23 June 2011
Bookshelfing in the Rockfish Conglomerate
Posted by Callan Bentley
Another thing I saw last week in the Rockfish Conglomerate were several nice examples of “bookshelfing,” which is when a rock or mineral fractures into pieces, and the pieces slide down relative to their neighbors, like a set of encyclopedias slumping outward on a bookshelf. This has the effect of shortening the grain/clast in one direction, and elongating it in another direction:
Ever seen anything like that? If so, where? What was the context?
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’s a cracked-pebble conglomerate on the north end of Humbug Mountain on the southern Oregon Coast which shows similar structure. The Mt. butts right up against the shore, and there’s a campground/park right where 101 turns inland to go around the peak. Getting across the creek can be dicey if you don’t want to wade, but there’s normally enough driftwood to make it possible. The clasts are smaller- I’d guess average is about 0.5 cm. I don’t recall the unit’s name (Humbug conglomerate, maybe?), but it’s Cretaceous in age. Very cool!