KF7TBA+K7LWA's Friday Insomniac-Net BLOG: [WIP] -- LW [K7LWA]'s Back East Rock Picks (Sep 2014)

LW [K7LWA]'s Back East Rock Picks (Sep 2014)

Updated 07-2015 (get it? -- like a rock pick.... argh!)
(click on photos to enlarge)

1) (PA) COLUMNAR JOINTED VOLCANICS -- in South Mountain area -- about 10 miles west of Gettysburg, PA along Rt 30: Rare for Pennsylvania, these well-preserved columnar joints occur in meta-rhyolite (Precambrian age) and are exposed along the west wall of Carbaugh Reservoir, in the Michaux State Forest. The columnar structures exposed here are about 20 feet high and as much as 2 feet wide, and have hexagonal and pentagonal sides. (PDNR)

PDF LINK
It's a little tricky to find -- forest roads, but the best way is to use Google Earth or maps to find Carbaugh Reservoir. MAP LINK
There is a "causeway" path that leads from the gate down to the Reservoir that has broken meta-rhyolite columns on either side -- massive and fragmented. Very unique specimens (looks like "spotted dick", or English bread pudding). Wide open along the path, but if you get near by, it might be worth several photo close-ups of this Precambrian Catoctin Formation (~600 mya).
The meta-basalts (greenstone) and more meta-rhyolites are found further south in Maryland along I-70 (for about 10 miles) between Catoctin Mountain and South Mountain (see below). Photos: Gate leading to Carbaugh Reservoir | "causeway" path | broken meta-rhyolite shards | meta-rhyolite chunk

2) (MD) OLD ROCKS in MARYLAND (see pp 87-92 in Roadside Geology of MD, DEL & WA,DC)

   Shelley will tell you that I go crazy about getting some really, really old rocks.
     So this part of the trip was pretty crazy, because I was looking for specimens of both some 1,100 mya (1.1 billion-year-old) granitic gneiss (Grenville basement rock) as well as some meta-lavas from around 320-340 mya.
     As you know, when ancient African plate collided with the eastern part of the North American plate about 1,200 mya during the
   The Middletown Gneiss and Quartzite along Catoctin Creek -- MD Rt 17 (see pp 87+)
    Gaging Station is on south side of Catoctin Creek bridge below roadside pull off on left.
     PHOTOS: Catoctin Creek (p88_Rt17-A) | P1040916_SM_Catoctin_Q+G | Catoctin Creek (p88_Rt17-C) |

3) (MD) METAs of South Mountain & Catoctin Mountain -- Rt 40 (pp 55-62; esp pp 57-58)
The Roadside Geology of MD, DEL & WA,DC has a good introduction of the formations along US 40 (p 55).
We were continuing our drive from Catoctin Creek on Rt 17 South and backtracked northwest on US 40 until we passed Greenbrier State Park. Just pass the Park Office (NW) we found a small outcrop of meta-basalt, with some meta-rhyolite along the lower roadside.

PHOTOS: Meta-Basalt_NW of GreenBrierSP(pp58-60)_P1040816.jpg | Meta-Basalt_P1040648.jpg | Meta-Basalt+Meta-Rhyolite_P1040659.jpg |

I'm a sucker for greenstone whatits (namely Catoctin Formation's metamorphized basalts), and there are many veg-covered outcrops along US 40 on both sides of the road.
Just about 300' south of the State Highway Facilities entrance, beautiful green-gray laminated meta-rhyolite specimens are found on both roadsides (see top Figure on p 59).
PHOTOS: Laminated_Meta-Rhyolite_GreenBrierSP(p59)_P1040813.jpg | Laminated_Meta-Rhyolite_SE_GreenbrierSP.jpg |