[1][2] These are interbedded with volcanic tuffs and flows that are assigned to the Mogollon Group.

[3] The formation has been interpreted as sediments eroded from caldera rims of the Mogollon-Datil volcanic field.

[4] Oxygen isotope ratios suggest that the magma from which the caldera rims solidified had its origin in the Earth's crust as a partial melt of granite source rock.

Kottlowski in 1953 for exposures around Bell Top Mountain, northwest of Las Cruces, New Mexico.

[1] Steven M. Cather and his coinvestigators argued in 1994 for a revision of southwestern New Mexico stratigraphy in which the Bell Top Formation was restricted to the sedimentary beds and included in the Spears Group.