Friday, September 20, 2013

Remarkable Finds Amidst Cobbles in Baltimore, MD's Herring Run

Alana Benkowski has an eye that spots four leaf clovers pretty much at will. Her eye has proven just as sharp for spotting crystals in water-polished cobbles. She collects them in the alluvial deposits of Herring Run near where it flows beneath Baltimore's Inner Harbor Tunnel Thruway. The site is but a short walk across the flood plain adjacent to her home in East Baltimore's Armistead Gardens neighborhood.

Alana has been gathering these cobbles with a passion for several years. Driving her hobby is an intense curiosity regarding the identification of what she's been finding. Her Bible has been a much pageworn edition of the National Audubon Society's Field Guide to Rocks and Minerals, and more recently the Internet. That is where she found our Maryland Minerals website, and contacted us last month.

With no training and little formal education, Alana has the sensibility of a locality expert regarding what to look for and how to find it along this brief stretch of Herring Run. With no bedrock exposures in the area, however, everything she finds originated upstream and beyond in places where the geology is different. The diversity of the species along with curiosity as to where they originated is what most draws her to this hobby.

Pictured below are a few of her finds.

Gemmy Epidote in Quartz

Clinozoisite (Micro crystals in vug in water-polished quartz)

A container of Schorl (Tourmaline Family)crystals in water-polished quartz.

Chromite in serpentinite (Peacock Ore)

Micro Pyrite Crystal on Garnet in Cockeysville Marble


Graphite (lower right of specimen)

When speculating as to where such non-indigenous  rocks originated, knowledgeable authorities can often be expected to hedge their bets. Here are a few thoughts:

After studying some maps, our friend Bob Conkright, who specializes in hydrology for the Maryland Geological Survey, offered the following theory regarding a possible source for the epidote and possibly the clinozoizite. That is, of course, on the assumption that our "clinozoisite" isn't simply a lighter coloured epidote. He noted that several miles upstream, Herring Run flows through exposures of the Baltimore Gabbro complex, which is known to host epidote.
As a member of the epidote family,  clinozoisite could be present in the Baltimore gabbro as well. Both epidote and clinozoisite tend to crystallize inside vugs where they enjoy some protection as water and sand polish the surfaces of the rocks bearing them.  

Almost certainly, these  pieces originated in the Bare Hills serpentine barrens in Baltimore County. The Barrens are adjacent to the Baltimore Gabbro Complex. It is reasonable to assume that over many years, such material could have traveled by stream or through flooding in a southerly direction toward where Herring Run flows through the Baltimore Gabbro Complex. 

This is the only no-brainer: Herring Run cuts through plenty of schorl bearing pegmatite. The specimen in our post of May 11, 2012  was collected about three miles upstream from where Alana collected these cobbles. 

This specimen is curious because the matrix, which our photomicrograph all but ignores, appears to be Cockeysville Marble. It is unlikely that streams or flooding leading from Cockeysville Marble outcrops would carry any it in a direction leading to Herring Run. Further, such soft and relatively soluble material would likely be water polished and/or dissolved away long before it could travel this far. Almost certainly this piece and a couple others that Alana collected made their way south by truck before landing somewhere upstream in Herring Run.  
Assuming our identification is correct, this one  raised some eyebrows, simply because fulgurites are rarely found hereabouts. However, they are formed when lightning strikes sand, and there's plenty of sand along Herring Run, particularly in the area where this piece was found. For that matter, a hot electric wire could have done the trick. 
By all accounts, this is the most mysterious find of all. To the best of our knowledge, graphite has never been reported at any locality from which one would expect that it could end up in Herring Run. The closest place of which we're aware that graphite has been collected is in Harford County. From there, it would most likely find its way into the Chesapeake Bay before it could get to Herring Run.

Regardless of various theories as to origin of any of these rocks, Bob Conkright, shortly after mentioning the the Baltimore Gabbro,  offered another insight: "Of course, Mother Nature has had millions of years to erode bedrock in the Piedmont, transport it and polish it along the way, to be deposited on the outwash deposits of the coastal plain." And after a moment's hesitation, he added: "But not that (Cockeysville) marble."