The practice of drawing landscapes in graphite pencil was probably more common in the 19th century than it is now, as most people nowadays think of plein-air work in terms of full-on painting.
Here he sorts out the light and dark puzzle of the plank railing on a log bridge.
The Metropolitan Museum has a large sampling of William Trost Richards' pencil, watercolor, and oil landscapes in their collection, which is available online.
Here's a well-illustrated survey of his work, based on a recent museum show: William Trost Richards: True To Nature: Drawings, Watercolors and Oil Sketches