An isolated hill, with steep sides and a flat top, it may be the remnants of a mesa, which has a flat top and at least one clifflike side but is broader.
The cone-shaped hill forms downwind of a volcanic vent. Lava flying out of the vent solidifies as it hits the air and accumulates in a neat pile.
It’s a surface layer of pebbles packed closely into the hard soil. One theory holds that it develops as wind and rain remove the finer-grained topsoil.
Illustrations by Roger Hall