Starks first recognized that the larger of these basins are approximately elliptical to elongate in shape. The larger and semi-elliptical of these basins typically measure about by . Most basins are closed depressions that hold runoff from the surrounding landscape to form seasonal lakes and wetlands in them. Some of these basins have been naturally "breached", or drain to an outside watershed, and no longer hold precipitation run-off. Many of the large semi-elliptical to elliptical Rainwater Basins have a crescent-shaped ridge that Stark referred to as a "lunette" located on the southeast side of them. These distinctive ridges are about in relief.