Many rocks show layers.
Sometimes the layers are twisted and bent.
Sometimes they’re perfectly parallel.
Sometimes strange things happen
Just because the layers look similar, does not mean that they are the same thing.
In SEDIMENTARY ROCKS, these layers are called BEDDING, and are the result of slow accumulation of sediments, whether they be little bits of rocks or shells or calcite precipitating out of water.
In METAMORPHIC ROCKS, these layers are called FOLIATION, and are the result of the alignment of mineral grains and/or the segregation of minerals into different bands.
The important first step in distinguishing between bedding and foliation is recognizing whether the layered rock you are holding is sedimentary or metamorphic.
Sedimentary rocks are typically formed of mineral grains that appear to be glued together (because they are). The grains may be rounded due to transport down a river. Mineral grains in a sedimentary rock will not appear to have grown together, merely stuck together. Organic bits, like coal or plant fossils are found only in sedimentary rocks. Other fossils, bones and shells for example, are also strong evidence that you’re observing a sedimentary rock and that the layering you observe is bedding.
Foliation is the result of heat and/or pressure on a rock, causing mineral grains to align and in some cases for minerals to segregate themselves into bands. Continued heat and pressure can cause the bands themselves to distort and bend. Bent layers alone is not sufficient to determine if layers are foliation, as bedding can bend as well. However, if the mineral grains in the rock appear to be aligned, that is one clue that it’s a metamorphic rock showing foliation. If the crystals interlock, as though they’ve grown together, that is another strong sign that your layered rock is metamorphic and is showing foliation.
While similar at the first pass, bedding and foliation are layers representing completely different processes on and within the Earth. Being able to distinguish between the two can be helpful when trying to discern a region’s geologic history.