Between forty million years ago and about five million years ago, what is now southwestern Colorado was the site of numerous volcanic eruptions. Old lava flows and beds of volcanic ash underlie the surface of much of this part of Colorado. One particularly large eruption, about 28 million years ago, left in its wake a huge caldera, or crater-like basin, about 15 km in diameter and about 2 km above sea level. Filling with water, the basin became a shallow mountain lake; streams ran down into the lake, and plants colonized its shores. The plant communities near the ancient lake were mixed, but in general similar to plant communities in the Rocky Mountains today. There were forests dominated by pines, spruce, and firs;woodlands dominated by pines and junipers; and nearly treeless patches of chaparral dominated by broadleaf shrubs, such as mountain mahogany (Cercocarpus), hawthorn (Crataegus), barberry (Berberis), and Oregon grape. The lake largely dried up in droughts or in warm summers, refilling from the runoff of melting show. Runoff into the lake brought silt and mud, and later eruptions in the area filled the north end of the lake with volcanic ash. The silt and ash buried and preserved fossil leaves, stems, and seeds of the plants that once lived there.
Today, the beds of ash and silt, known as the Creede Formation, have been exposed by the erosive action of the Rio Grande River. Flowing southwards out of the Rocky Mountains, the Rio Grande has cut through the ancient caldera from north to south. In the rolling hills of the Rio Grande Valley near the small town of Creede, Colorado, shown in the picture above, the Creede Formation has yielded a rich assemblage of fossil plants, known as the Creede flora. The Creede flora has been precisely dated radiometrically at 27.2 million years; thus it is late Oligocene. In several respects the Creede flora resembles the somewhat older Florissant Formation plant assemblage. Both resulted from volcanic ash and mud filling in and choking a lake, and several plant genera and species are common to both locations. Unlike the Florissant Formation, however, the Creede Formation appears to lack diverse insects.
Paleobotanist Daniel Axelrod collected thousands of specimens from the Creede Formation, and described and documented the entire paleoflora (Axelrod, 1987). Axelrod described 73 plant species from the Creede flora, and his collections are now part of UCMP. Much of Axelrod's taxonomy has since been revised (Wolfe and Schorn, 1990); only about thirty plant species can be validly distinguished in the Creede flora. Nevertheless, the Creede flora remains paleobiologically significant. It is the only fossil plant locality from the Oligocene in the southern Rocky Mountains. Different locations around Creede have different fossil assemblages: environmental differences along the rim of the ancient lake are reflected in the different plant communities seen in the fossil record, ranging from montane conifer forests to pine-juniper woodlands to chaparral and scrub. Furthermore, Creede fossils have documented some evolutionary changes in plant lineages. For example, the living plant Chamaebatiaria (fernbush) now inhabits dry, open environments. However, in the Creede flora, a likely ancestor to Chamaebatiaria is found associated with fir and spruce, suggesting that the lineage has evolved a new habitat preference in the past 27 million years. (See Wolfe and Schorn, 1989)
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