Saturday, August 3, 2019

The Garnets of Stony Run in Baltimore City

A new garnet locality has emerged in Wyman Park along Stony Run. It has produced crystals that visually resemble those pictured above. They were collected about a third of a mile upstream at the historic long closed and built over Wright Quarry.  The new spot  is along a short stretch of Stony Run between the Wyman Park Drive Bridge and where it enters a tunnel before flowing into  Jones Falls en route to Baltimore Harbor.  

Mineral Bliss  was here before in conjunction with a 2014 post.  Its subject was a piece of quartz we found in the stream that hosted several white concentric circles, which rendered us "stumped." We now suspect these circles were once a contact point for small stalactites or stalagmites that had  long ago originated within a cavity in rock somewhere upstream.

On a recent steamy July Sunday,  local aficionado and collector of regional minerals Stuart Herring directed our second visit. We came to look for garnets, and we succeeded in finding them.  

Jones Falls Schist and hornblende rich Baltimore Gneiss share the country rock everywhere that Stony Run flows through Wyman Park. Intruding the schist and gneiss are at least three separate pegmatite dikes. They differ according to  varying proportions, hues, and varieties of quartz, mica, and feldspar minerals. Interestingly and importantly, all three pegmatites have yielded similar gemmy red almandine garnet crystals. 
Heading downhill  through overgrown brush to reach the stream  from the west side of Remington Avenue below Wyman Park Drive was easier than it was 5 years ago. Since then, a diagonal swath extending about halfway to the stream was created to allow heavy equipment to descend, then dig and blast as  necessary to replace a decaying sewer line. The blasting dislodged a significant amount of pegmatite rock from beneath the soil. Some of the pieces made their way further downhill toward and into the stream. 

Directly across the stream from where we approached is a much steeper embankment that exposes a contact point between the Jones Falls schist and a pegmatite dike. The recent blasting suggests that very likely this same pegmatite once extended to our side of the stream. Comprising it are differing amounts of microcline, albite, and plagioclase feldspar along with quartz and mica. 

The garnet crystals within rarely measure more than 1/2 inch. Mostly they reveal themselves inside the pegmatite rock when it is broken, which unfortunately is likely to damage many of the crystals. We obtained our garnets, such as pictured at right, by hammering away at a few  rocks and cobbles plucked from along the stream bank and in the stream. 

After a little more than an hour, we headed upstream to the area where Stony Run flows past the former Wright Quarry site.  Our approach was longer than the previously mentioned 1/3 mile and roundabout to avoid impenetrable  vegetation. Along the way, Stuart pointed through a wooded  area where several years ago, the City had blasted into a different pegmatite  where a tannish microcline was dominant. Once again, the City's  purpose had been to replace a portion of aging sewer line,  The blasting produced and revealed numerous garnet bearing pegmatite boulders. They remained at the site long enough for Stuart to accumulate an attractive selection of specimens  bearing attractive almandine crystals resembling those we'd just collected. Ultimately the material was  hauled away with no trace remaining. 

The former Wright Quarry site was more approachable. A pegmatite consisting primarily of white albite once intruded through its walls.   In earlier times, reports show this pegmatite yielded not only garnets, but beryl, zoisite (var.) thulite, fluorapatite, and autunite.  Long after the Wright Quarry closed and even as the Johns Hopkins University campus extended into and over its site,  a few garnet yielding  pegmatitic rocks from the old Wright Quarry dumps remained above and along the stream banks. 

They disappeared in recent years during a Baltimore City Department of Public  Works project that reinforced the stream bank to slow erosion. When the work was complete, the few rocks and boulders once part of the Wright Quarry dump were nowhere to be seen. Sometime thereafter Stuart noticed a few pieces of material that looked like Wright Quarry albite in the stream near its now reinforced bank. He suspected that some of the Wright Quarry rocks and boulders  could have  ended up beneath the recently reinforced stream bank from which storm currents could have dislodged them. 

We pulled two such rocks out of the stream, each weighing several pounds  When cracked open, portions of both rocks revealed hundreds of  tiny, mostly broken gemmy red garnet crystals measuring from one to two millimeters. Such ubiquitous small garnet crystals have long been considered definitive of Wright Quarry pegmatite and never known to be present in other  nearby pegmatites. Pictured at right is is a large fragment from one of the rocks we collected. It is  reasonable to believe that it could be one of the last examples of Wright Quarry garnet to be found. 

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