Yardangs are normally defined as spectacular streamlined, sharp and sinuous ridges that extend parallel to the wind, and are separated by parallel depressions. They are sometimes said to resemble upturnedships’ hulls. Yet the form of yardangs varies. Two size classes are distinguished – mega-yardangs and yardangs. Mega-yardangs, which are over 100 m long and up to 1,000 m wide, are reported only from the central Sahara and Egypt, some good examples occurring in the Boukou area near the Tibes...
Potholes are roughly cylindrical or bowl-shaped depressions IN ROCKS OF RIVER AND inshore platforms that are ground out by the swirling action of sand, gravel, pebbles, andboulders associated with wave action.
Penetration Twins - have an irregular composition surface separating 2 individual crystals. These are defined by a twin center or twin axis. Shown here is a twinned crystal of orthoclase twinned on the Carlsbad Law with [001] as the twin axis.
Contact Twins - have a planar composition surface separating 2 individual crystals. These are usually defined by a twin law that expresses a twin plane (i.e. an added mirror plane). An example shown here is a crystal of orthoclase twinned on the Braveno Law, with {021} as the twin plane.