Foliation and Bedding in Rocks. Totally not the Same Thing.

Many rocks show layers.

Sometimes the layers are twisted and bent.

Gneiss – A metamorphic rock showing FOLIATION
CREDIT: Siim Sepp CC By-SA 3.0

Orthogneiss from the Czech Republic – A metamorphic rock showing FOLIATION
CREDIT: Huhulenik CC By 3.0
Cross-bedding and scour in a fine sandstone; the Logan Formation (Mississippian) of Jackson County, Ohio – sedimentary rocks showing BEDDING
CREDIT Wilson44691 Public Domain
Manhattan schist outcropping in New York City’s Central Park. Metamorphic rocks showing FOLIATION
CREDIT Loadmaster (David R. Tribble) CC By-SA 3.0

Sometimes they’re perfectly parallel.

Tilted sedimentary BEDDING in shales of the Cretaceous Salto del Fraile Formation, Peru. Sedimentary rocks showing BEDDING
CREDIT Miguel Vera Leon CC by 2.0
Touchet beds in the “Little Grand Canyon” near Lowden in the Walla Walla valley. Note distinct layers. Sedimentary rocks showing BEDDING
CREDIT Citypeek CC By-SA 3.0

Sometimes strange things happen

Cross-bedding in a fluviatile sandstone, Middle Old Red Sandstone (Devonian) on Bressay, Shetland Islands – sedimentary rocks showing BEDDING
CREDIT Anne Burgess CC By-SA 2.0

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.

Fossil-rich layers in a sedimentary rock, Año Nuevo State Reserve, California. These sedimentary rocks show BEDDING. One bed is full of fossil shells.
CREDIT: Brocken Inaglory CC by-SA 3.0
Limestone formations in the Torcal de Antequera. These sedimentary rocks show BEDDING
CREDIT: Juan Fernández CC By-SA 2.0

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.

Schist specimen showing the characteristic “scaly” schistose texture, caused by platy micas – A metamorphic rock showing FOLIATION
CREDIT Michael C. Rygel CC By-SA 3.0
Blueschist, Ile de Groix, France – a metamorphic rock showing FOLIATION. The different colored bands are made of different minerals.
CREDIT: Arlette1 CC By-SA 3.0

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.

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