The Pilar Formation consists of a dense gray-black to black hard carbonaceous phyllite or schist with thin white schistose layers that are interpreted as a metatuff.

The rock has very irregular slaty cleavage, with tiny muscovite flakes visible in the hand lens.

Montgomery rejected its designation as a slate, as it has undergone middle-grade metarmophism along with the underlying staurolite-bearing Rinconada Formation.

[3] Because of its distinctive appearance, the formation is an important structural marker, which provided evidence that the Hondo Group of which it is a part fills an inverted syncline.

[5] The formation was originally designated as the Hondo Slate by Evan Just in his 1937 survey of pegmatites in northern New Mexico.