6 June 2016
Cleavage refraction in Konnarock Fm. rhythmites – a virtual sample
Posted by Callan Bentley
Here’s a visualization combination that leverages the advantages of the GIGAmacro system with the 3D ‘virtual sample’ perspective of the Sketchfab-hosted model: the same sample presented in both formats.
In this case, it’s a lovely example of cleavage refraction going from meta-clay-shale (now ‘slate’) through a graded bed of fine sand and silt.
Link GIGAmacro by Callan Bentley
Photoscan model by Marissa Dudek
The sample is of the Neoproterozoic Konnarock Formation‘s lower “rhythmite” unit (maybe varves??) near Mount Rogers, in Virginia’s Blue Ridge. The deformation overprints the primary sedimentary features, and is certainly related to Appalachian mountain building – probably the terminal Alleghanian phase.
Students: can you tell which of these renditions is right side up? Explain. How does the cleavage angle reinforce this deduction?
Callan Bentley is Associate Professor of Geology at Piedmont Virginia Community College in Charlottesville, Virginia. He is a Fellow of the Geological Society of America. For his work on this blog, the National Association of Geoscience Teachers recognized him with the James Shea Award. He has also won the Outstanding Faculty Award from the State Council on Higher Education in Virginia, and the Biggs Award for Excellence in Geoscience Teaching from the Geoscience Education Division of the Geological Society of America. In previous years, Callan served as a contributing editor at EARTH magazine, President of the Geological Society of Washington and President the Geo2YC division of NAGT.
There are some interesting green “plumes” at the sand-silt boundary that seem to bleed out from thin wedges of a white mineral in the silt (now slate) and into the sandstone. I gather these are another result of metamorphism.