The Agate Bluff Archaeological Site is a collection of four Late Prehistoric rockshelters located in a large bluff in northwest Weld County near the Wyoming border. In 1951–52 siblings Cynthia and Henry Irwin excavated the rockshelters and determined that they had been occupied by Plains Woodland and Upper Republican people, but in 2009 Michael K. Page reanalyzed artifacts from the site and determined that the supposed Upper Republican occupation was probably Itskari. A single kernel of corn suggested the possibility of agriculture in the area, but if so, cultivation formed only a small part of an economy based more on nomadic hunting and gathering.

Site Discovery and Excavation

In 1950 young siblings Cynthia and Henry Irwin—who became involved in archaeology thanks to Marie Wormington of the Denver Museum of Natural History (now the Denver Museum of Nature & Science)—discovered the Agate Bluff site during a field survey. The next year they performed an extensive excavation of three of the site’s rockshelters, and in 1952 they started to excavate the site’s fourth shelter. Working on weekends, the Irwins completed their excavation in June 1953. The artifacts they recovered went to the Denver Museum of Natural History, and in 1957 they published a report of their findings.

The rockshelters at Agate Bluff were situated near the start of an intermittent stream at an elevation of about 5,900 feet. The first three were within about 150 feet of each other. They contained stone artifacts such as projectile points, scrapers, knives, drills, and flakes, as well as 125 ceramic sherds. The Irwins identified most of the projectile points and ceramics as characteristic of the Upper Republican phase. In addition, the rockshelters contained bones from bison, elk, and several smaller mammals and birds. Four and a half feet below the surface of one of the rockshelters, the Irwins found a single kernel of corn that resembled corn specimens found at Basketmaker sites and prehistoric Ozark sites. At the fourth rockshelter, located about 250 feet west of the first three, the Irwins found a large collection of chipped stone artifacts, largely stemmed projectile points dating to the Late Plains Woodland period.

The Irwins determined that the Agate Bluff rockshelters had been occupied by two distinct cultural groups, both of which had ties to the eastern plains. First, Plains Woodland people, who relied on hunting and gathering but had no agriculture, occupied the rockshelters (primarily shelter 4) between about 1000 and 1200 CE. Next, Upper Republican people moved into the shelters between about 1200 and 1500 CE. The presence of a single kernel of corn without any other agricultural evidence suggested that these people may have cultivated some crops but could not have relied on agriculture as their main source of food. Instead, they had more of a nomadic hunter-gatherer economy than their relatively sedentary Upper Republican counterparts farther east.

Recent Reanalysis

Since the time of the Irwins’ work, archaeologists have speculated about the relationship between supposed High Plains Upper Republican sites like Agate Bluff and the more substantial Upper Republican settlements found on the central plains. Early theories proposed that the High Plains sites represented hunting parties from the central plains. Later scholars hypothesized that a local, perhaps unrelated, hunter-gatherer population used the sites. More recently, Laura L. Scheiber and Charles A. Reher have suggested that High Plains Upper Republican sites could represent seasonal hunting or scouting rounds that eventually resulted in the permanent migration of some Upper Republican groups from the central plains to the High Plains.

In 2009 Michael K. Page completed a comprehensive reanalysis of High Plains sites previously classified as Upper Republican—including Agate Bluff—and showed that the High Plains were occupied by a variety of Central Plains Tradition peoples in the early 1000s CE, especially the Itskari variant. Page’s work reclassified several sites—including Agate Bluff—as probably Itskari rather than Upper Republican and suggested a new framework for looking at High Plains sites dating to that period. Itskari people probably traveled regularly to the High Plains to procure stone and hunt bison, while Upper Republican people used the area less intensively, probably when periodic droughts forced them away from the central plains.