Sunday, March 29, 2009

Stb Rock Outcrops

A few weeks ago I was wondering about the unusual rocks along a stretch of the Paugussett Trail in Monroe, even before I read the CFPA trail description that identified an old silver mine in that very spot. I took a look at the USGS bedrock geology map (click to enlarge) and learned there is a rock formation called the Straits Schist Basal Member, abbreviated Stb on the map and shown in orange. The Stb formation is defined as a gray schist with amphibolite, marble and quartzite and is known for containing metals.

This is the same formation that pops up at Old Mine Park in Trumbull, location of a former Tungsten Mine. Back in the 1990's they were blasting nearby to build a corporate park and rock hounds were going in on the weekends to see what they could salvage of the metal-bearing rock (I collected a crate of rocks myself and will be using one for my first rock stash).

This photo is a close-up of one of the Trumbull rocks I collected in the 1990's at the corporate park blasting site. The face of the rock was coated with pyrite (fool's gold). Click to enlarge so you can see it better. The Stb formation has metals in it due to geothermal activity. Water came from magma carrying metals and dropped those metal into the surrounding country rock as it cooled. Theoretically, anywhere you see the Stb symbol on the geological map would be a place to look for metals such as copper, silver, gold, tungsten, etc. The metals might be elemental (and look like a metal) or they might be in the form of a mineral. Pyrite is a mineral composed of iron-sulfide.

The formation is also highly visible along the Paugussett Trail in Shelton in the north end of Indian Well State Park, just south of the aqueduct. All of these locations have enormous boulders with an unusual appearance, some creating caves (I recently heard in passing that the Indian cave in Monroe is "missing" but used to be the site of Indian soapstone quarrying. The entry collapsed and now people can't find it.) This photo shows one of many huge boulders on the Paugussett in Shelton. The aqueduct that cuts through the area has lots of interesting freshly broken rock if you are crazy enough to risk sliding down the hillside looking for them (guilty).

This photo shows the Stb formation in Monroe along the Paugussett at the silvermine. The mine is located in the East Village section of the trail where the trail descends into a hemlock ravine with the Boys Halfway River right next to the trail. I found some marble and possibly silver nearby (I didn't have a hand lens with me, but it sure looked like silver so I dropped it in a letterbox I was planting in the area).

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