A northeast-trending wrinkle ridge cuts across the plains of
Mare Imbrium. The overall ridge is about 4.5 km wide. The
contact between the ridge (right) and the surrounding mare
basalt plains (left) runs diagonally up the image. Wrinkle
ridges are relatively common on the Moon and are mostly found in
the maria; smaller versions are occasionally found in the
highlands. They are called "wrinkle" ridges because wrinkling is
exactly what happened. Rock (in this case mare basalt) was
squeezed from opposite directions, as if it were in a vise, and
the rock failed and buckled, or wrinkled, in a direction
perpendicular to the squeezing (compression). In detail
scientists have found that they are formed by a combination of
faulting and folding. Generally there is a broad swell -- the
ridge -- and a narrower feature -- the wrinkle. It is the
wrinkle that is shown in this frame. In full frame entire
structure can be observed. In this area, the overall ridge is
about 4.5 km wide and the wrinkle varies from a few hundred