Surface Folds

Sulci, sharply creased surface folds, are found in soft materials like bread or an infant’s arm.
Image courtesy of Evan Hohlfeld, Lawrence Berkeley National Lab and Harvard University (2011)

A sulcus is a sharply creased surface fold made by compressing or pushing on a soft material. It begins and grows like a crack, but unlike a crack it is reversible. Sulci are found in the brains of humans and monkeys.  Your brain has many sulci, the folds in the surface of your brain which help increase the surface area of your brain.  Sulci can also be seen directly in a slice of bread (a) or in the arm of an infant (b), or indirectly as a disturbance pattern of glass dust on a folded and unfolded rubber slab (c) or as a pattern in refracted laser light passing through a folded rubber slab (d) as shown above.