The Roberts Ranch Buffalo Jump in northern Larimer County is a Protohistoric period (1540–1860 CE) bison kill and butchering site that dates to about 1663–84 and represents one of the southernmost bison jump sites on the Great Plains. Discovered in 1957, the site was excavated in the late 1960s by amateur archaeologists from Denver and students under the supervision of W. James Judge of Colorado State University (CSU). Starting in 2012, Christopher Johnston of CSU reanalyzed the site and its artifacts using recent research to reconstruct the spatial division of task areas at the site.
Discovery and Excavation
The Roberts Ranch Buffalo Jump is located on Roberts Ranch near Livermore in northern Larimer County, about fifteen miles south of the Wyoming border. It is at the base of a cliff on the south bank of the North Fork of the Cache la Poudre River. Ranch owner Evan Roberts discovered it in 1957 when he saw bones sticking out of the riverbank while using a bulldozer to open the river channel. He reported his find to archaeologists and encouraged them to excavate the site.
The first excavations took place in 1966 under amateur archaeologist Raymond Barker, who led a team from the Denver chapter of the Colorado Archaeological Society. They focused their efforts on the base of the cliff that rose from the south bank of the river and recovered many artifacts, including part of a flat-bottomed ceramic vessel. A new round of excavations started in 1969 under W. James Judge of CSU. Judge dug a test trench from the riverbank to the base of the cliff, which showed that the site contained a major bone bed. Judge returned the next year with the CSU Archaeology Field School. Students performed detailed excavations on a large portion of the riverbank.
Max Witkind reported the results of the 1966, 1969, and 1970 excavations in his 1971 CSU master’s thesis, in which he interpreted the site as a combination bison kill and processing site. His work was one of the first scholarly accounts of a Great Plains bison jump. Over the next four decades, significant new research into mass bison kills revealed more information about when and how they occurred, leading Jason LaBelle and Christopher Johnston of the CSU field school to revisit the Roberts Ranch site to reanalyze its artifacts starting in 2012. This work culminated in Johnston’s 2016 master’s thesis, in which he confirmed Witkind’s basic analysis but added substantial new details and context to help interpret the site.
Site Description and Significance
Communal bison killing played an important role for human groups on the Great Plains from the area’s earliest inhabitants to the nineteenth century. Kills used a variety of methods, but the basic goal was always to gather bison in a group and drive them to a kill point such as a trap or jump. These coordinated kills often occurred in late fall or early winter. By the Late Prehistoric period, most communal bison kills seem to have taken place north of Colorado, but a few kills from that period—including the Roberts Ranch site—have been found in northern Colorado. The kills are important for archaeologists because the large collections of bones and artifacts found at the sites provide valuable information about prehistoric human behavior.
In his investigations at the site, Johnston found a series of fourteen small rock cairns running for more than 350 feet along the crest of a gentle slope near the top of the cliff. He suggested that the cairns were perfectly positioned for bison hunters to turn a stampeding herd north toward the cliff. The bone bed at the base of the cliff contained 3,005 bone elements, indicating that a minimum of nineteen bison jumped at the site.
Johnston’s spatial analysis of the bones and artifacts recovered from Roberts Ranch showed that the site was divided into three clearly defined task areas: primary butchery, secondary processing, and immediate consumption. Some bison survived the jump and had to be killed using projectile points. The carcasses were then divided into sections at the spot of the kill. The hindquarters were carried to a separate part of the site for further processing. The herd included at least eight fetal bison; these were considered delicacies and were carried to a spot upslope from the main processing area to be consumed on-site.
Based on radiocarbon samples from five bison bones and the lack of European trade goods at the site, Johnston dated the kill to roughly 1663–84 CE. The gestational age of the fetal bison indicated that the kill probably occurred between mid-December and mid-April.