During survey of this quadrangle it was found that the main Algonkian quartzites and slates were like those of Uncompahgre section, but from structural complexities the portion represented was less, and, as in Uncompahgre Canyon, the upper and lower parts were not seed. In Vallecito Canyon, however, at southern margin of quadrangle, a heavy conglomerate was discovered and traced to wast, where it was found such relations with Uncompahgre quartzites and slates as to appear unquestionably to be lower part of the group. To this conglomerate the name Vallecito conglomerate is given, and to the group which includes it and the Uncompahgre, together with higher formations that my hereafter be recognized, the name Needle Mountains group is applied. Total thickness of group unknown. Vallecito conglomerate is 2,000 or more feet thick 2 miles east of this quadrangle, and overlying Uncompahgre quartzites and slates are 5,000+ feet thick in Grenadier Range. [Age is pre-Cambrian.]
[GNC remark (ca. 1936, US geologic names lexicon, USGS Bull. 896, p. 1472): For many years this group was classified as Algonkian, but the terms "Algonkian system" and "Archean system" having been discarded by the USGS the group is now classified as pre-Cambrian.]

Source: US geologic names lexicon (USGS Bull. 896, p. 1471-1472).

Abandoned herein. Included metasedimentary rocks of two vastly different sequences: Uncompahgre Formation and Vallecito Conglomerate, which are separated stratigraphically by Irving Formation, Twilight Gneiss, and granites of circa 1,700 m.y. age group. Listed as a unit of Needle Mountains of the pre-1,700 m.y. (Precambrian X) metamorphic complex.

Source: GNU records (USGS DDS-6; Denver GNULEX).