It is possible to do landscapes from memory, especially since many terrains have large stretches of similar features and colors. You won't duplicate the exact contours of a "real" landscape but you will, if lucky, create something which is the "ideal" version of it, which you probably would do even if you were sitting out there looking at the real thing and trying to make a picture of it.
These are images of the Virginia landscape done from memory after I got home from my short vacation. They are tiny, almost business card sized, which also helps when you are doing things quickly.
These two images are originally markers on sketchbook page. Then I worked on the lower one with colored pencils, and then I did stuff to both of them in Photoshop. So, mixed media. Size of the upper one is about 2" x 2 1/2", lower one is about 2 1/2" square, August 17-18, 2014.