Note: This is the final version of the Trail Guide, as best I can determine for Sept 6, 2011.  The version that Eric used to construct the most recent tri-fold (Sept 5, 2011) was an old version that contained virtually none of the corrections that Alice suggested in her first editing. Aside from that the tri-fold Eric sent last night looked really good, and we should be able to swap text out relatively quickly? 
	Don’t know what happened there, but I have gone back and reconstructed the version below from the copy Eric had, and the Track Changes copy I have where I originally made all the changes and corrections.  They are now reconstructed in the version below.  Peter, I do not think the version below has the disclaimer you mentioned, so you need to check to make sure that is there.  I suspect we have just had too many versions of this crossing in e-mails, and we got really lost.
Lynn

Stop 1: Trail Head Parking Lot

As you stand in the parking lot with your back to the kiosk, look at the view in front of you. Notice the broad, flat valley floor and compare it with the more rugged hills and mountains all around. Directly across the field is a white house, Elk Hill, located on a low hill. To the far left is Horseshoe Mountain, and in the distance just to the right of Elk Hill is Three Ridges.

The contrast in topography between the valleys and mountains tells us that different geologic processes form each. The mountains are undergoing active erosion: ice wedging in the winter that breaks the rocks into angular fragments; running water in the summer that carries the particles down the slopes and rounds them into gravel; and chemical weathering all year round that decays the minerals in the hard rocks to soft  saprolite (deeply weathered rock), which crumbles into sand and soft clay.

When this weathered rock erodes it ends up in the valley as sediment where streams and rivers pick it up and transport it downstream toward its ultimate destination, the Chesapeake Bay (Stop 2). Over thousands of years the streams and rivers sweep back and forth across the Rockfish Valley spreading the sediment out in layers that have produced the broad, flat, flood plain valley in front of us. During a major flood the entire valley can go under water. This may happen every 100 years or so, or 10,000 major floods in a million years. We do not know how deep the original valley was, but the river sediments may be tens to hundreds of meters deep (see cross section). Dig a hole almost anywhere and you will soon encounter gravel.

The rocks forming the surrounding mountains are folded into a broad anticline: an upward–arching mountain of rock whose eastern flank rises out of the earth along a northeast–trending strip of land that includes the Monticello area east of Charlottesville. The western edge of the anticline descends back into the earth in the Blue Ridge mountain area. The anticline formed about 300 million years ago as part of the Alleghanian mountain building event when Africa collided into North America to form the Pangaea supercontinent. The mountains today are the eroded remnants of the Alleghanian mountains. The Rockfish Valley now exists because as the rocks folded they stretched and cracked, allowing water to penetrate and erode the top of the anticline. 

Stop 2: Grenville Granite of the Rockfish Valley 

From the main trail walk a short distance to your right down the path to the Rockfish River. Today it runs along the west side of the valley, but in the past has wandered from one side of the valley to the other. 

Observe the gravel in the stream bed. The gentle flow of water you probably see today is not nearly strong enough to bring these large pieces of gravel here or move them further. The presence of the gravel tells us that significant floods passed this way. Indeed, on April 16, 2011, most of this area was under water and the water was moving with enough speed and energy to have moved all these gravel pieces a long distance downstream. 

On the other side of the Rockfish River is a wall of thinly layered rocks sitting up almost vertically (picture). Originally hard and tough, today they are deeply weathered into saprolite, especially toward the top. If you were to pick up a piece, it would crumble in your hand. This is the result of the chemical weathering that creates the sand and clay that carries the mountain grain by grain down to the ocean. To your left, and upstream a short distance, you can see other examples of these rocks sticking vertically out of the river bed.

The saprolite is so badly weathered that determining what the original rock was is hard. They are 1-billion-year-old granites formed during the Grenville mountain building event. They have been intensely ground up and recrystallized in a shear zone—a compressive scrunching that only forms at great depth and high temperatures—where two bodies of rock slide past each other. The rocks you see in front of you contain evidence of at least three mountain-building events: the1-billion-year-old Grenville mountain granites, an unnamed 340-million-year-old event that formed the shear zone, and the 300-million-year-old Alleghanian mountain building that folded these rocks into the Blue Ridge anticline.

This is a good place to do a scavenger hunt among the river gravels looking for the different kinds of rocks brought down by the river from the surrounding mountains. At a quick glance the gravels in the river bed may look all alike, but on close examination they vary quite a bit, indicating the complex geologic history preserved here. On the back of the Trail Guide are descriptions of the rocks you are likely to find.

Stop 3: The Rockfish Braided River

From the main trail, again turn right and walk down the path a short distance to the Rockfish River and look upstream. Notice how the river is divided into channels by gravel bars (picture).

The river might look rather peaceful the day you are here, but it is subject to periodic floods that will completely rearrange the bars and channels, sometimes even moving the river sideways into a new part of the flood plain while abandoning the old channel.  When the vegetation is down you may see old abandoned channels along the row of trees on the far side of the river.

If you look across the river (picture) you will see at a slightly higher elevation than where you are standing a flat surface gently sloping to the right. This is a terrace, a remnant of an ancient, eroded flood plain. In the past that surface extended across the entire valley to the other side, suggesting that at some time the river contained more gravel than it could carry and deposited it, filling the valley in. It also suggests that the flood plain you are standing on is the result of erosion that has removed everything except for the terrace.

If you turn around and look across the flood plain toward Elk Hill you will notice the mostly flat surface the house is sitting on (best seen when the trees have lost their leaves). This is a monadnock, an erosional remnant of granite. The flat surface is a pediment. For the pediment to form, the valley had to be filled enough to just bury Elk Hill, preventing erosion from going any deeper while shearing the exposed granite surface flat.

Based on evidence elsewhere, at one time the entire area might have been buried in sediment all the way to the top of the Blue Ridge Mountains resulting in a broad, high, flat, featureless plateau far different from the beautiful and rugged country we have now. 

Erosion is relentlessly removing the mountains around us, with the rivers carrying all pieces to the ocean.  Eventually everything will be eroded down to sea level, and everything you see before you will be gone.  It will be an ending, and a beginning, for history tells us that after a mountain has been eroded new mountains will be built, as they have many times in the past.

Scavenger Hunt 

Let’s have a scavenger hunt. Search carefully among the gravels looking for the following varieties at each stop and along the trail. Each of these rocks represents a different chapter in the story of the Blue Ridge Mountains and the Rockfish Valley.  After you have identified the rocks, make sure you put them back into the stream channel. 

Grenville Granite: abundant and easy to find cobbles with large white and dark crystals. Other samples may have weak layering, indicating that the granite was metamorphosed into gneiss. Washing them in the river will help you see the crystals better. The source rocks for the cobbles were intruded into the roots of the Grenville Mountains about 1.1 billion years ago and metamorphosed about 1 billion years ago, miles below the surface. 

Catoctin Greenstone: dull, dark green to pale green, fine grained. These gravels are much rarer than the granite and will take some hunting. They are basalt lava flow fragments now metamorphosed to greenstone. They flowed across a land surface of eroded Grenville granites about 560 million years ago. Today these lava flows are seen mainly along the Blue Ridge Parkway, so these river gravels will have been transported down from there. Catoctin lava flows erupted when the supercontinent Rodinia rifted apart.

Mylonite (shear zone): very thinly layered, fine-grained rocks like the ones on the far side of the Rockfish River at Stop 2. These are also rare, but if you find one look for severely stretched and thinned pencil-lead-like mineral grains. Formed in a unnamed mountain-building event about 340 million years ago, this is the only surviving evidence of the existence of that mountain.
Unakite: intergrown large pink and pale green crystals. These are even rarer than the greenstone pieces and mylonite, but if you look carefully you may find some. Smaller pieces are prized as gemstones, while larger pieces are carved into bookends and other objects. Unakite crystals are metamorphosed Grenville granites, probably by the same event that metamorphosed the Catoctin lava flows. 

Quartz: clear to white glassy mineral, sometimes mixed with muscovite mica, which makes flat very shiny flakes. Quartz is injected as veins (dikes) throughout the Grenville granites. Sometimes people find some fairly large chunks.

Keep an eye open for other gravels different from any of these that are occasionally found. 

So how many did you find?

WARNING; Do not allow children in the river, be careful on slippery rocks. Swift currents are dangerous.
Proceed at your own risk.  PLEASE do not disturb river banks

Credits:
Lynn S. Fichter and Eric J. Pyle, 
Department of Geology and Environmental Science at James Madison University, designed and created the trail materials. They were assisted by Chip Morgan, Rockfish Valley Foundation trustee and vice president of the Virginia Flora Project, and Martha Teu, a 2011 graduate of James Madison University.

The Rockfish Valley Foundation is a 501© 3 public nonprofit organization founded in 2005. Donations are appreciated and may be made through the website.

Rockfish Valley Foundation
P. O. Box 235
Nellysford, Virginia 22958
Tel: 434-226-0446
http://www.rockfishvalley.org