LUNAR SURFACE FEATURES
Highlands constitute about 80 percent of the surface of the moon. Although fractures are observed in the lunar crust, there is no evidence of folded mountain belts (as on the earth) nor other indication of thermaltectonic activity. Lunar mountain ranges were apparently produced in conjunction with the formation of the impact basins that are now seen as maria. Most mountain chains are on or near the periphery of the roughly circular maria. The mountains bordering the maria rise more steeply on the side facing them than on the other side. Many have lofty peaks, occasionally rising over 7000 meters above the surrounding plains.
Beyond the eastern edge of Mare Imbrium a narrow valley cuts across the lunar Alps Mountains. This feature has long been known from photographs taken from the earth. From photgraphs taken by an orbiting spacecraft we now know that the Alpine Valley is a deep trough some 3 to 10 kilometers wide and over 100 kilometers long. Narrow channels (rilles), which resemble chasms or gorges, cut many ki lometers across the lunar terrain, frequently without interruption. Running lengthwise down the middle of the Alpine Valley is a very conspicuous rille. Rilles may be lava channels, part or all of which were roofed when filled with flowing lava. Now these tubes have collapsed and are partly choked with rubble from the days of active lava flows.
