The upper beds contains considerable latite to andesite breccia interbedded with tuffaceous claystone and siltstone.

[6] More recent high-precision U-Pb dating gives an age of 45.0±0.7 Ma for an ash fall tuff in the lower part of the formation in the Robledo Mountains and an age of 39.6±0.5 Ma for an ash fall tuff in the upper part of the formation in the Sierra de las Uvas.

The interbedded ash fall tuffs and volcaniclastic beds of the formation are interpreted as volcanic activity after the end of Laramide mountain building but before the opening of the Rio Grande Rift.

Kelley and Caswell Silver in 1952 for exposures in Palm Park, southeast of Caballo.

[1] Steven M. Cather and his coinvestigators assigned the Palm Park Formation to the Spears Group in 1994.