While on a field trip working in the Grand Staircase Escalante National Monument (GSENM) , a few members in our group came upon several cobbles of chert and quartzite cobbles with an unusual surface that I can best describe as a rind. Each cobble of chert and quartzite had a black rind 1- 2 cm thick around it. The cobbles are part of units such as the Canaan Peak. The same cobbles are found Dakota Formation near Capitol Reef National Park. The units were deposited in streams during the Cretaceous period of time. The cobbles have since been reworked and deposited in the GSENM in alluvial terraces.
These rinds, sometimes referred to as patina apparently form from weathering on the outside of the cobbles that were deposited in braided streams deposits. Analysis of similar rinds show iron, manganese and other elements like silica leaching out of the rock over time. Some suggest that the microflora aid in this process.
Try as we might, we can't find any papers that are specific to the rinds on these Cretaceous conglomerates. We would like to know more - the composition of the rinds, why they are so common in the Cretaceous (Sevier Orogenic) conglomerates. Looks like a great future research project.
Baker, J. C., Edmonds, W. J., Ogg, C. M. (2001) Research Gate. Retrieved from http://www.researchgate.net/publication/232149104_Quartzite-Cobble_Weathering_in_Alluvial-Fan_Soils_of_the_Virginia_Blue_Ridge
Rajamani, V., Tripathi, J. K. (1999). Current Science. Retrieved from http://www.currentscience.ac.in/Downloads/article_id_076_09_1255_1258_0.pdf
Viveen, W., et al., Reconstructing the interacting effects of base level, climate, and tectonic uplift in the lower Miño River terrace record: A gradient modelling evaluation, Geomorphology (2013), http://dx.doi.org/10.1016/j.geomorph.2012.12.026
Wagner, G. A., (1998). Google Books. Retrieved from http://books.google.com/books?id=ADuZDCa08kwC&pg=PA37&lpg=PA37&dq=iron+rinds+on+chert+in+alluvial+terraces&source=bl&ots=txI_5azP_I&sig=PtZAyR89-EaP4nBfxXuCLq8ehVc&hl=en&sa=X&ei=6iVRU_6PJqbq2gXF0IGQAw&ved=0CEYQ6AEwBQ#v=onepage&q=iron%20rinds%20on%20chert%20in%20alluvial%20terraces&f=false