The region is also traversed by the Sur-Hill fault, which is noticeable at Pfeiffer Falls in Pfeiffer Big Sur State Park. The waterfall were formed when the stream flowed over the hard gneiss of the Salinian block and encountered the softer Santa Margarita Sandstone. The falls were formed when the softer sandstone was worn away. The interior canyons are typically deep and narrow, and even in the summer sunshine only reaches many of the canyon bottoms for a few hours. The land is mostly steep, rocky, semi-arid except for the narrow canyons, and inaccessible. The Little Sur River canyon is characteristic of the Ventana Wilderness region: steep-sided, sharp-crested ridges separating valleys. At the mouth of the Little Sur river are some of the largest sand dunes on the Big Sur coast.