The Rock Island site (NR C9-5 aka 42Sa4542) in SE Utah, is a large Grand Gulch phase Basketmaker II (BM II) site. It is located in a very defendable location on an isolated mesa remnant between the North Road canyon forks on the east side of Cedar Mesa, Utah (Figures 1, 2). This site was discovered and partially collected during the “Drainage Canyon Inventory and Sampling” subproject of the Cedar Mesa Project in 1974 (Lipe and Matson 1971, Matson, Lipe and Haase 1988, 1990). It was reported very briefy by Matson (1994) and even more briefy by Matson and Cole (2002). It is the only BM II defensive site found on Cedar Mesa or yet identifed anywhere. In terms of lithics, it was by far the largest BM II site found on Cedar Mesa by Lipe and Matson of about at total of 300 documented BM II sites and the approximately 200 substantially collected by the Cedar Mesa Project (CMP) and related research.