Cast shadows can help show the surface changes in a landscape. In this small plein-air sketch, the tree shadows define the curve of the hills by wrapping around them.
The three shadows get progressively narrower as they go back, which helps suggest depth. Sunlit figures are arranged to stand out in front of the darker tones.
Watch for these long cast shadows late in the afternoon or early in the morning. The shadows cast by trees and utility poles can really help you define the contours of roads, sidewalks, and buildings.