FIGURE 1. Glaciated terrain east of Hellas Planitia, at latitude 42° S, longitude 252° W.

This image from a Viking orbiter mission shows a scene about 180140 km². At the top is the lineated valley fill of Reull Vallis, which may be an extant or relict debris-covered glacier about 10 km wide. The uplands at the centre and bottom of the image were eroded to produce forms typical of glacial alpine sculpture⁹⁴. At the base of several uplands are prominent lobate debris aprons. The longest of these, near the centre of the image, extends for 40 or 50 km from the brightly illuminated walls of the sculpted uplands. Prominent flow lineations show that the debris moved by viscous flow, probably facilitated by the plastic deformation of underlying ice. The lack of craters on the flow-lineated surfaces indicates a remarkably recent (possibly continuing) occurrence of the responsible flow processes.