<b will be acquiring some sizable new residents. No, it’s not buffalo. Rather, some magnificent samples of local geology will soon be residing on the crest of prairie in the Lower Arb. These rocks represent the second phase of the Marc Von Trapp memorial. Visitors to the lower Arb are likely familiar with the bench and oak grove that are dedicated to Marc Von Trapp, class of 2000, who died in an automobile accident during a spring break trip to Utah. This new phase is meant to provide a memorial to Von Trapp and to allow for visitors to the Arb to gain a better understanding of the geology of the Arb. Geology students in Mary Savina’s geomorphology class chose four rocks from within the Arb that represent different aspects of the local glacial history.
The first rock selected was a sample of Prairie du Chien Dolomite, a grayish sedimentary rock typical of bedrock in the Lower Arb. Outcrops can be found in several places such as the bed of the Cannon River. Prairie de Chien is fascinating because it is a link to the Arb’s oceanic past. Five hundred million years ago, Northfield, and the whole of Southeastern Minnesota was underneath a shallow inland sea!
A Duluth Gabbro was chosen as the second sample. This rock is part of the Duluth Complex which is one of the largest gabbro sources on Earth and yielded many of the gabbros found throughout the area. Approximately 1,100 million years ago, as part of the Mid-Continental Rift, large pools of magma poured out and covered the western edge of Lake Superior. Parts of these deposits were then carried south by the Superior Lobe of the Wisconsin Glaciation.
A large granite is the third member of the quartet. It currently resides near the tennis courts in the Lower Arb and might prove a mite difficult to move, but it is an excellent example of the types of rocks that were brought down by the Des Moines Lobe of the Wisconsin glaciation.
The final rock is currently preventing errant drivers from driving around the gate near Highway 19, but in its illustrious new role as part of the memorial, it will provide an informative sample for petrology students. This rock not only shows a contact between granite and granitic gneiss, but also contains garnets! Though the Arb probably won’t be making any money off these semiprecious stones, this sample will be valuable to future geology students.
The relocation of these rocks is expected to be completed later this spring. So don’t be too surprised when there are four new vantage points from which to view the Arb.
-Jasmine Cutter, for the Cole Student Naturalists
