This smooth hole was part of a system of alignment with the gap between the two big rocks. Presumably a stick or post was placed in the hole & used as a sightline through the gap between Companion & Chimney Rocks. There are two similar holes on distant ridges. At a certain time of year, the rising sun shines straight down one wall of the upper Great House and through this point, too.
I have heard of things like this being used by the Maya. I am unfamiliar with them in the context of the Anasazi, but the guide said there are such things at Chaco, too.
At the western end of the High Mesa, 164 feet from the Great Kiva, is a partially dismantled stone circle containing a circular basin, 14 inches in diameter, cut into the bedrock. These stone circles and their associated basins appear to be primarily a Chacoan phenomenon. The stone circle and basin at Chimney Rock reaffirm the Chaco connection and may have marked a ceremonial location for viewing the high chimney back-lighted by the sun at summer solstice dawn. — from Malville