Geodetective: instructions 1

A reliable recipe for photogeological maps: 1 Outlines. Observe where distinct landforms and patterns appear in the photograph. These are the units. Draw their boundaries. Use a dashed line for the crater rim and a continuous line for the edge of its ejecta blanket.

2 Colours. Fill each outline with a different colour: crater, out to its ejecta edge, yellow; crater-filling sediment, green; plain, pink; wrinkle ridge or island, red; rugged terrain, yellow-brown; winding channel, blue with dots; straight trough, dark brown. Use stronger colours for younger features and paler colours for older ones. Hatch units that are similar in character but differ in age.

3 Symbols. Label units of the same colour with letters according to their relationships, cross-cutting and superposition. The youngest, uppermost layer is unit A, the earlier layer is B, and so on.