Monday, August 8, 2016

New Finds: Falls Road Corridor near Baltimore City Line

Stuart Herring

There is not a field collector anywhere who has introduced this writer to more localities in our native Maryland than Stuart Herring of  Baltimore. For the past two years, his passion has been seeking out unexpected or long forgotten localities by studying old maps. He particularly enjoys exploring unheralded spots  along the Falls Road corridor near the Baltimore City Line just a few minutes from his home. 

On a recent hot August Thursday morning, we visited a talc deposit at the southeastern fringe of the Bare Hills Serpentine Barrens near the contact point between  serpentinite country rock and quartz, schist, or pegmatite, depending upon immediate  direction,  The locality also offers small quantities of  attractive micaceous green chlorite as pictured at right and a few traces of magnetite (rather than the expected chromite). More abundant than the chlorite or magnetite is what appears to be anthophyllite.

Demonstrating a particularly interesting manifestation of the apparent anthophyllite presence is the specimen pictured at left. It bears a stunning visual resemblance to a genre known as Hermanov spheres, eponymous with the locality at Heřmanov, Velké Meziříčí, Vysočina Region, Moravia, Czech Republic.These spheres consist of a phlogopite core surrounded by anthophyllite crystals. In our specimen, the core is actually talc. The crystals surrounding it are talc pseudomorph after anthophyllite. This is the first and only anthophyllite occurrence of which we are aware at the Bare Hills Serpentine Barrens and the chrome pits dotting it. Interestingly, anthophyllite was once quite common in a different geological environment less than two miles away. The locality was the historic Bare Hills Copper Mine located just past the opposite end of the serpentine barrens. At this point, the  serpentinite  has given way to gabbro with hornblende schist and  amphibolite. For the last 55 years, the Bonnie Ridge Apartments has stood where its dumps were previously accessible. 

From the talc locality, we drove to a spot in the Mount Washington neighborhood at a point on Western Run  (not to be confused with Western Run in northern Baltimore County). The location is about 100 yards above where it flows into Jones Falls beneath the Kelly Avenue Bridge. Days before, the area had had endured a flash flood severe enough to extirpate and dislodge many hundreds of previously unrevealed rocks and cobbles.  After parking on Forge Avenue, we walked to the stream. Its banks were strewn mostly with water-polished cobbles of the same gabbro and amphibolite 
that once hosted the Bare Hills Copper Mine.  Another hundred yards above us, a little creek leading  from the site of the former copper mine dumps empties into Western Run. Thus, we kept our eyes peeled for sulfides and malachite patinas, but observed no traces. Of more interest was the occasional epidote group material gracing some of the cobbles. It showed a visual resemblance to  zoisite or clinozoisite. Analysis would be necessary to make the determination. It could also be epidote, which was known to have occurred in similar material less than a mile away at the copper mine.

Another  interesting spot to explore newly uncovered rocks could be farther south along the Falls Road Corridor, especially near Woodberry where Jones Falls flows through the well mineralized Baltimore Gneiss. The same could be said for the Patapsco and the Patuxent Rivers, especially near pegmatite areas.

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