As described earlier, fossils are nearly always found in sedimentary rocks. So regions of igneous or metamorphic rocks are of little interest to fossil hunters. The age of sedimentary rocks is also important. If you have become interested in, say, fossil mammals, then it' s little use targeting rocks from the Palaeozoic Era. Likewise if you are looking to build a collection of trilobites or ammonites, then Tertiary rocks are too recent and will not contain them.