Any rocky areas of rapids interrupting the flow of the Nile River, caused by granite abruptly interspersed in the Nubian sandstone belt. There are six numbered and several minor cataracts between Aswan and Khartoum, which are hazards to navigation. The 2nd Cataract, the most formidable, was impassable except during the annual inundation. Cataracts 1-4 and the Dal Cataract were political frontiers at different times.