Fossae on the Alba's western flank (pictured right). The pits likely formed by the collapse of surface materials into open fractures created as magma intruded the subsurface rock to form dikes. Valleys and gullies The northern slopes of Alba Mons contain numerous branching channel systems or valley networks that superficially resemble drainage features produced on Earth by rainfall. Alba's valley networks were identified in Mariner 9 and Viking images in the 1970s, and their origin has long been a topic of Mars research. Valley networks are most common in the ancient Noachian-aged southern highlands of Mars, but also occur on the flanks of some of the large volcanoes. The valley networks on Alba Mons are Amazonian in age and thus significantly younger than the majority of those in the southern highlands. This fact presents a problem for researchers who propose that valley networks were carved by rainfall runoff during an early, warm and wet period of Martian history. If the climate conditions changed billions of years ago into today's cold and dry Mars (where rainfall is impossible), how does one explain the younger valleys on Alba Mons? Did Alba's valley networks form differently from those in the highlands, and if so, how? Why do the valleys on Alba Mons occur mainly on the northern flanks of the volcano? These questions are still being debated. In Viking images, the resemblance of Alba's valley networks to terrestrial pluvial (rainfall) valleys is quite striking. The valley networks show a fine-textured, parallel to dendritic pattern with well-integrated tributary valleys and drainage densities comparable to those on Earth's Hawaiian volcanoes. However, stereoscopic images from the High Resolution Stereo Camera (HRSC) on the European Mars Express orbiter show that the valleys are relatively shallow ( or less) and more closely resemble rills or gullies from intermittent runoff erosion than valleys formed from sustained erosion. It seems likely that the valleys on Alba Mons formed as a result of transient erosional processes, possibly related to snow or ice deposits melting during volcanic activity, or to short-lived periods of global climate change. (See Surface characteristics, above.) Whether the eroded material is an ice-rich dust or friable volcanic ash is still uncertain. Geologic history Alba's well-preserved lava flows and faults provide an excellent photogeologic record of the volcano's evolution. Using crater counting and basic principles of stratigraphy, such as superposition and cross-cutting relationships, geologists have been able to reconstruct