Wednesday was my last day for exploring this week. I decided to hit a few new places. I do not have pics from the locations or samples yet, so I will just provide some details of my search.
Cabin John Creek and Rock Run in lower Mont. Co. are known for gold deposits, so i decided to check the area out for myself. I started at the corner of River Road and Seven Locks Road in Potomac. Heading NW from there along River Road, there is a bridge crossing of Cabin John Creek. I parked on the north side of the road right at the bridge. There is a small area for about 2 cars there. The State Highway Administration has posted signs for "No Trespassing" to the right of these spaces, but Cabin John Creek Stream Valley Park begins at this location and heads north past Bradley Blvd, Seven Locks Road, and Democracy Blvd. There is a walking path that begins at this location as well.
I started in the stream itself right at the bridge and began walking upstream. I was immediately surprised by the sheer volume of Granite in the stream. Nowhere else in the county had I seen this much granite. I later learned that Tri-State Stone (http://www.carderock.com/) operates their granite quarry about 1/4 mile downstream and that this was the top of the local formation. It was an amazing site as it looked like the stream had actually cut dimensional stones, pavers, and veneer pieces. These rocks looked like the type of material people pay good money for. No surprise that there are three quarries within a stones throw of this site. Unfortunately for me, this meant very little in the way of large quartz cobbles to search and break. I will have to return one day to sift the abundant gravel in the stream though and search for interesting granite specimens.
Being a bit disheartened by the lack of targeted samples, but still fascinated with the local geology, i searched my brain for another spot. I didn't have to go far. Crossing to the other side of River road is Carderock Springs Dr. Just downstream from the location i started from is a fork in Cabin John Creek. Another tributary joins the stream not 100 yards below the bridge. I went and parked in a small pull off just uphill from the intersection of River and Carderock Springs and climbed down into the much smaller tributary. At first glance, I was worried as there was a large granite outcrop right next to me. This proved to be a temporary thing, as just upstream, I found what I was looking for. There is a rise in elevation from the bridge to this location of about 30 feet. This area marked the top of the granite formation and the local soil began in earnest on top of it. The stream bed is littered with many cobble sized pieces of Quartz ranging in color from almost clear to smoky quartz appearance. Most of the samples are Milky in nature and tinged yellow to brown by the Iron. Many pieces vary in crystal structure from fine to large/mixed crystals. I assume that the samples come from further upstream and uphill (they are eroding out of the dirt stream banks) and come from several locations. The show varied cooling times with the difference of crystal size.
From these two sites, I have about 40 hard rock samples ranging from several ounces to 20 lbs. I also filled up a coffee can with selected sand/gravel from behind one large granite boulder. This stream bed has plenty of sand and gravel, but a very low water flow. I don't think there is enough for sluicing at all, and panning spots were hard to find. It looks like it would have good erosion during flood stage. Carderock Springs comes to a T intersection at Fenway Road. The stream is diverted under Fenway via a 6 ft. concrete tunnel that is about 30 ft. long. I will check upstream from this location after I go through my current sampling.
As the evening wore on, and the mosquitoes began to feast on my pale legs, I decided to get out of the darkening little stream bed and go west a bit. I decided to check out a location known for gold, but one I had never actually visited. I turned NW on River Road and took it into Potomac Village. A left turn onto Falls Road and I was pointed in the right direction. Falls Road dead ends at MacArthur Blvd. A left will take you to DC, a right takes you to the C&O Canal Towpath and the National Park Service Headquarters @ Great Falls MD. Before you turn though, look straight ahead into the woods. You will see several fenced off areas and the remains of the old water tower. These are shafts 1-3, the water tower, and the remains of the Assay Office. Turn right to head to the park. Normally, it is paid entry, but they are doing some construction and entry is free (for now at least). I parked on the left side of the road (where it was safe) and walked back to the area. The tailings and dumps are all grown over, and most prospectors tell you there is no gold to be found anymore. I suggest walking around in the woods looking for pieces of quartz that the trees have naturally pushed to the surface.
A few key notes about this site. It is an interesting part of Maryland history. The Quartz veins were up to 200 ft below ground and the site was set up to crush dense quartz for the tiny specs of gold intrusions. The US GS has an online publication about the site. It is an interesting read if you like that sort of thing (which I do!). Also, the recent construction has unearthed a rather large quartz boulder that is right next to the road. I suggest you stop by and take a look to see what exactly they were working with! Some recent tree falls right on the hiking path also show distinct mining signatures and marks from drilling and pick marks.
Even though I have yet to really inspect my samples, I have noticed a few things from this day of searching:
1 - The area is very diverse in it's geological structure.
2 - It is better to be outdoors than behind a desk all the time.
3 - I still like playing in creeks and streams and digging in the dirt!
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