May 12, 2012

Georneys with Dana- Part III: The Chesterfield Gorge

Posted by Evelyn Mervine

The gorgeous gorge.

A gorgeous gorge.

Note: I convinced my Geokittehs co-author Dana Hunter to fly from Seattle to New Hampshire to visit me for a few days. I handed in the final version of my PhD thesis last Friday afternoon, and Dana arrived last Saturday to help me celebrate. This is Part III of my description of the fun georneys we had together during Dana’s visit.

After our visits to the dinosaur footprints and The Rock, Fossil, & Dinosaur Shop, Dana and I headed back to New Hampshire to our secret lakeside geologist lair. On the drive back, we stopped at the Chesterfield Gorge. Although there has recently been some illegal activitiy at the gorge, during our visit Dana and I didn’t encounter anything more dangerous than an overly friendly golden retriever who wanted head scratches. I was happy to see that the gorge is back to being a peaceful natural retreat. I think my mother’s claims of the “danger” of the gorge were perhaps somewhat exaggerated.

Gorge Trail Map. Image taken from here:

Dana and I hiked the 0.7 mile loop trail along the gorge, stopping frequently to investigate the gorge’s intriguing geology. The gorge contains a most unusual looking stream. Most of the time, the bedrock found in streams and rivers is smooth (and often covered in sediment), and the rocks found within streams and rivers are rounded. River and stream rocks are generally worn down and rounded by physical erosion caused by the passage of water over them. However, the rocks at the gorge are not very round. They generally have sharp, square edges that indicate that the stream is fairly young (or just hasn’t been in contact with this bedrock very long) since there has not been enough time for physical smoothing of the rocks.

Sharp, square edges on those gorge rocks.

More sharp, square rock edges.

More square rocks.

Dana and I made our way along the trail to the first bridge across the stream:

The first bridge. Thou shall pass.

Before crossing the first bridge, we decided to take a closer look at the gorge’s rocks. The rocks are highly-weathered, and many are covered in moss. So, we had to exercise the good science of rock-breaking:

A weathered, moss-covered rock with a freshly-broken rock on top. Penny for scale.

Unfortunately, my rock hammers and other rock-breaking supplies are all in South Africa, and Dana packed carry-on for her trip and couldn’t bring her rock hammer. So, we had to improvise a bit with the rock breaking:

Throwing one rock against another.

Rock breaking by throwing is not as effective as rock breaking by hammer, so we also gathered up some rock samples to break later. The next afternoon, we borrowed a hammer from my dad’s stash of tools and did a little more advanced rock breaking. A word of advice: in the below photos, I set a bad example. Rock breaking should ideally be done with closed-toed shoes and long pants (to prevent injury from stray rock shards), but I did remember the essential eye protection (wrap-around sunglasses, in this case).

Breaking rocks in sandals. Don't try this at home, kids!

A little more aggressive rock breaking.

Dana, breaking rocks in more sensible rock-breaking attire.

Excited about the rock-breaking results.

Here’s what we saw when we broke the gorge rocks:

Gorge rock #1.

Gorge rock #2.

Gorge rock #3.

So, what type of rock is found in the gorge? Looks like a metamorphic rock (fairly low grade) that perhaps used to be a granite or granodiorite. I managed to find a little more information about the gorge rock type by plotting the location of the Chesterfield Gorge on top of a geologic map of New Hampshire. I found the New Hampshire geologic map on the USGS website here, and I imported the map data into Google Earth. Pretty neat, huh? It’s actually quite easy to do– just download the .kml file, put it into Google Earth, and then you can look at the geologic map as Google Earth layer. Then, you can just click on a geologic formation, and an internet browser window will open with the information for that particular formation.

Below are some maps that I made in Google Earth showing the gorge location. First, here are some regular Google Earth images:

Google Earth image, showing the location of the Chesterfield Gorge in the state of New Hampshire.

A zoom-in of the previous Google Earth image.

Now, let’s take a look with the New Hampshire geologic map added as a Google Earth layer:

The geologic map for New Hampshire as a Google Earth layer. Pretty neat, huh?

A zoom-in on the geologic map, showing the location of the Chesterfield Gorge.

An even closer zoom in. The gorge parking lot area is indicated by the marker.

The marker in the above maps indicates the location of the parking area for the gorge, which is located in the orange formation. If I click on the orange layer in Google Earth, I find that the parking lot is located on the Ammonoosuc Volcanics, which is a Middle Upper Ordovician unit described as,

Part of the Central Maine Composite Terrane (Central Maine Trough) – Variably metamorphosed sedimentary and volcanic rocks of greenschist to granulite facies, locally migmatized. Area includes structural belts between the Monroe fault on the west and the Campbell Hill fault on the east; that is, the Bronson Hill anticlinorium, Piedmont allochthon, Kearsarge-central Maine synclinorium, central New Hampshire anticlinorium, and Rochester-Lebanon (Maine) antiformal synclinorium. Ammonoosuc Volcanics = 461+/-8 U/Pb per J.N. Aleinikoff, oral commun., Feb. 1994.

Since the trail to the gorge leads away from the parking area, I believe that at least part (all?) of the actual gorge is actually located in the pink layer. When I click on the pink layer in Google Earth, I find that the pink layer is Granite/Granodiorite/Tonalite of Late Ordovician Age. The unit is described as,

Part of the Oliverian Plutonic Suite (Late Ordovician) – Pink, weakly to moderately foliated, locally porphyritic biotite granitoids found in mantled gneiss domes. Mafic varieties contain hornblends. Variably metamorphosed up to amphibolite facies. Oliverian Plutonic Suite: Keene and Surry dome intrusive rocks = 444+/-8 U/Pb per NH020. Warwick dome intrusive rocks = 444+/-8 U/Pb per NH020.

So, I believe the rocks that Dana and I saw at the gorge are from the pink formation. The rocks aren’t particularly pink-colored at the gorge, but they do seem to be granites/granodiorites/tonalites (the rocks seem to contain a fair amount of plagioclase and biotite, so my guess is granodiorite although I’d really need to take a closer look, perhaps employing a thin section– any opinions based on the rock pictures?) that have been metamorphosed somewhat. I’m assuming that the ages above (based on U-Pb dating) are in millions of years, so the gorge rocks are nearly half a billion years in age. Such an old age isn’t unusual for bedrock. Nevertheless, I still find myself somewhat in awe that these rocks originally formed nearly half a billion years ago. What history is exposed in the gorge!  As a quick aside, when people ask me why I became a geologist, I sometimes say, “Well, I always liked history. And geology is really ultimate history, if you think about it. Geology is the study of the history of our planet– and other planets and planetary bodies.”

So, why do such old rocks host such a young stream? Clearly, the stream hasn’t been in contact with the bedrock very long or else the metamorphic gorge rocks would be worn smooth. I searched and searched for some academic papers on the formation of the Chesterfield Gorge. However, despite spending quite a bit of time using search tools such as GeoRef and Google Scholar, I didn’t find any scientific papers on the gorge. If anyone knows of any publications on the gorge’s geology, I would be most grateful if you would direct me to them. I couldn’t even find the gorge in my Roadside Geology of Vermont and New Hampshire book.

The best information I could find on the gorge’s origin is a PDF of an information pamphlet published by New Hampshire State Parks. The brochure indicates that the gorge is located along a fault (I’d love to bring Callan to the gorge to take a look at the structural geology) and that Wilde Brook (never knew the stream was called that until I read this brochure) is a superimposed stream that originated from glacial meltwater ~12,000 years ago. The sediment and gravel that once covered the metamorphic bedrock have been eroded by the young stream (I presume 12,000 years is fairly young for a stream? Perhaps not?), and now the metamorphic bedrock is slowly being eroded.

There is certainly some evidence that there was once a retreating glacier in the Chesterfield Gorge area. For example, just look at this enormous glacial erratic:

A glacial erratic boulder, just a few feet from Wilde Brook.

After Dana and I spent some time breaking rocks at the first bridge, we made our way down to the second bridge. Along the way, we saw some beautiful scenery:

Gorgeous gorge scenery.

More gorgeous gorge scenery.

After a short hike, we found ourselves at the second bridge. When we arrived, we found that we had a slight problem:

The second bridge. Thou shall not pass.

Since we are intrepid geologists, we decided to ford the stream rather than backtrack:

Venturing off the beaten path.

Caulking the wagon and floating didn't seem necessary.

Dana looked a little nervous before the stream crossing, but she made it just fine.

After fording the stream, we headed back up towards the parking area. On our way out, we had another look at the first bridge:

Peaceful and pretty.

And we also found some interesting biologically-assisted rock erosion:

Tree root style rock erosion.

More tree root style rock erosion.

And we saw some interesting mushrooms:

Pretty fungi.

And we found a tree growing in a neat shape:

Strangely shaped tree, with geologist for scale.

All in all, Dana and I had a good day of geologizing. We saw dinosaur footprints, visited a kitschy rock shop, and explored the geology of a gorge. After this full day, we picked up a pizza and headed back to our lakeside geology lair to watch Dr. Who. Next up: Day 2 of our georneys!