Campus Landscaping By Constructing Mock Geologic Outcrops

Journal of Geoscience Education, Sep 2006 by Matty, David J

Structural Geology, Unconformities, and Sequence of Events - The original mock anticline constructed on the east side of Brooks Hall can provide potentially hundreds of lower-division students with a variety of learning opportunities. First, it provides an opportunity for students to observe similar rock types in separate outcrops and to make the intellectual connection that rock types often are continuous and correlatable between exposures. Second, it poses a question as to why fossiliferous sedimentary rocks formed as originally horizontal layers are now exposed as beds that tilt in different directions. Third, it allows students to determine, in a very simple way, the strike and dip of beds (see Figure 6) and from plotting these data on a map, to determine what structure is represented. Finally, it forces them to visualize the fold even though its axial portion has been "eroded."

When originally constructed, this mock anticline was built, in part, by burying dipping dolostone slabs in a low berm just east of Brooks Hall. Later, a single boulder of bedded, fossiliferous Devonian limestone was placed on top of the berm with its beds oriented horizontally. Consequently this portion of the CGA now represents an excellent example of a mock angular unconformity. Students in introductory courses are routinely challenged to interpret these relationships, and we consider this representational grouping of specimens to provide another valuable learning opportunity. The opportunity to challenge introductory students to identify a nonconformity also exists in the CGA, specifically between the granite/gneiss /BIF exposures and the dolostone foldbelt, but because this feature is more difficult to visualize, it is often absent from exercises in larger introductory courses. Instead, we often challenge students with this feature in smaller courses where we can provide better guidance.

It follows from the previous examples that many parts of the CGA provide students with opportunities to make observations, critically assess their observations, and make interpretations based on their assessments. Not only can specific geologic features be recognized in our representations, but many individual groupings and especially the CGA as a whole provide excellent opportunities for students to investigate complex sequences of events. Such exercises involving the whole of the CGA in our earth history course has been presented by Benison (2005).

Upper-division Vourses - The CGA is also utilized in other mid- to upper-level geology courses. Students in mineralogy and petrology often revisit many of the boulders to investigate lithologies, textures, and other features in more detail. Students in paleontology and stratigraphy/sedimentology study many or the sedimentary and metasedimentary rocks to learn more about various textures, fossils and sedimentary structures. Likewise, students in earth materials (a course for non-majors and pre-service earth science teachers) often utilize a wide variety of igneous, sedimentary, and metamorphic samples from the CGA as part of their course requirements. However, perhaps the most intensive use of the CGA comes from students enrolled in our sophomore-level introduction to geologic investigations course. The first eight weeks of this course focus on geological fieldwork and culminate in a four-day field trip to the Precambrian exposures of Michigan's Western Upper Peninsula. The field trip exposes students to a complex folded terrane consisting of highly metamorphosed rocks and associated intrusions, and the students are expected to construct a geological map and history of their area. Prior to the trip however, the same students use the CGA to hone basic field geology skills. In an exercise spanning 3 to 4 weeks, they 1) accurately measure, record, and plot on a map the strike and dip of beds, fold axes, dikes, and metamorphic foliation; 2) correlate rock types and measure sections across poorly exposed dipping beds; 3) determine and plot on a map the location of various lithologic contacts and unconformities, both known and inferred, 4) construct a map and cross-section based on their observations and measurements of the CGA representations, and 5) prepare a report of the geology of the CGA based on their observations, measurements, and interpretations. This exercise requires students to learn to distinguish between known and inferred contacts, and to begin to make sound interpretations of complex structures. In particular, students are challenged by change of fold axis orientation, which may be interpreted as resulting from a later generation of folding or from some form of rotational faulting; we also expect our students to note that the orientation of the "dike" is consistent with either interpretation.