Sunday, August 31, 2014

STUMPED! on a Rock from Baltimore's Stony Run

Many of us  collectors have rocks lying around  that we are unable to identify even when we know their locality. Ultimately, if determined enough, we figure it out. Often we find someone who can tell us, or we encounter a similar rock that's been identified. To date, no such answer has surfaced regarding the most definitive aspect of the rock pictured above.  Baltimore artist Rick Shelley---I was collecting with him at the time---pulled it  from the stream bed of Stony Run a short distance south of the Remington Avenue Bridge.

This is a section of the stream where mica schist, quartzite and quartz account for most of what is around. The latter two often bear stains of iron oxide. A few cobbles, when cracked open, show vugs bearing weathered hematite and/or goethite.  Less frequently, one might find a rock similar to this one where the quartzite bears a coating of drusy quartz that's become variably smoothed since entering the stream, The drusy quartz may or may not be botryoidal, as it is with this specimen

What everyone who  has seen this rock finds to be curious are the concentric circles. A likely conclusion could be that they are fossils, or fossil imprints. Although it's possible, even with with a matrix of quartz or quartzite, such an occurrence would seem highly unlikely at this location in the eastern section of Maryland's Piedmont.

Much if not most of the approximately one square mile of land surrounding where Mr. Shelley found this rock was once the site of various  filled in and built over gneiss quarries that yielded a diversity of interesting mineral species. These quarries and their minerals were the subject of numerous classical mineralogical reports. We are not aware of any literature noting the existence of  fossils in the area.
Very recently, we showed this rock to my friend John S. White, who having been Curato-in Charge of the Smithsonian's Division of Mineralogy for nearly eight years, could be the source of a credible opinion. After checking it out under the scope, he was as mystified as everyone else who has seen the specimen regarding its circles. His one firm opinion was to rule out the likelihood of a fossil.  His reasoning was as follows:
  • I didn't see any organic looking surfaces.
  • While there are a couple of prominent concentric circles, there are plenty of circles  that are quite irregular.
  • There does  not appear to be any evidence of a sedimentary environment. 
What, then, expllains these circles?  We're hoping that someone will submit an answer about which he/she feels confident.