Across the Late Eocene Erosion Surface (LEES) of the southern Front Range, erosionally isolated ignimbrite outcrops recently correlated with the Wall Mountain Tuff (36.7 Ma), along with deposits of mega-boulders of several rock types, pose questions about the changing topography of the area in the middle Paleogene. How was the Wall Mountain ignimbrite eruption in the Sawatch Range near Buena Vista, Colorado, able to spread ~150 km east to Castle Rock ~50 km south of Denver across an area now segmented by mountains and valleys? What instigated and facilitated the transport of mega-boulders as large as 5 m in diameter for up to 10 km across an erosion surface of supposedly low relief?

Recent geochemical work has confirmed that undated ignimbrite exposures on the LEES in the Lake George/Florissant area, none thicker than ~10 m, are correlative with radiometrically dated proximal occurrences of the Wall Mountain Tuff in western South Park. This verification indicates that potential topographic barriers like the Puma Hills—which would have impeded, ponded, and thickened the ignimbrite flows from the west—were not in place at eruption time. The exact timing of the subsequent emergence of the Puma Hills is difficult to constrain, but a relative age can be derived using drainage reorientations and stream incision through the Puma Hills. The former southeasterly course of the ancestral South Platte River was deflected 90 degrees to the northeast and became antecedent in a steep canyon through the Puma Hills as they were elevated. Sterne (2019, 2021) and Sterne et al. (2023) attribute this and other nearby stream reorientations to the development of a major structure known as “Pikes dome” beginning in the latest Eocene. This uplift could also have instigated debris flows that transported immense boulders across the erosion surface to the east of the Puma Hills.

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