Monday, August 27, 2012


 In a blog on August 11, I reported on the existence of a quite large piece of mammillary chalcedony observed during a trip to South Park Basin, Colorado.  The blog also contained information about the geology/formation of the Park so that will not be repeated here.

The northern part of the Park (north of US 24) has numerous mountain ranges in the east such as the Kenosha and Tarryall Mountains where the bed rock is generally Precambrian in age.  On the western flank rocks of Paleozoic age front the Mosquito Range.  South of the highway the landforms are more subdued and a wide variety of Cenozoic, volcanic-related rocks overlie Mesozoic bedrock; however, the Mesozoic rocks often crop out in north northwest trending ridges (easily seen near Hartsel and extending north) (Scarbrough, 2001 ).  My interest, in various collecting trips to South Park, has generally been in the southwestern part of the Basin where Scarbrough (2001) has outlined the Cenozoic history as follows:

Deposition of Denver (South Park?) Formation concurrent with Laramide tectonism (compressional mountain and basin building) in the early Tertiary.

Middle Tertiary erosion, then deposition of lake beds, volcanism in the form of lavas and extensive airfall deposits, igneous intrusions, and fluvial deposits.

Pleistocene glaciation in the adjacent mountains producing outwash deposits.

Deposition associated with Holocene fluvial systems.

One of the best-known volcanic-associated units is a formation usually mapped as the Florissant Lake Beds. These beds crop out near Lake George and represent deposition is a basin partially occupied by a late Oligocene Lake.  Thousands of fossil plants and insects (and various other vertebrates and invertebrates) have been extracted from these beds and have produced a wonderful snapshot of life during this time period.  Today, Florissant Fossil Beds National Monument preserves several of the outcrops.

Heading south from Hartsel on CO 9 and 53 Rd, the Antero Formation of Oligocene age (probably equivalent to Florissant Lake Beds) crops out.  However, good exposures are somewhat rare since the rocks are highly weathered at the surface; the landscape is a gently rolling surface.  But, a little prospecting and walking will likely produce specimens of petrified wood.  In fact, I was able to even locate wood in ditches along a gravel road.  However, a word of caution---the land ownership situation in South Park is a jumble of Colorado State land, BLM land, and private land.  In fact, a representative from a federal agency told me that the only way for a novice (like me) to determine land ownership was to take my GPS, get a latitude and longitude reading, and compare such with a federal data base.  An easier way is probably to visit with the ranchers and request permission to prospect.

The Antero Formation is a clastic and volcaniclastic unit that contains water-laid ash, air-fall tuff, siltstone, sandstone, and algal limestone and …contains fossil plants, insects, mollusks, and vertebrates (Epis and others, 1979; Scarbrough, 2001).   A number of writers have noted the presence of petrified wood in South Park, perhaps beginning with Orvando Hollister in 1867:  This Park has salt springs, beds of gypsum, coal shales, veins of chalcedony, carnelian, and other curious stones and minerals.  It has not been thoroughly explored and no one fully knows its resources or curiosities.  Silicified wood abounds in its lower portion, and at one point, about 30 miles west of Pike’s Peak, there is a small patch of petrified stumps still standing, one of which is fifteen feet in diameter [now Florissant Fossil Beds National Monument].

Although I saw much wood in the area, one particular partial log really attracted my attention, mainly for its seemingly internal structures—as seen on the photo.  I don’t know what these structures represent but could guess they might be some sort of activities related to insects.  I am hoping that someone in cyber world will notice these structures and help me out! 

A more detailed account of South Park geology, and the petrified wood and chalcedony, will appear in the CSMS Pick & Pack (probably September) 


Epis, R.C., Wobus, R.A., and Scott, G.R., 1979, Geologic Map of the Guffey Quadrangle: U.S. Geological Survey Miscellaneous Investigations Map I-1180.

Hollister, O. J., 1867, The Mines of Colorado: Samuel Bowles & Company, Springfield, MA. reprinted 1974, Promontory Press, New York.

Scarbrough, Jr., L. A., geology and Mineral Resources of Park County, Colorado: Colorado Geological Survey Resource Series 40.

Wallace, C. A., J. A. Cappa and A.D. Lawson, 1999, Geologic Map of the Gribbles Park Quadrangle, Park and Fremont Counties, Colorado:  Colorado Geological Survey Open-File Report 99-3 (with map).