This image shows a series of small ridges in Hesperia Planum. Similar ridges occur in the plains just north and south of Valles Marineris. Such ridges are formed when the crust is compressed by forces inside the planet (see Fig. 2b). In addition, there are numerous impact craters present (for more information on impact craters, see slide #17). Some of the impact craters are superimposed on top of the ridges, but none of the craters are deformed by the ridges. Thus, the sequence of events in this region began with formation of the plains, probably by lava flows. Next, the plains were compressed, forming the ridges. This deformation must have occurred soon after the plains were formed. Finally, meteorite impacts produced the craters. These ridges are similar in shape to wrinkle ridges found on the Moon. This image is 100 kilometers across.
Viking Orbiter image 418S39