Geology of the Site

The following information is from Gordon A. Lothson's 1976 book, The Jeffers Petroglyphs Site

      The Jeffers Petroglyphs site is located 5 miles east and 3 miles north of the town of Jeffers in north-central Cottonwood County (Fig. 1). It lies on the crest of a ridge of metamorphosed sandstone, known as Red Rock Ridge, which extends for 23 miles and rises some 100 to 300 feet above the landscape from Watonwan County on the east into Brown County on the northwest Although carvings occur on Red Rock Ridge in section 12, Amboy Township, and in sections 5, 8, and 9, Delton Township, the largest concentration of 1,885 glyphs exist at the preserved site located in the N1/2 of the NW1/4 of section 9, T107N (Delton), R35W. The carvings appear on a rather flat-lying outcrop of rose-colored quartzite that is over 1,000 feet long and 150 feet wide (Plate 1; Winchell, 1884:499-501).
      Adjacent to the outcrop is one of the state's rare remaining examples of true tall-grass prairie. The only native trees in the region are confined to the banks of the Little Cottonwood River located about 300 yards to the southeast. There the river winds its way through one of the few breaks in the Red Rock Ridge, passing through small groves of American elm, bur oak, box elder, cottonwood, basswood, hackberry, and willow trees. Within this passageway are two very prominent game trails - one following each side of the river. Deer, racoon, rabbit, fox, and other animals travel along these paths as evidenced by myriad tracks.
      The Sioux quartzite found at the Jeffers Petroglyphs site is one of the oldest bedrock formations in Minnesota and was deposited originally as red sand and mud during the Proterozoic period. It is a metamorphic rock formed from sandstone cemented with silicon dioxide and re-crystallized by heat, pressure, and chemical solutions. The quartzite varies in color from nearly white to red and also appears as deep brown, lavender-brown, lilac, or red-purple. The variety in color is due to iron oxides that occur as thin films around and between the quartz sand grains of which the rock is composed (Schwartz and Thiel, 1963:284, 286).
      At the Jeffers site the rock surface has a consistent 5-degree south-southwest dip and west-northwest/east-southeast strike. The quartzite is of two types: (1) a very hard, indurated, pale pink stone of low porosity featuring secondary enlargement of the quartz grains and deposition of quartz in the intergranular spaces and (2) a pale gray-red stone consisting mainly of angular quartz grains set in an aggregate of muscovite, feldspar, and hematite exhibiting no secondary growth of the quartz grains (Bowles, 1918:202-204). The second type is much softer and easier to work than the first.
      There are several unusual physical features on the rock surface at the Jeffers site that are totally unrelated to the carvings but easily confused with them. Among the more interesting are ripples that resemble the wave marks seen on the beaches of many lakes. They were produced by waves acting upon ancient Proterozoic shorelines nearly 500,000,000 years ago. Also present are curiously formed polygon figures produced by processes similar to those that formed the ripples.
      The third geological feature of note is glacial striations that appear as large and small parallel grooves in the rock surface (Plate 3). These were produced by the advancing Des Moines ice lobe during the Mankato phase of the Wisconsin glacial advance some 12,000 to 14,000 years ago (Wright, 1972). The ice front was about a mile thick at that time and, judging from the northwest-southeast orientation of the striations, moved in a southeasterly direction. The smaller striations which crisscross at various angles probably represent minor advances of the Mankato ice front or previous advances of the Wisconsin ice sheet, rather than advances of the earlier Nebraskan, Kansan, and Illinoian glaciers. This explanation is supported by the presence a few miles south of the site of a prominent terminal moraine. Moraines of this type are associated with ice stagnation and seasonal advance and retreat of the ice front, which could account for the shallow, multidirectional striations on the quartzite outcrop.

Main Page