Cactus Rock

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay

Location: 3 miles (5 km) south of New London, SE 1/4, NE 1/4, Sec. 26, T22N, R14E, Readfield 7.5' Quadrangle.

Follow W through New London, proceeding south and west on W (Wyman Street and then Pershing Road) through several turns. After leaving the built-up section of New London, County W heads south, then curves west. Bean City Road branches off from County W at the curve; there is a small cemetery east of Bean City Road at the intersection. Continue south on Bean City Road about 1.6 km to the outcrop.

Cactus Rock is an ice-sculpted landform with a steep glacially-plucked southwestern face. 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. A low ridge extends as a "tail" southwest from the eastern end of the outcrop for about half a kilometer; it is apparent on topographic maps but not very obvious from the ground. 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. The cactus is a ground-hugging variety, usually growing as small thorny globes about 2-3 cm in diameter in mossy crevices. Larger growths look a little like miniature cholla cactus, with stubby finger-like branches. It has been largely eradicated from the most heavily- traveled areas of the rock; if you find any, please do not disturb it.

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. The oldest rock unit in the area is a suite of granitic and rhyolitic rocks about 1760 million years old (Smith, 1978; 1983; Van Schmus, 1978), exposed in about ten outcrops in south- central Wisconsin. 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. The southernmost unit of the Wolf River Batholith is the Waupaca adamellite, an extremely coarse-grained intrusive rock with feldspar crystals up to 3 cm in diameter. Boulders of this rock occur along Highway 54 west of New London.

Joints are concentrated in three main sets oriented at about 30 degrees, 155 degrees and 115 degrees. The WNW-ESE-trending joints are widely spaced but very long. These joints parallel the trend of the outcrop and probably determine its shape. The other joint sets are commonly very closely spaced. They often occur in bands a few meters wide in which the joints are spaced only about 10 cm apart. Both sets of joints show occasional offsets of a few centimeters.
Cactus Rock displays glacial polish, striations, and chatter marks (more properly called crescentic fractures or friction cracks). At the summit of Cactus Rock, two cross-cutting sets of chatter marks occur: an earlier set trending about 281 degrees and a later set trending 220 degrees. The trend of the early set is nearly perpendicular to the western margin of the Green Bay ice lobe, while the later set is approximately parallel to the axis of the ice lobe. These relationships suggest that the earlier set of chatter marks formed as the ice lobe was expanding outward from its axis, while the later set formed after the ice lobe had reached its full width and ice flow was predominately along the axis of the lobe.
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Brecciation has been observed in one of the NNE-trending joints. A quartz-magnetite vein occurs at the summit of the outcrop, filling a joint trending 028. About 5 meters southeast, tastefully highlighted by the graffiti "Chris 93", is a quartz vein offset by several small right-lateral offsets. [As geologists, we may take some solace from the fact that Cactus Rock should outlast both the graffiti and its author.]

As the case with many crystalline outcrops in glaciated areas, evidence of wind abrasion is evident on close inspection.

 

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Created 19 May 1999, Last Update 11 January 2020