General terms:
* Fissure: A narrow crack or opening in rock.
* Crevice: A deep, narrow opening in rock.
* Fracture: A break or crack in rock, often caused by stress.
* Fault: A break in the Earth's crust where there has been movement.
* Cleft: A narrow opening or split, often with steep sides.
More specific terms:
* Canyon: A deep, narrow valley with steep sides, often carved by a river.
* Gorge: A narrow valley with steep sides, often with a river at the bottom.
* Chasm: A deep, wide opening in the earth's surface, often caused by a fault.
* Gully: A small, narrow ravine or ditch, often formed by erosion.
* Cave: A natural underground chamber, often formed by the dissolving of rock.
The best term to use depends on the specific characteristics of the gap in rock you are describing.
