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Our transcription: The many categories of folds and faults were developed long before geologists had a clear knowledge of how and why these structures formed. Even today, the origin of many geologic structures is not fully understood; but geologists do know that stress, which is as concentration of force, plays a role. Compressional and tensional stresses are. We're really talking about the stresses around a point or the stresses operating on a block of rock. I might use my little foam model here. In the case of compressional stresses, that's where we have stresses directed towards our low block of rock in every direction so it's squeezing the rock in all three dimensions. Tensional stresses are the exact opposite. Instead of squeezing the rock together, what we're trying to do is just pull the rock apart so the stresses are directed outward away from the rock.
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