You can make fast landscape studies with markers. Instead of dealing with all the colors, try a small set of gray markers (here I use Copic toner gray markers). Be careful, these markers will bleed through thin paper to leave marks on the next pages in a sketchbook. Try heavy paper or placing a blotter sheet below the one you are working on.
Click on one of the steps below to see the full size sketch and step-by-step instructions.