Why are geodes found in some places and not others?
The presence of geodes appearing on the surface of a given locality is dependent on the geologic history of that area. Geodes are usually found in areas of exposed and eroded sedimentary rock. The shell of the geode, often quartz, weathers less vigorously than the entrapping rock, usually calcitic or dolomitic limestone. In northern lattitudes, glacial deposits may have covered up the geodes and sedimentary rock, or perhaps the sedimentary rock layer did not exist previously or has eroded away completely.
