The graben is a pronounced feature because of the black Cardenas Basalt that forms the middle section of the graben, presumably free of debris accumulation by its cliff face steepness, and winds, and airflow drainage that course through the Colorado River's canyons; unprotected side canyons of Cardenas Basalt show accumulations as a slope-forming geologic unit, with little showing of black basalt.

[2] The south face of the Tanner Graben block shows three distinct geology units, all conformably laid members of the Grand Canyon Supergroup-(and Unkar Group), and are at a paleo-tilted (fault) angle of about 15 degrees.

caprock section of distinctively, multi-layered Nankoweap Formation, which is a unit of the Grand Canyon Supergroup.

The black Cardenas Basalt, makes up much of the Tanner Graben; it sits on erodable, softer, reddish, and layered-(inter-bedded) Dox Formation.

The Dox Formation forms the west and east sides of the narrow southeast flank of the Tanner Graben, and shows its down-dropped distance.