Continue south on Bean City Road about 1.6 km to the outcrop. The northeast side of the rock is somewhat gentler than the southwest side, but still quite irregular. The gentle slope leading up to the rock on the northeast side is probably ice-sculpted glacial deposits, banked up against the outcrop by ice flow. This feature may have resulted from ice being diverted southeastward around the end of the ridge, depositing drift in the lee of the outcrop. The site is a scientific reserve owned by Lawrence University and named for growths of a wild cactus, Opuntia fragilis .
Larger growths look a little like miniature cholla cactus, with stubby finger-like branches.
Cactus Rock is the easternmost Precambrian outcrop in east-central Wisconsin. The granite at Cactus Rock is a uniform, fine-grained pink granite composed mostly of quartz and potash feldspar. Bedrock contacts in the area are concealed by Pleistocene deposits. Although Cactus Rock has not been radiometrically dated, it is lithologically similar to this suite and probably belongs to it. North of New London is the Wolf River Batholith, about 1485 million years old.