Other regions of the surface of Ganymede are clearly the product of intense internal geologic activity. Generally, these regions are of higher albedo and consist of many straight parallel lines of mountains and valleys. The Voyager geologists call these the grooved terrain because of their appearance from a great distance. Typically, these mountain ridges are 10-15 kilometers across and about 1000 meters high, similar in scale to some sections of the Appalachian Mountains in the Eastern United States. No higher relief exists, presumably for the same reason it is absent on Callisto. In many places the grooved terrain forms between areas of the older, darker surface, giving the appearance of mountains extruded between separating plates of ancient crust. In other areas the relationships are much more complex, with curved systems of grooves and ridges overlying each other, displaying intricate crosscutting relationships. Apparently Ganymede has experienced a series of mountain-building events. The grooved terrains show a substantial range in age, as indicated by the crater densities. The oldest have nearly the same density as the ancient, dark plains, suggesting that formation of the grooved terrain began early, perhaps 4 billion years ago. The youngest grooved terrain has only about one-tenth as many craters, but this is still as many as are seen in the 3.5-billion-year-old lunar plains. The Voyager geologists believe that even in these areas geologic activity ceased billions of years ago. [Illustration: The hemisphere of Ganymede that faces away from the Sun displays a great variety of terrain. In this Voyager 2 mosaic, photographed at a range of 300 000 kilometers, the ancient dark area of Regio Galileo lies at the upper left. Below it, the lighter grooved terrain forms bands of varying width, separating older surface units. On the right edge, a prominent crater ray system is probably caused by water-ice splashed out in a relatively recent impact. [260-671]] [Illustration: The sinuous nature of some of the narrower Ganymede groove systems can be seen in this oblique view, obtained on March 5 by Voyager 1. The area shown is about the size of California, with features visible as small as 5 kilometers across. The ridges appear to be the result of deformation of the crust of Ganymede. [P-21235]] Other types of surfaces are seen on Ganymede. Some regions are lightly cratered and smooth, with no indication of mountain building. In one place, there is a rough mountainous area that looks more like the jumbled lunar mountains than the long ridges and valleys of the rest of Ganymede. Many of the larger craters are distinguished by brilliant white halos and rays that suggest that impacts may have splashed large quantities of water or ice over the surface.