The formation consists of volcaniclastic sand and gravel eroded off exposures of the Keres Group of the southern Jemez Mountains.

[1] The formation is restricted to sedimentary strata composed purely of volcaniclastic sediments that overlie Keres Group volcanic rocks and Keres-age volcanic sediments south of the Jemez Mountain, and which in turn are in turn overlain by Pliocene and early Pleistocene gravels.

[2] The formation was first defined by Bailey, Smith, and Ross in 1969 as part of their work establishing the stratigraphy of the Jemez Mountains.

[3] Gary A. Smith and Alexis Lavine argued in 1996 that the original definition was inconsistent and that volcanic and sedimentary basin stratigraphers had worsened the problem by their different approaches to regional stratigraphy.

[5] Smith and Lavine identified a mappable, distinct stratigraphic unit that included most of the original exposures mapped by R.L.