Young fold mountains like the Himalayas are characterized by active geological processes. Deep gorges are carved by rivers eroding through uplifted land. U-turn river courses, known as 'angs', indicate rivers following tectonic structures and continuing to flow despite mountain building. Parallel mountain ranges are a defining feature of fold mountains. Steep gradients and the associated instability leading to landslides are common in tectonically active, young mountain belts due to ongoing uplift and faulting.