ravine gorge canyon pass defile gulf gulch gap couloir flume gill notch kloof chasm gully saddle linn fissure clough col crevasse cut bunny chine crevice rift arroyo abyss cleuch heugh barranca nullah quebrada break valley donga sloot khor khud clove ditch glen hollow dale vale channel void cavity crater opening watercourse cleft pit hole basin dell dene depression trough combe dingle slade cwm strath wadi vega coomb plain bottom swale lowland dip crack concavity fracture trench shaft indentation interstice rent passage rupture cranny slit breach dent separation dint recess excavation cave indenture pothole cavern aperture division well borehole chamber dugout