The mountains consist of plateaux separated by valleys and pinnacles. Topographically the deserts consist of alternating tablelands and massifs dissected by river valleys and canyons. Agriculture is restricted to the few upland plains, open valleys, and terraced hillsides. Lengthy, troughlike valleys and basins characterize the mountains. The larger islands have some fertile valleys and plains. They are constructed to convey watercourses across gaps such as valleys or ravines. Some of these rivers have trails running along their valleys. Plains rise to the north of the valleys. Overall, it receives less rainfall than the surrounding valleys and mountains. The hills are less than higher than the adjacent valleys. Intense erosion has resulted in the formation of steep valleys and ridges. Heavy rains caused flooding in many towns built along narrow river valleys. AD who migrated between winter camps in the valleys and summer grounds in the mountains. Debris flows move as granular flows down steep mountain valleys and washes. The western half is mountainous, interrupted by numerous large valleys.