Our last major stop for the day on the Gold Belt Byway was Indian Springs Ranch, a ranch and campsite owned by the Thorson Family. The Indian Springs Trace Fossil Site is a National Natural Landmark (NNL) protected by the federal government, but it is owned by the Thorson Family and you must have permission and be accompanied by a family member to visit the site. Tours may be arranged by calling ahead.
Carly Thorson, who led our tour, is very knowledgable about the many plants of the ranch, as well as its history. The flower to the left is chinchweed (Pectis papposa), a lemony-scented plant with several medicinal uses. We also saw a petroglyph and some historical sites, including the cabin used by Wild Bill McKinney of Quantrill’s Raiders.
The Middle Ordovician (c. 450 million years old) trace fossils were found by Thorson’s father, Bennie C. Thorson, who worked the site for five field seasons with now-retired Colorado College professor William A. Fischer. The site was designated as an NNL in 1979. The voluntary NNL program, administered by the National Park Service, provides a way to recognize and protect important natural resources on any type of land, including private, local, municipal, state, and federal. Not all NNLs are open to the public, and many others require landowner permission to visit.
The Indian Springs Trace Fossil Site consists of a shallow excavation exposing a single bed of the Harding Sandstone, representing an “instant” in geologic time. The quarry is 16-18 feet above the base of the Harding. Much of the overlying sandstone has been eroded at this locality. The Thorsons keep the site covered to protect the exposed trace fossils, which remain in situ according to family policy.
Body fossils are rare in the Harding, including at Indian Springs, but trace fossils are abundant. Bony plates from ostracoderms (armored fish) and conodont elements are found there, but the site’s primary significance lies in its spectacular trace fossils from various animals moving across the Ordovician seafloor. At right, you can see a track left by a sea scorpion (eurypterid).
Ostracoderms were some of the earliest vertebrates. They lacked jaws, but were covered with bony plates, and were also the first organisms to use gills exclusively for respiration. Ostracoderms were bottom-feeders and probably relatively slow-moving. Here you can see a feeding trail of an ostracoderm. The close spacing of the feeding marks indicates that food was abundant on the Ordovician seafloor.
Tracks of several species of horseshoe crabs are quite common. The feeding traces typically curve either left or right according to the species. Horseshoe crabs are not actually crabs–they are more closely related to sea scorpions, and distantly related to the spiders–and they have evolved little since they first appeared in the fossil record. They are tolerant of harsh environments and protected by their sturdy shell.
This trace fossil shows an abruptly ended ostracoderm feeding trail, as the fish became a meal for a predator. The preservation of interactions like these is one reason the Indian Springs fossil site is so important. Other trace fossils at this site include tracks from trilobites, merostomes, and polychaete worms.
After Indian Springs, we drove back through Phantom Canyon (Stops 19-20), the location of several historical railroad sites, as well as more outcrops of Precambrian granodioriate and metamorphic schist and quartzite. We didn’t manage all of the stops on the Gold Belt Byway–that would take at least two days–but we had a great field trip.
-Melissa Barton
Photo Credits: Melissa Barton
Geologic Guidebook to the Gold Belt Byway, Colorado, by Thomas W. Henry, Emmett Evanoff, Daniel A. Grenard, Herbert W. Meyer, and David M. Vardiman (Gold Belt Tour Scenic and Historic Byway Association, 2004), is available at the Florissant Fossil Beds National Monument Visitor Center for $19.95 plus tax.
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