The Florissant Formation was deposited upon the eroded surfaces of the Pikes Peak Granite and the Wall Mountain Tuff. Six informal units make up the Florissant Formation and include from bottom to top: the lower shale, the lower mudstone, the middle shale, the caprock conglomerate, the upper shale, and the upper pumice conglomerate (Evanoff, McIntosh & Murphey, 2001, p. 8).

Multiple lines of evidence indicate a late Eocene age for the Florissant Formation, which accords with the current placement of the Eocene-Oligocene boundary at 33.7 Ma. The Florissant Formation is dated at 34.07 Ma based on volcanic minerals in the formation (Evanoff, McIntosh & Murphey, 2001, p. 14). The overlapping range of brotontheres and Mesohippus indicates a Chadronian age (37-34 Ma) for the mammalian fauna of Florissant. The 70 meter Florissant Formation is almost entirely reversed in polarity and is most logically correlated with Chron C13, which spans 33.7-34.7 Ma (Prothero & Sanchez, 2004, p. 145). Thus, radiometric dating, mammalian fossil fauna, and the magnetic stratigraphy of Florissant corroborate a latest Eocene age for the Florissant Formation.

The Florissant Formation represents both fluvial (river) and lacustrine (lake) environments. An initial lake in the paleovalley became filled with sediments over many years, after which a stream valley developed. Evidence suggests lahars traveling down side tributary valleys eventually formed a natural dam, re-establishing lake conditions. Renewed volcanic activity resulted in a lahar deposit that entered parts of the second reservoir-like lake. Eventually, the deposition of a pumice conglomerate marks the end of lake conditions. Let’s take a closer look at the major units in the Florissant Formation.