Semi-desert, foothills. Openings. Spring, summer.
Canyons of the Ancients National Monument, April 11, 2010.

This Opuntia would qualify as O. erinacea if we were to key it with Weber's Colorado Flora: Western Slope, for it has spines that are flattened at least at their base.  But, as explained elsewhere , the Synthesis and the Flora of North America indicate that O. erinacea is a variety of O. polyacantha and it does not grow anywhere near the Four Corners area.

Most O. polyacantha pads, such as the ones pictured at left, are circular.  Some of the spines pictured at left are over four inches long, but spines on O. polyacantha can also be just one or two inches long.

Semi-desert, foothills. Openings. Spring, summer.
Canyons of the Ancients National Monument, April 28, 2006.

Semi-desert, foothills. Openings. Spring, summer.
Canyons of the Ancients National Monument, April 25, 2009.

Everything has its beginnings.