Last spring, I read a hilarious post by Callan Bentley, in which he displayed his structural super power, which would be to shoot en echelon tension gash arrays from his fingertips. Which is pretty cool, if you are into that sort of thing

After thinking about this for a while, and since no one got me a structural geology super power for Christmas this year, I thought I’d go ahead and spell it out.

What I would like is really pretty simple, just the ability to generate a blazing-hot plumose structure which I could launch from the palms of my hands in a spectacular, fiery display of geologic comprehension. Not only would this be a useful method to drive off  my geologic arch-enemies, who continually sabotage my pursuit of science by obscuring good outcrops (lichen), but it could also be handy for highlighting the direction of joint propagation.

Here is what I’m talking about.

Yeah, I know. Pretty sweet, right?

Welcome all, to the first installment of the Not Necessarily Geology Mountain Bike Guide.

The NNG Mountain Bike Guide is an idea that has been bouncing around in my head for a few years now. Basically, the concept unifies two aspects of my life that I’m really passionate about; geology and bicycles. I started thinking about this back when I lived in San Francisco, California. I was enjoying an extended summer break, employed as a professional (a term that is certainly debatable) bicycle tour guide. On my days off from “work” I liked to pedal across the Golden Gate Bridge into Marin County to explore some amazing geology and enjoy some world-class mountain biking. As a tool for exploring the area, I purchased a mountain bike guide, which contained a wealth of useful information leading me to outcrops and points of view inaccessible from roadways. During this time I also tried to read whatever I could find on Bay Area geology to familiarize myself with the complex regional picture. After spending a lot of time reading field guides in both geology and mountain biking, I thought I would like to combine the two concepts. So here it is.

The first in this hopefully long series of posts is from Conway Robinson State Park in Gainesville, Virginia.

This hidden gem of a park is literally a stone’s throw from Rt 66 in Northern Virginia and just over 30 miles from Washington DC. It is one of the few parks that allow mountain biking in the area. For me, this place has long been a wooded refuge from the sprawling suburban colossus that is Northern Virginia. Taking a ride at Conway Robinson is an easy way to escape all the traffic, McMansion town-homes and chain restaurants, which have become all too common around here. Or, if you’re the type who would like to get away from the present reality in space and time, if only for a few hours, you can do that too.

Take a visit to Virginia’s Jurassic Park.

It is pretty unlikely you will find any dinosaurs here (living or non), nothing like the depiction of the Jurassic Period made famous in the Classic 1990s film. Yes, there were dinosaurs on Earth at that time, and there is even good fossil record of dinosaurs nearby, just not in this park (that I know of). This post has nothing to do with dinosaurs because… well, personally I find the dinosaur inhabitation of Earth to be far less interesting than the tectonic history during that period. There, I said it.

Before we get into the mountain biking, here is some background on the tectonic history and regional geology of the area.

The following 3 images are from The Paleomap Project by Christopher R. Scotese. This is a great resource for visually exploring reconstructions of global Plate Tectonic history.

This first image is of the Earth during the early Triassic period, Pangea is seemingly still a happy marriage of the continents. The North American Plate is still in contact with the African Plate. However, you can see some shaded areas in the middle, which likely represent the beginnings of the end of this continental assemblage.

The next image, also from Scotese, is a reconstruction of the early Jurassic period. This looks pretty similar to the first image but actually a lot has changed, especially on the North American/African boundary.

In this image the spreading is now very visible. The northern end of the rifting zone is opening up like a zipper and has admitted seawater where the new Atlantic Ocean is forming. Africa and North America, now mostly separate from each other, are spreading away which will eventualy result in their present placement on the globe.

The last picture in the 3 photo sub series (also from Scotese) is the from the late Jurassic, 152 million years ago.

At this stage the Atlantic Ocean is established and on the way to becoming a major oceanic basin. The view at this stage in Earth’s history is beginning to look more like the Earth we know today. The continents are starting to take on their current shapes and there are more recognizable features, like the Atlantic and Pacific Oceans.

Now back to Virginia.

Conway Robinson is located in Virginia’s Culpeper Basin, a tectonically induced rift basin, which was activated during the early stages of the break up of Pangaea in the Triassic. It is basically a giant tear in the crust that failed to become the new Atlantic Ocean, and remained attached to the North American Plate. This basin, along with several others of similar age along the east coast of North America, filled with sediments and flood basalts.

I have previously written about a conglomerate from the Culpeper Basin, in which I described some local geology. Also, last year I wrote a rather cheesy post using pieces of my sandwich as a rudimentary analogue for the extensional strain which formed the Culpeper Basin. Though, one of these posts may (or may not) be good background reading for this, here are a few recycled images for those who did not read those posts.

This graphic is just a simple Google Map image of the area, which I annotated to mark the approximate location of the Culpeper Basin.

Here is a general cross-section of the Culpeper Basin by C.M Bailey of The College of William and Mary. The two units that are featured inside Conway are Jurassic Diabase, shown in red, and Triassic/Jurassic sediments, shown in green.

There is a lot of useful information in the above diagram. Illustrated are the structural boundaries of the Culpeper Basin, both of which are normal faults, creating a down-slumping block of crust. The action along these faults created space for sediments to be deposited (TJs). As spreading continued, eventually mafic feeder dikes (Jd) cut through the older sedimentary units delivering flood basalts to the surface.

Now here is a closer view of the park itself. The main trail is marked in red and follows the perimeter of the park.

I created this trail map using MyTracks, a wonderful and really useful app for my Droid smart phone. I just started the app, which tracks GPS location, tossed my phone in my bag and rode the trail. The app does the hard work calculating the moving statistics, all I had to do was enjoy myself and look out for rocks!

The trail marked above is actually the longest possible loop around the edge, which is probably several trails and there are several more trails which cross the middle of the park. I plan on improving this map to include more specific trial info and updating this post. Just as soon as I get more time to ride and record more trail data. There are nearly 7 miles of riding within the park. So there is a good amount of area to explore, but not enough to constitute any real danger of getting lost. As you can see the park is basically encased by highways, on ramps, off ramps, neighborhoods and golf courses. You can basically experience the joy of getting lost (and found) several times in one afternoon.

As far as the difficulty of the mountain biking, it’s mostly moderate with a few difficult single track sections near the backside of the park. One of the things I’ve come to learn about the Culpeper Basin (also visible in the “outline of the Culpeper Basin” graphic) is that it is pretty darn flat, not a lot vertical relief. So there are not many sustained climbs or long miles of technical downhill.  Dont worry though, there are lots of smaller hills to make the ride fun and challenging enough for those looking for good workout.

As far as the geology goes, it’s pretty great.

Here is the same trail map image from above, which I imported into Google Earth, paired with the USGS Virginia State geologic map overlay (available here) and added some additional annotations to indicate the bedrock geology.

You can see in the above image, the park is underlain by two general rock types. The red is the Jurassic Diabase and the light green is Triassic shale and siltstone. Most of this is covered by vegetation and soil, but there is one really good outcrop of Jurassic Diabase near the “backside” of the park marked on the above map.

Here are a few pictures of the diabase outcrops.

Some of these exposures appear very columnar to me, like the one above and the first picture featured in the post. If they are in fact columns, then they owe their existence to the cooling of this once molten rock. Columns form (mainly) in igneous rocks (usually basalts) as contraction fractures. As the rock cools it shrinks and fractures in order to accommodate the volumetric change.

Here is a link to some other pretty world-famous columns I saw last summer in Ireland at the Giant’s Causeway.  Also here is a post by my former Structural Geology professor Callan Bentley, about some great columns in meta basalt in Shenandoah National Park.

This next picture is an up close look at the vertical faces of these “columns”. Notice the horizontal fractures perpendicular to the vertical joint faces. Or maybe these are arrest lines, I’m not sure really.

Here is an annotated version of the first diabase photo illustrating the relationship between the vertical joint faces of the column and the horizontal fractures.

Here is a look at a one of the fractures, notice the zig-zag shape of the fracture and the overwhelming lichen coverage.

And here is a look at a fresh surface. You can see the “salt and pepper” look to the rock. The pepper being black clinopyroxene and the salt, white plagioclase feldspars.

At several points the trail crosses obstacles which represent the meeting between geologic consideration and mountain bike trial description. A contact, if you will in both a metaphorical and geologic sense.

Here is what I’m talking about. While most of the riding is pretty tame, at points the trail crosses what local mountain bikers call “rock gardens” ranging from 10-50 feet in width.

These rock gardens are made of rounded and weathered pieces of the Jurassic Diabase and outcrop as linear features cutting across sedimentary units. However, I imagine they have a more tabular nature in the subsurface. I think these could be the base of feeder dike systems that supplied the basalt flows on the surface. Or another idea is that they indicate the geometry of the igneous body and how its shape influences the exposure. I guess I’d have to get off my bike and map these features in greater detail to get closer to the answer.

On the bike this can actually be a pretty treacherous crossing,  plenty of opportunities to scrape your chainring or bash your foot into rocks. The best way to handle these is; try to pick a line on the approach, maintain decent speed, and keep your weight centered on the bike, you should cruise right through. Or you can just get off and walk for a bit.

I took the two photographs above a few years ago with my long-lost favorite bike. Don’t let the drop handle bars and “painter’s blue” bar tap fool you, that is no road bike and certainly not a work in progress. That, my friends, is a Surly Cross Check, and pretty much an indestructible swiss army axe of bikes. The Cross Check is perfectly suited for both road and trail, although not ideal for serious single track. Unfortunately this fine machine was stolen from me two summers ago. The culprit is still at large.

One more picture to finish this up. I was lucky enough to meet this little fellow on one of my trips who I thought was worth featuring here.

Additional Links:

The Virginia Department of Forestry page on Conway Robinson

Mountain Bike Trail Reviews for Conway Robinson

In honor of Halloween today, here are some frighteningly cool photos of a spider I caught sneaking across the floor yesterday. I took these photos using my new USB digital microscope, that I’ve been having a lot of fun playing with.

In the above photo you can see 6/8 eyes, there are two more on top.

Here is the spider’s face maximum zoom.

There is something about those cold black eyes, mixed with the reflection of the circular LED light array on the microscope that makes this extra-creepy.  It almost looks like the face of some sort of cinema-inspired alien cyborg.

Here is a shot of the whole spider.

Judging by the markings and color I think this is probably a Wolf Spider and not a Brown Recluse, which are pretty dangerous. But I’m not sure, if there are any spider-people out there reading ( as in people who know about spiders, not people dressed as spiders, or alien cyborgs today for Halloween) maybe they could help out.

Here you can see all 8 menacing eyes, Two big ones on top, then two smaller eyes in the middle, then the row of 4 smaller eyes on the bottom.

This last one is with a sense of scale and hopefully makes this experience a little less frightening to see that it is actually relatively small – about the size of a penny (which actually still seems pretty big too me).

Happy Halloween !

Here are a few shots that I took using my new, super-cool USB microscope which I purchased from EmCal Scientific. I first saw this unit on display at GSA a few weeks ago, and after playing around with one, I decided I had to have it.

Here is the unit at work, it’s about the size of a small flashlight, but has some serious zoom power.

Here is a dime. The detail around the eye/brow region is pretty impressive.

I wrote a post last year about using my camera phone+hand lens to photograph some pretty small items. This is so much better than that. This powerful little USB microscope connects to my Macbook (or PC) and takes some pretty sweet pictures and video.

Here is some beach sand I collected last year on the Southern coast of Puerto Rico, near Ponce.

A lot of sub-rounded quartz grains but also some strange shell and coral fragments. For scale is the 0.7mm lead from my mechanical pencil.

Here is a closer view at one of the more interesting shell fragments with the microscope on full zoom.

This next sample of sand is from a different beach. There are some clear differences with this one and the first sand sample. This one seems to have more rock fragments and also some different shell and coral fragments.

I took a few other pictures of things around me at the time.

Here is the shirt I’m wearing today.

I actually had no idea that there was any yellow in this shirt. But whatever, it works.

Here is another closer shot. 200x zoom.

Here is the beard I’m wearing today.

Yikes !  Is that really what my face looks like ?

I’m really excited about this new toy of mine and have several samples that I’m looking forward to sharing in the near future.

After spending most of the summer in Ireland and the past few weeks writing about Ireland and things I saw at field camp, I kinda need to get back to Virginia.

Here is an old friend. The Waterfall Conglomerate, from the Jurassic age Waterfall Formation in the Culpeper Basin, Virginia

Aside from being charming and beautiful, this rock has a very interesting personality as well as an interesting source area. The discovery of this unit in outcrop last spring, was a major achievement for myself and my partner in regional geology, Aaron. This is a charismatic rock which we had seen as float, but never in out crop.  As a goal in our study of regional geology, we challenged ourselves to find it in situ.  We finally found this elusive conglomerate after some reading and exploring. When we took a closer look and cut a few pieces open, it raised many more questions than it answered. This rock launched me and the geology super-crew (me and Aaron) into an ongoing search through several miles of section, stacks of literature and millions of years of geologic time to find the source area.

But before blitzing into a discussion of source area, which is heavy with regional geology jargon, I’m going to first attempt to provide a little bit on the geologic history and regional geology.

A bit about the Culpeper Basin:

On the surface, the Culpeper Basin is recognizable as an elongate, NE-SW trending, mostly flat region, located in the Piedmont province of Virginia. The southern extent of the basin is somewhere around Culpeper, Virginia and continues northward into Maryland.

Here is a Google map image on which I drew the approximate location of the Culpeper Basin.

Geologically, the Culpeper Basin is an ancient rift basin, a down-slumping block bound by normal faults on both sides.  This basin formed in the Mesozoic period as the first phases of rifting which took place during the break up of Pangaea.

During this period, extensional forces on the Earth’s crust tore giant rips across the continent. These rift basins spread to the point at which continental flood basalts spilled out onto the surface, but not far enough that they allowed the admittance of seawater.

Here is a general cross-section of Culpeper Basin. By C.M. Bailey of William and Mary.

Here is a view of the 5 geophysical provinces of Virginia. From William and Mary’s website The Geology of Virginia.

And here is a Google Earth image that I created using the USGS Google Earth overlay for the state of Virginia. This has pretty much been an indispensable tool in my campaign of local geologizing. The white star located on the edge of the Blue Ridge Province (Blue Ridge) and Piedmont Province (Piedmont) marks the approximate location of the Waterfall Conglomerate.

Before we move on, there are just two more diagrams that I feel obligated to put up.

One is this general cross-section of the Blue Ridge from James Madison University’s website The Geological Evolution of Virginia and the Mid-Atlantic Region.  I included this to illustrate how the Blue Ridge is an up-folded anticlinal structure.

I think this is a cross-section from the central section of Blue Ridge and does not show the Culpeper Basin. However, it is a region with in the Piedmont Province, and in Northern Virginia rests against the division between the Blue Ridge and Piedmont.

The next graphic is the same Google earth image from before, except this time it is made to correspond with this above cross-section diagram.

Ok, whew… Now that we have that out of the way we can now begin to talk about the source area for this conglomerate.

The Waterfall Conglomerate is a western border conglomerate and the youngest unit in the Culpeper Basin, it has been interpreted as alluvial fan deposits also known as a fanglomerate. The Waterfall Conglomerate is not laterally continuous across the basin margin, rather there are several different fan conglomerates along the western border.

The source area for these Triassic-Jurassic border conglomerates is interpreted to be primarily from the older rocks to the West, mostly from the Blue Ridge province (Lindholm, 1979).

So we would be expecting to find (listed in order of proximity).

• Quartzite and other metasediments from the Chilhowee Group
• Metabasalt from the Catoctin Formation
• Igneous and metmorphic rocks from the Basement complex

And maybe

• A few pieces of the Valley & Ridge thrown in.

So here is the rock once again, but cut open this time to reveal the internal mysteries.

What a beauty!  A lovely conglomerate containing a lot of different clast types. Among the more interesting to me are the finely laminated red and grey/black calcareous/ fine-grained siliclastics, and the unidentified pale-pink and off-white mystery-clast in the upper right corner that fizzes in hcl. Actually everything here fizzes…everything.

What a minute…  What ?

I don’t see anything in there that looks like pieces of the Blue Ridge.  The next calcareous unit moving westward from here is the Valley and Ridge.  But I don’t think that the dominant source area for this rock is from the west at all. Here’s why.

1. Lack of Chilhowee clasts.

The first thing that jumped out at me once I cut this rock open was the lack of any quartzite chunks. The next unit immediately to the west of this location is the Chilhowee group, a “tough” package of rocks, made of quartzite and other resistant sedimentary rocks which stand topographically higher than the surrounding lithologies.  If this conglomerate was brought from a western source area, wouldn’t we expect to find pieces of its nearest westward neighbor?

A similar western border conglomerate, the Leesburg Conglomerate is to the north and has quartzite clasts derived from the Chillhowee group, but there are little to none at this location in the Waterfall Conglomerate.

2. Is that Catoctin?

This conglomerate has beautiful chunks of basalt in it, but where are they from? The relatively unaltered nature of the basalt clasts strikes me as odd.  Sure, I haven’t seen all of the Catoctin in existence, but I’ve seen a fair bit. I know from field experience that almost all of the Catoctin around here is really easy to spot as it has the characteristic green color.  I think all of the Catoctin I’ve seen out on the “flanks” of the anticline was green and clearly altered. The only place I’ve seen really black Catoctin, is in the feeder dikes intruding into basement rocks exposed along skyline drive.  So it seems mysterious to me, that if these are in fact Catoctin, how they escaped metamorphic alteration received by everything else in the local source area.

3. What about the red and grey/black finely laminated calcareous siltstone/mudstone clasts?

Here is a closer picture at the original image of the cut sample. Apologies for lack of scale. Ball park, the laminated clast on the bottom left is about as wide as my thumbnail and just a little bit longer.

Here is another sample I collected and cut.

This clast type is very common in this conglomerate accounting for I’d estimate more than 50 percent in this sample.  The problem I find is that if we look westward for a source area, we must go all the way to the Valley and Ridge to find carbonate clasts.  And we need to go deep into the Valley and Ridge, the first couple units adjacent to the Blue Ridge like the Tomstown Dolomite and the Beakmantown are both dolomitic and in outcrop look nothing like these clasts in the conglomerate.  Although I have less personal experience in the Valley and Ridge than in the Blue Ridge, I don’t know if I’ve ever seen anything in outcrop in either province that looked like these finely laminated clasts.

Hmmmmm?

Perhaps this is a matter of an inconsistency between my own current regional picture and the actual regional picture during the deposition of this conglomerate… A lot has changed since the Jurassic Period. There is also a lot I still don’t fully understand about the sequence of events in East Coast Geology. I am not yet an expert at mentally reconstructing ancient environments at a given point in geologic time and then pressing the “play” button to watch things unfold. I work with more or less a growing set of snapshots, that continue to build the larger narrative for me.

That being said, I still think there is something odd about these clasts.

Maybe the source area is not from the west, but instead from the east. Or I should say rather, intrabasinal instead of extrabasinal.  The red and black/grey laminated clasts seem to resemble the fine-grained siliclastics described lower in the strat column. With in the Waterfall formation there is laminated lacustrine sediments (Gore, 1988). These lacustrine sequences record the history of a 40 million year lake during the early Jurassic Period, which was located on or adjacent to, the western boundary of the Culpeper Basin (Hentz 1985). I think that some of these clasts are intrabasinal lacustrine sediments which were reworked and deposited during periods of shoreline fluctuations. If these are who I think they are, then the tiny laminations record cyclical changes in sedimentation in the ancient lake.

An intrabasinal source area would also explain the presence of the unaltered black basalt. In the western Culpeper Basin in the Buckland Formation there are several basalt flows which seem to have been a part of the source area for this rock (Lindholm 1979).

This could also explain the identity of the strange mystery clasts. Remember these?(The pink and off white clasts in the upper right corner)

I think these could be related to the lacustrine calcareous fine siliclastics and basalt occurring right next to each other.  Perhaps these could be thermally altered calcareous lime mud. They fizz just like everything else and have a strange crystalline type structure.  This is sort of a leap for me, and pushes me right to the limit of my geologic understanding on the topic.  I don’t really know what these are and cant find anything in my research that really nails it down. If anyone with more experience in this area has any ideas I’d be very interested to hear them.  With out thin sections it is hard for me to say.

Here are a few more pictures of the mystery clasts.

As an undergraduate geology student and enthusiast of local geology, I can say that this rock unit did more for my understanding of regional geology (especially the Mesozoic) than almost any other single unit. For several reasons. 1 This rock is a conglomerate on the border of 2 geophysical regions in Virginia and potentially contains pieces of both of them. So I learned a lot about a broad geographic and geologic range.  And 2) Because (most) sedimentary rocks are made from the broken down and reworked material of other rocks, I think there is a natural mystery that begs to be solved in each one.  I’m still not all the way sure about everything in this rock, but I’ve learned a lot in the process that has led me here.

References

Gore, P.J.W.(1988) Lacustrine sequences in an early Mesozoic rift basin: Culpeper Basin, Virginia, USA. Geological Society of London Special Publications. 40,247-278

Gore, P.J.W. (1988) Toward a Model for Open and Closed Basin Deposition in Ancient Lacustrine Sequences: The Newark Supergroup (Triassic-Jurassic), Eastern North America. Paleogeography,Paleoclimatology,Paleoecology, 70, 29-51

Hentz, T.F. (1985). Early Jurassic sedimentation of a rift-valley lake : Culpeper Basin, Virginia. Bulletin of the Geological Society of America,96,92-107

Lindholm, R.C. (1979). Geologic history and stratigraphy of the Triassic-Jurassic CCulpeper Basin, Virginia. Buttetin of the Geological Society of America,90,995-997, 1702-1736

Lindholm, R.C. Hazlett, J.M. (1979). Petrology of Triassic-Jurassic Conglomerates in the Culpeper Basin, Virginia. Journal of Sedimentary Petrology, v49, n4, 1245-1262

Over the past two weeks, I’ve been working through a lot of the photos from my summer field camp in Ireland.

For anyone keeping score at home so far I’ve covered:

• The Lakes Marble Formation
• Folded quartzite and the Dalradian Supergroup
• Pillow Basalts
• Columnar Jointing at the Giant’s Causeway

Today I’m featuring some great invertabrate fossils from a Mississippian-aged reef (about 340 million years ago).

Firstly, here is a photo of the field location, on the shore of Lough Mask in County Galway Ireland.  Lots of limestone, heavily jointed with an egg shell weathered topping.

I’ll just get right to the good stuff. Here is a giant colonial coral.

If you zoom in, you can see how the individual corals all radiate outwards, some of them in sort of “wiggly” fashion. This location had a great impact on me, I think because the exposures like the one above look so “alive”.  For myself as a geology student, it has at times been hard to connect with fossil remains of ancient organisms that dont exist anymore.  I dont think it is easy to look at a coral sample in a box and imagine it as a living thing.  Not like looking at the remains of a dog or a bird or a fish.  This reef really set some of these creatures to life for me. You can see how this was a biologically diverse and rich environment.

Here is another little colonial community

These are Siphonodendron, a colonial rugose coral that lived in warm shallow marine waters during the Carboniferous.  They were common in shallow seas beginning in the Ordovician and disappear by the Permian.

This next one is a different type of coral, a solitary rugose coral, Siphonophyllia.  And this is definitely the biggest rugose I have ever seen.  They are also called horn coral because they resemble the shape of a bull or sheep horn.  This one is like a Texas Longhorn coral.

Here is a nice cross sectional view of one you can see the internal tabulae structure.

This next one is not is big but is still pretty spectacular, I think.

This next one I’m pretty sure is a bryozoan of some sort.

The rock unit which these fossils are in is from the Mississippian or what the Irish would call the Carboniferous, the Visean period of the Carboniferous to be more specific.  During this time Ireland was positioned near the tropical latitudes and this location would have been a marine shelf environment.

Here is where I found some useful info on Irish Fossils.  After looking through some of the info there, I think I might have just enough to earn my Irish fossils geology merit badge last summer. I think I found nearly all of them.

So of course I wasn’t going to write about primary igneous structures and Ireland with out covering the Giant’s Causeway.

This is probably Ireland’s most famous geologic feature and a place I’ve wanted to visit ever since I was just a wee intro student.

These columns, located in Northern Ireland, are part of a cenozoic aged tholeittic basalt unit which erupted on the surface about 60 million years ago.

Photo by Aaron B.

Here are some of my fellow field students, probably discussing something like how they were going to pull off recreating a man-sized version of the famous Led Zeppelin shot from the album cover to Houses of the Holy. ( no, really)

These columns form as the basalt cools and contracts creating the hexagonal shape. I wrote about some columnar basalt in the Blue Ridge Province of Virginia last spring, which are much older. But of course they really cant compare to the Giant’s Causeway.

Most of these have 5 or 6 sides, although I have been told there are 7 or even 8-siders to be found.  I spotted only a few 7-siders, but nothing greater.

Like this one.

And this one.

Here is a view of the columns from the side along with myself proudly displaying my shirt from Citizen Chain, my good friends and the very best bike shop in San Francisco, maybe even the world.

Another student, making his way across the columns.

Here is another view of a column that is pretty wavy.

Maybe this is something that can be explained by weathering, or maybe is the result of a varied cooling front.

For more on cooling fronts and such, here is a link to my former structural geology professor Callan Bentley’s great post on cooling fronts and column formation.

Here is one more shot of Ireland’s iconic igneous structures.

Photo by Aaron B.

From the peak of Bencorragh.

Over the previous two posts I discussed a few members of Western Ireland’s Neoproterozic age Dalradian Super Group, the Lakes Marble and the Bennabeola quartzite, all of which I encountered during my 6 week field course in Ireland.

Today I’m moving onward and upward in the geologic column, to the base of the Ordovician system and looking at some great pillow basalts in the Bencorragh formation of the Louch Nafooey group.

Sure, I have seen pillows before. However, I think this is the first time I’ve ever seen such a clean cross-section of piled pillows. Notice how the tops are all convex up, shaped like muffin tops, and the bottoms taper to a thin point conforming to the tops below.  Here is a quick annotation to emphasize the point.

Pillow basalts are a primary igneous structure, formed as lava erupts into water which cools the outside of the pillow at a much faster rate than the inside.  They are also a useful “up” structure, providing the up direction at the time of cooling.

These pillows have (apparently) no interstitial sedimentary material in between them, it’s just all basalt.

Here is one that looked good enough for a rest, a picture every geologist’s mother should have.

Here are some more pillows with a huge bowl-shaped Cirque in the background, evidence of Ireland’s glacial history.

One last one to illustrate my boundless love for Ireland….or well, lets say, my sensibly bounded love for Ireland.

Here is a link to a video of pillow basalts in action I found on you tube.  There are lots of these.

And some good reading material on the Lough Nafooey Group.

-Ryan P.D, Floyd P.A. & Archer J.B. (1980). The stratigraphy and petrochemistry of the Lough Nafooey Group (Tremodaocian), western Ireland. Journal of the Geologic Society, London. Vol 137 pp443-458

More from my mapping area in the beautiful Connemara region of Ireland.

A few days ago, I wrote about the Lakes Marble formation, a folded marble and interesting member of Ireland’s Neoproterozoic Dalradian Super Group. Today, I’m continuing the discussion with another member from the DSG and more details about the Connemara metamorphic complex.

Here is once again our celebrated interpreter of local geology, a truly distinguished Irish gentleman, Dr Martin Feely (along with another gentleman who is apparently less confident in his choice of headgear for the day), standing on top of a folded quartzite.

Here is a pretty “blocky” fold I found nearby, with a great example of a rounded hinge and straight limb fold.

Here is what I mean.

And another tighter fold.

This is the Bennabeola formation, a “predominately clean” orthoquartzite containing more than 90% quartz, which was metamorphosed from sandstone deposits. This unit is more resistant to weathering than its local lithological neighbors, so it stands “proud” forming the hills in the core of the Connemara Antiform. I say “predominately clean” because while the description above makes the Bennabeola seem relatively trustworthy and safe, it is anything but.  I can say after several days of mapping this unit, there are enough gradational changes and shifts to make even the most enduring mapper want to break their map-board in two and launch their Brunton into the nearest, deepest bog.  Usually, that was a good time to break for lunch.

We are on the western coast of Ireland in County Galway, looking at the metamorphosed sedimentary rocks of the Dalradian Supergroup. This is to refresh everyone’s memory, or to introduce the concept to those who didn’t read the last post.

Here is once agin the Geologic map of Ireland, simplified from the original produced by the GSI.

In the last post, I discussed how the larger anticlinal structure was super-imposed onto the landscape in the shape of the lakes. Here are some similar large-scale structures seen in the Bennabeola quartzite hills.  This one should be easy to spot.

As a proponent of both Irish geology and improving safety on Irish roadways (if you’ve ever driven in Ireland, you know this is not a joke), I’m happy to report that the Irish Department of Transport found a clever way to warn motorists of hazardous road conditions, while simultaneously drawing their attention to some killer folds in the quartzite hills.  Somehow I feel like these two concepts are at odds (if you are a geologist/geology student and drive a car, you know this is also not a joke).

Though cleverly positioned, it is not entirely correct.  Anyone can clearly see  a “Z” fold showing a dextral sense of shear. I took the liberty of making the correction for them.

All joking aside.

The Bennabeola is stratigraphically below the Lakes Marble and together with a package of other sedimentary rocks form the DSG. This package was laid down on the Laurentian passive margin, after the break up of Rodinia around 600 million years ago.  After deposition, these rocks experienced a series of 4 separate deformation events.  These events started with the Grampian Orogeny (we call the Taconian) in the mid to late Ordovician and ended with the Hercynian Orogeny (we call the Alleghanian) which produced the Connemara Antiform.

Well wait just a minute… that kind of sounds like a Cambrian package of sedimentary rocks deposited on the Laurentian passive margin after the break up of Rodinia I know!   The Chilhowee group from my beloved home region, The Blue Ridge Province in Virginia. While the Chilhowee was less impacted (or not at all) by the Taconian Orogeny (what they call the Grampian), was majorly impacted by the Alleghanian, which formed the anticlinal structure and westward thrusting which defines the Blue Ridge today. Not that these two rock units are the exact same, because they are not, but they share a similar story. Like cousins with a similar upbringing on opposite sides of town, these two formations are alike in maybe more ways than they are different.

This was a serious “ah ha” moment for me personally, relating rocks of my home region to global geologic processes.

Ah HA !

Making the connection.

Image from University of Michigan school of Public Health.

So here it goes. Back to school, back to blogging.

It was just about the very best summer a geology student like myself could hope for. It all began with geology field camp, where geology undergrads go to complete their geologic training and take place in the sacred art and practice of field mapping.  I was lucky enough to attend field camp in Ireland’s beautiful county Galway.

Among some of the strange and wonderful things I saw at camp were:

•  Charismatic metasedimentary rocks of the Dalradian Super Group,
•  All sorts of sweet glacial features.
•  Some amazing igneous structures such as pillows and some pretty famous columnar basalt.
• The site of the “last stand of the neptunists”, which in terms of last stands was no less thrilling than Thermopylae, The Alamo or Little Bighorn.
•  World-class Carboniferous marine sedimentary successions in the Clare Basin.
• Sheep in numbers best quantified with scientific notation.

I also learned all about safely navigating across expansive man-eating bogs and turf harvesting. I learned how a pint of Guinness can be used as a semi-accurate analogy for practically any geologic concept. And I felt the painful, fiery sting of Ireland’s most serious natural threat to a field mapper, the dreaded Stinging Nettle ( as there are no snakes, bears, or poison ivy).

All in all it was a great experience and now that I’m back home I’m planning on synthesizing all the information and  blogging a bit about all the things I saw this summer (with little breaks to write about Virginia geology here and there).

I will start at the beginning of the trip and hope to move through my  stockpile of geology photos more or less as I experienced the geology of the Emerald Isle. Today, I’m featuring some of the most spectacular folded marbles I have ever seen.

Ladies and gentleman, boys and girls, friends of all ages, I give you…  The Lakes Marble Formation.

Photo by Lori W.

Located in the Cur region of Connemara, this marble is part of a Neoproterozoic metasedimentary package of rocks, the Dalradian Super Group which have been folded into an anticlinal structure and mineralogically altered from several stages of mountain building.  The white bands are almost pure calcite and the darker bands contain detrital quartz and other siliclastics, zooming in on one of the pictures will provide a better look and  you can see how the black bands stand “proud” as they are more resistant to weathering.

Here is a geologic map of Ireland, simplified from the official map produced by the GSI that can be found here.  I modified this image to highlight the area of interest. Right away one can see the highlighted area is just about the most complex area on the map, or at least one showing a great deal of lithological variation.

Here is a closer look at the highlighted region. I drew in a green line to show the antiformal shape of the rock units.

The trace of this antiform can be seen in the terrain as well. Here is a google image of the region.

The chain of lakes that arc eastward of Connemara National park basically provide an outline of the Connemara Antiform.  There are several other geologically interesting aspects of this image that I’m intentionally not mentioning as I hope to discuss them in a future blog post.

Here is another green dashed line to drive the point home.

The amazing Dr Martin Feely, our guide to Irish regional geology, hailing from The National University of Ireland, Galway, discussing the mineralogy of the marvelously folded marble. 

This last photo is of the surrounding area, visible are the more resistant quartzite ridges which make up the core of the antiform ( more on that later) and some of my favorite friends from Ireland.  

Some references on the topic:

• Geoffrey and Shackleton.Structure and stratigraphy of the Dalradian rocks of the Bennabeola area, Connemara, Eire. Geological Society,London, Special Publications 1979, Vol. 8, pp 243-256
• Leake. The geology of SW Connemara, Ireland: a fold and thrust Dalradian and metagabbroic-gneiss complex. Journal of the Geological Society, London,1986. Vol.143, pp 221-236