Black Mesa, a flat-topped formation created by a horizontal lava flow, is separated from a similar mesa to the north by a curving ridge of poorly consolidated lake sediments. Below is a telephoto view of where that ridge abuts Black Mesa, and a sunbeam that is truncated by the ridge, casting a shadow line.
The photo below was taken 7 minutes later. This Black Mesa should not be confused with another Black Mesa, that is located 15 or more miles to the south, on the San Ildefonso reservation, south of Espanola. That Black Mesa is a volcanic plug. This Black Mesa is located just north of Espanola, and sits within the angle formed by the confluence of the the Rio Chama and its tributary, the Rio Ojo Caliente, with the Rio Grande.