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There may be a layer missing in the strata, or a set of sedimentary rock on top of metamorphic rock.
These interfaces between discontinuous layers of rock are called unconformities.
It's called the Principle of Original Horizontality, and it just means what it sounds like: that all rock layers were originally horizontal. As you can imagine, regular sediments, like sand, silt, and clay, tend to accumulate over a wide area with a generally consistent thickness.
It sounds like common sense to you and me, but geologists have to define the Principle of Original Horizontality in order to make assumptions about the relative ages of sedimentary rocks. Say you have a layer of mud accumulating at the bottom of a lake. More sediment accumulates from the leaf litter and waste of the forest, until you have a second layer.
Inclusions are always older than the sedimentary rock within which they are found.
Other times, geologists discover patterns in rock layers that give them confusing information.
Learn how inclusions and unconformities can tell us stories about the geologic past.We'll even visit the Grand Canyon to solve the mystery of the Great Unconformity!