The absence of light is darkness, but when light is present, yet blocked, we get a shadow.
To be technical:
A shadow is a region where the source of light is obstructed by an object. The shadow occupies all of the three-dimensional volume behind an object with light in front of it. The cross section of a shadow is a two-dimensional silhouette, a projection of the object blocking the light. (adapted from Wikipedia)
Light has been said to be the bread and butter of photography. Many of us believe that the shadows are as important as the light. Everybody has photographed light and shadow, just some do it more deliberately than others.
Google “shadow photographs” or “light and shadow photographs” and zillions of examples come up, including some classics.
I’m no different. On both my trip to Iceland in 2013, and to California and the West in 2014, I came home with some shadow pictures among the favorites from the trips.
I really could make a whole series of this type of image, but it’s been done. It’s still very appealing, though, as it makes light and the effect of light the actual subject of the photograph.
Shadows are inescapable unless the light is very, very flat. Shadowless light is not very flattering, though, either to the landscape or to people.
Look for the light, look for the shadows, and make more photographs.
Oh, and place the shadows on Zone III …
See: More Light, Candle