The term side valley refers to a small valley or ravine that branches off from a larger valley or series of hills or mountains. There are several synonyms that can be used to describe this geological formation, including gully, coulee, ravine, hollow, notch, cleft, defile, and glen. Each of these words has a slightly different connotation, but all refer to a narrow, often steep-sided channel carved by water or other natural forces. Side valleys are common features in mountainous regions and are typically home to a variety of plant and animal life, including rare and endangered species.