical geology map for a US state, that of Kentucky, presented here with thanks to the Kentucky Geological Survey.

geology map of Kentucky

This map gives more specific information than the big US map possibly can. For instance, notice my home area again (central western area), which in this map is represented in dark blue with fingers of yellow, instead of the gray blob on the big US map. The key at the left in the Kentucky map identifies the yellow areas as alluvial material of the kind found along river floodplains, and that's exactly right. I grew up in a swampy area on alluvium, but I was surrounded by coal-bearing, sandstone-hill uplands, exactly as the map shows. I also have a county map of my home county, and that shows the location of oil fields, minor faults, hills and streams, and other things even the above map can't show. Geology maps are wonderful.

The colors in the above map, as is typical of most geological maps, relate to the geological age of the rocks outcropping at the ground's surface. Thus the dark blue area covering the easternmost, or right side of the map, is keyed as "Pennsylvanian." We would say "Eastern Kentucky's outcropping rocks (rocks at the earth's  surface) are Pennsylvanian in age." Here the word "Pennsylvanian" refers to the "Pennsylvanian Period" of geological history. Now you need something to tell you how old "Pennsylvanian rocks" are. You can find this out at our Geological Time Scale Page.

View of the Grand CanyonWhen you think of "geological processes," maybe the first thing that comes to mind is the Grand Canyon, pictured at the right. You know that the Grand Canyon got the way it is mostly because of erosion -- the wearing away of the land by water. But of course there are several kinds of erosion, and other processes than erosion that alter the geological landscape, and many of these processes can be seen and studied in your own area.

These processes are so numerous and varied that we can't go into them in detail here. However, just to give you a taste of the world of geological processes, more technically thought of as the study of geomorphology, here's a chart put together from my notebook from back when I took a college course in geomorphology:


  1. Caused by events at the Earth's surface
    1. Processes that bring things down to a common level
      • Weathering (disintegration of rock in place)
        1. Physical
          • Expanding because of heating
          • Crystal growth
          • Removal of heavy weight above
          • Activity of plants & animals
        2. Chemical
          • Hydration
          • Hydrolysis
          • Oxidation
          • Carbonation
          • Solution
      • Mass wasting or removal by gravity
        1. Landslides
        2. Mudflows
      • Erosion
        1. Running water
        2. Groundwater
        3. Waves, currents, tides & tsunami
        4. Wind
        5. Glaciers
    2. Processes that build things up
      • Running water
      • Groundwater
      • Waves, currents, tides & tsunami
      • Wind
      • Glaciers (more info)
    3. Work of living things, including humans
  2. Caused by events inside the Earth
    1. Continental drift (more info)
    2. Volcanism
  3. Caused by events off the Earth
    1. Meteorites