The geological setting of Fort Necessity, Pennsylvania

October 11th, 2014

Great Meadows 101114On July 3, 1754, colonial lieutenant Colonel George Washington fought and lost a small battle on this site in southwestern Pennsylvania. He and his 400 men had built this makeshift fort about a month before in anticipation of an attack by several hundred French soldiers and their Indian allies. The French were incensed at Washington and his troops after they killed or captured most of a French party at the Battle of Jumonville Glen two months before. (Accounts vary as to who was at fault for that deadly encounter as France and Britain were not at war.) The Battle of Fort Necessity was just one day long, and the British under Washington had the worst of it. Washington accepted French surrender terms and he and his men were allowed to march home. This pair of skirmishes between the French and British started the French and Indian War,  known outside of the USA as the Seven Years’ War. It quickly became a global fight between empires; in many ways it was the first modern world war. And it all started in this lonely part of the Pennsylvania country.

Washington chose to place his ill-fated fort, a reconstruction of which is shown above, in a high grassy spot known as the Great Meadows. It is situated near two passes in the Allegheny Mountains, and thus sits strategically beside major trails. Washington liked the area because there was plenty of feed for his pack animals and horses, lots of available water (too much, it turned out), and it was not in the midst of the endless woods of the region.
Screen Shot 2014-10-12 at 5.19.55 PMThis geological map of the area (from the National Park Service) shows that the fort was situated on the Upper Carboniferous Glenshaw Formation. This unit has much clay, trapping water in the thin soil above (“Philo Loam“). Further, the area is a floodplain, thus making the area a kind of wetland with grasses and sedges. Great for horse grazing, not so good for walls, buildings or trenches.
Entrenchments 101114Here we see the shallow entrenchments made by Washington and his men as they awaited attack. The clayey soil and pouring rain made a mess of these boggy trenches.

Fort inside 101114Inside the fort was a simple square building used mainly to keep supplies and wounded men dry, During the battle it was partially flooded with rainwater.

British view 101114This is the British view from the fort of the surrounding woods. Washington miscalculated his placing of the fort because the French and Indians could easily hit it with musket shots while hiding among the trees.

French Indian view 101114The French and Indian view of the hapless fort. It was easy to rain bullets on the British from the woods with little fear of the return fire.

Braddock road trace 101114Nearby is a trace of the military road Washington’s unit had blazed through the Pennsylvania woods on their way to the French Fort Duquesne in what is now Pittsburgh. The British General Edward Braddock enlarged this road the next year in his famous march to a spectacular defeat nearby (the “Battle of Monongahela“).

We can’t fault Washington too much for his choice of a fort location. He did not have the resources to clear a large patch of forest, so the meadow would have to do. He expected to be reinforced soon, so he saw the fort as a temporary measure of protection. The rain was beyond his control that July day, and the clay-rich meadow floor ensured wet misery and ruined supplies. The French surprisingly gave good terms for surrender because they were wet, too, in those woods, and they also expected more British and colonial troops would arrive soon. They feared being surrounded, and so thought their message to Washington and his countrymen had been sufficiently made. How different our world would be if the French were not so generous here in southwestern Pennsylvania!

Additional Reference:

Thornberry-Ehrlich, T. 2009. Fort Necessity National Battlefield Geologic Resources Inventory Report. Natural Resource Report NPS/NRPC/GRD/NRR—2009/082. National Park Service, Denver, Colorado.

Wooster Geologist at Valley Forge, Pennsylvania

March 20th, 2013

ValleyForgeHuts032013BRYN MAWR, PENNSYLVANIA–While visiting my friends and colleagues Katherine and Pedro Marenco at Bryn Mawr College, I visited the nearby Valley Forge National Historical Park. Everyone will remember, of course that this is the place outside Philadelphia that the Continental Army made its rough winter quarters in 1777-1778. The huts above are reconstructions of the soldiers’ quarters on the windy and cold fields. Commander-in-Chief George Washington chose this place because it was easily defensible, had plenty of timber for construction and fuel, and was close enough to British-occupied Philadelphia to keep an eye on the enemy — yet not so close to be likely attacked.


As a geologist, of course, I also looked for the rocky bones beneath the landscape. They were easily found in the above cliff near the main parking area. This is the Ledger Dolomite, a Cambrian unit found throughout this part of eastern Pennsylvania.

LedgerDolomite032013The Ledger Dolomite here is distinguished by these fine laminations visible on its weathered cross-sections. These are apparently stromatolites: laminar structures built by bacterial mats. We’ve met Cambrian stromatolites before in this blog.

Smilodon_gracilisI was surprised to learn that there is also a significant middle Pleistocene fossil deposit in Valley Forge called the Port Kennedy Bone Cave. This is a sinkhole deposit within the Ledger Dolomite. A particularly large sinkhole apparently trapped a variety of animals, including the gracile sabre-tooth Smilodon gracilis, the skull of which is on display in the Valley Forge Historical Park visitor center. S. gracilis was the smallest and earliest member of its genus. The Port Kennedy Bone Cave was one of the first fossil assemblages that the famous paleontologist Edward Drinker Cope studied. The location was lost to science until its rediscovery in 2005.

ValleyForgeCannon032013This is the requisite cannon image, even though no battle was fought here. It is nevertheless a dramatic place for the privations the soldiers suffered during the darkest days of the Revolutionary War. It is hard to imagine the conditions in 1777-1778 now since highways and casinos surround the old encampment.

Wooster Geologist at Fort Ligonier, Pennsylvania: Choosing your ground geologically

June 5th, 2012

Fort Ligonier was built by the British in 1758 during the French and Indian War (or Seven Years’ War) along the Loyalhanna River in what is now Westmoreland County of southwestern Pennsylvania. It is a spectacular site today with a fully reconstructed fortification and an excellent museum. It gives us a chance to see how a military engineer used the local geology to build a successful fort in a difficult terrain.
The purpose of Fort Ligonier was to serve as the forward base for the capture of the French Fort Duquesne at the forks of the Ohio River. This was the most strategic site on the western frontier. The French and their Indian allies desperately wanted to preempt this attack by destroying the advancing British columns in the woods before they could assemble. The British and American colonists needed a robust road through the wilderness approaching Fort Duquesne, along with defensible strongholds. Fort Ligonier was the most critical of these positions, then, for both sides.
You would expect a fort to be built on the highest ground, yet Fort Ligonier is in a valley surrounded by commanding heights. The British knew, though, that the French and Indians did not have significant artillery in this theater. They could give up the heights so that they could use the Loyalhanna River as a defensible barrier against the inevitable infantry attacks. The site of Fort Ligonier also has small ravines on its other sides, forming a kind of moat. Most importantly, sandstone cliffs on the river side provide an unbreachable wall and an overview of the most likely approaches to the fort by the enemy. The British placed their largest cannon at the top of this cliff, surrounding them with an elaborate wooden stockade and sharpened obstacles.
The exposed rock of the Fort Ligonier cliffs is the Casselman Formation, a Late Carboniferous (about 300 million years old) mixture of shale, siltstone, sandstone and occasional coal beds. The particular unit here is a fine micaceous sandstone with cross-bedding. It was formed in an ancient river system. The cross-bedding and abundance of mica is a clue to this environment: the cross-bedding shows high-energy seasonal flooding; the mica flakes (the white grains seen below) show ebbs in water energy to near zero.
The French and Indians attacked Fort Ligonier on October 12, 1758, and very nearly took it. The British artillery sited on the sandstone cliffs was the deciding factor, though, and the besiegers retreated. Fort Ligonier swelled in population as British troops assembled for the attack on Fort Duquesne. In fact, in November 1758 it was the second largest city in Pennsylvania! (Among the British forces was the young George Washington.) The French saw the score and retreated from Fort Duquesne. The British captured this most strategic location and renamed the site “Pittsburgh”. Building and defending Fort Ligonier was key to this victory. By March 1766 the fort had served its purpose and was decommissioned.


Fowler, W.M., Jr. 2005. Empires at War: The French and Indian War and the Struggle for North America, 1754–1763. Walker & Company, 360 pages.

Sipe, H.C. 1971. Fort Ligonier and Its Times. Ayer Company Publishers, 699 pages.

Stotz, C.M. 2005. Outposts of the War for Empire: The French and English in Western Pennsylvania: Their Armies, Their Forts, Their People, 1749-1764. University of Pittsburgh Press, 260 pages.

Glacial Features in Western Pennsylvania

March 22nd, 2012

SLIPPERY ROCK, PA – I’ve just returned from the March meeting of the Pittsburgh Geological Society. If you’re in the region and you’re not a member, you really should think about joining. On the third Wednesday of each month, a robust group of faculty, students, and professionals gather for a social hour followed by a tasty meal and a geo-talk. I was honored to have the opportunity to speak about my passion for basalt geochemistry and was fully impressed by the friendly, engaged audience. The questions from the students, in particular, were clever and insightful. (Attention potential employers: the PGS is your regional source for the next crop of professional geologists).

The PGS also runs field trips, and my host, Dr. Patrick Burkhart, took me on a tour of glacial features near his home institution of Slippery Rock University.

A google map image of the glacial features near Slippery Rock, PA. The numbers correspond to the images below. (Imagery from DigitalGlobe, GeoEye, USGS, USDA Farm Service Agency).

We began our trip at the Jacksville Esker (also known locally as the West Liberty Hogback or Miller Esker), which may be the best-preserved esker in Pennsylvania (Fleeger et al., 2003). The >6 mile-long esker is an elongate, sinuous ridge of sand and gravel that was deposited about 23,000 years ago by meltwater in a subglacial tunnel (Fleeger and Lewis-Miller, 2009).

Photo #1: View of the elongated ridge formed by the Jacksville Esker.

Adjacent to Jacksville Esker is Tamarack Lake, an ecologically important wetland. We viewed Tamarack Lake from Swope Road, which cuts through the esker, and noticed a school of trout in the water near the road. I imagined that they were enjoying the warm spring day.

Photo #2: View of Tamarack Lake from Swope Road.

Photo #3: Trout enjoying the warm water at Tamarack Lake.

Jacksville Esker ends in the Kame Delta, which formed in a proglacial lake. Sediment in the delta is generally well-sorted, ranging from fine sand to cobbles, and is currently being mined by the Glacial Sand and Gravel Company (Fleeger and Lewis-Miller, 2009). We didn’t get to tour the quarry, but according to the field guide, mining has exposed  crossbeds, ripples, and multiple foreset beds that suggest that the delta was built by a series of depositional events (Fleeger and Lewis-Miller, 2009).

Photo #4: View of the relatively flat-topped Kame Delta. The gravel mining operation is located near the center of the photo.

We saw a few other things on the trip, but I think I’ll save those for another post.


Fleeger, G. M., and Lewis-Miller, Jocelyn, 2009, Stop 7: Jacksville esker, delta, lake plain, and drainage diversion complex, in Harper, J. A., ed., History and geology of the oil regions of northwestern Pennsylvania. Guidebook, 74th Annual Field Conference of Pennsylvania Geologist, Titusville, PA. p. 146-159.

Fleeger, G. M., Bushnell, K. O., and Watson, D. W., 2003, Moraine and Mc- Connells Mill State Parks, Butler and Lawrence Counties—Glacial lakes and drainage changes: Pennsylvania Geological Survey, 4th ser., Park Guide 4, 12 p.

PA Geological Survey Field Trip

May 27th, 2010

Sorry to have kept you waiting so long for the ending of the PA diabase field trip. Last Friday, we spent a wonderful day in the field with a group from the PA State Geological Survey.

Our first stop was the Pennsylvania Granite Quarry.

Dr. LeeAnn Srogi was an excellent host. Here she is describing the orientation of the Morgantown Sheet on the geologic map.

The PA geologists had the opportunity to examine the plagioclase layers and cross-cutting dark channels up close.

They even had a chance to see the big saw in action. (The PA Granite quarry guys are so good to us).

After a good laugh (oh, those geologists and their humor!) and a nice lunch in a local park, we headed to the Dyer quarry.

Here we're discussing the fault patterns in the Dyer quarry. The wonderful thing about being in the field with a dozen other geologists is that the discussions are invigorating. We are so fortunate that these professionals took the time to visit our field area and add their observations and ideas to our own.

After a week in the field, I have a notebook full of observations, a head full of ideas, and a trunk full of samples! Sounds like a good week to me.

A Short Update from the Diabase Quarries

May 20th, 2010

We have an early morning tomorrow, since we’re running our NE-SE GSA field trip for the Pennsylvania Geological Survey. So, this post will be short and sweet. Here are a few of the highlights from today’s diabase escapades:

One of Betty Lou's cores that we cut for geochemistry and thin sections.

More awesome slickenfibers.

Contact between diabase and a felsic layer.

Quarry workers drilling holes to set up for the next blast.

My New Best Friend, Betty Lou

May 19th, 2010

The weather was beautiful today – slightly overcast, brisk, perfect field weather!

We began at an outcrop along the side of a road, where we debated about the origin of these coarse-grained layers in the diabase. The alternating bands of coarse- and fine-grained diabase seemed so regular that we even measured a section. (How unconventional for a petrologist!)

We spent most of the day sampling at the quarry. I could just imagine myself standing in the middle of a sill, surrounded by molten magma and crystal mush!

The beautiful textures in the wall are from layering of plagioclase and pyroxene in the diabase. This wall is about 15 feet tall. The quarry foreman told us that this stone is highly prized. They ship most of it to Italy, and it costs ~$4000 to make a countertop out of it.

Here's a closer view of the layers in a random cut block.

But the best part of the day was that we got to meet Betty Lou. Betty Lou is a Milwaukee Manta-III core drill, my new best friend! She came with Loretta from Lock Haven University.

Loretta and Tim (West Chester University) use Betty Lou to drill a core through the top of the diabase sheet.

Here's a close up of Betty Lou at work.

And here’s a video of Betty Lou in action.

Loretta shows off one of the many cores that we collected today.

The quarry workers were extremely friendly and accommodating, helping us in every way possible. Here’s a video of how they relocated the generator for us so that we could power Betty Lou on one of the lower levels.

I even managed to snap a photo of something my colleague, Dr. Judge, would have enjoyed: slickenfibers!

It’s hard to believe that I only have one more day of sample collecting and processing! We’ll be visiting another road cut and quarry tomorrow, then it’s off to the lab to slab the samples for geochem.

Planning for a Day in the Field

May 19th, 2010

I officially started my summer field work today! Unfortunately, here in West Chester, PA, it rained all day. That didn’t keep us from being productive, though. Lee Ann and Tim (from West Chester University) and Loretta (from Lock Haven University) and I began the day by visiting the quarries that we’ll be working in tomorrow. We spent the afternoon developing a plan for the rest of the week. It goes a little something like this:

The Plan

This really isn't the plan, but it is a map of all of the sites that we're interested in.

Lee Ann set out all of her samples from these locations and let us play in the lab all afternoon. We immersed ourselves in diabase, so much so that we nearly forget to break for dinner!

Diabase samples

A table topped with diabase hand samples, thin sections, maps, and chemical data = heaven.

Diabase thin section

Action shot of Lee Ann adjusting the microfiche viewer to get the best image of a diabase thinsection.

You would think that a day of discussion would clear things up, but I’m more overwhelmed than ever! I realize that I have so much to learn about the emplacement and evolution of these rift-basin dikes and sills. I typically think of these large intrusions as composite structures, formed by multiple pulses of magma, but I wonder if I’ll be able to recognize evidence of this in the field? How did the complex plagioclase-pyroxene layers form, and why are they different in different parts of the sill? And what are the mafic channels that cut across the plagioclase-pyroxene layers? Fortunately, I have wonderful and experienced (not to mention patient) colleagues who are seeking answers to the same questions. Some insights, I hope, will be gained by whole-rock geochemistry (as long as my sampling strategy works). Whew! Are all new projects as exciting as this?

Mineralogy-Structure Field Trip to Pennsylvania

September 15th, 2009

Last weekend, Dr. Judge’s and Dr. Pollock’s Structure and Mineralogy classes took a field trip to central Pennsylvania. It rained on Saturday, but that didn’t stop us from having a great time. We saw the most amazing pencil structures in the Reedsville Shale.

The intersection of cleavage and bedding create "pencils."

The intersection of cleavage and bedding in the Reedsville Shale creates "pencils."

In the Bald Eagle Formation just a short drive down the road, we found textbook examples of slickenfibres. Slickenfibres are elongated minerals that grow along a fault plane parallel to the direction of motion.

Colin Mennett, Dr. Shelley Judge, Megan Innis, Becky Alcorn, and Andrew Retzler excited about slicken-fibers along a fault surface in the Bald Eagle Formation.

Colin Mennett, Dr. Shelley Judge, Megan Innis, Becky Alcorn, and Andrew Retzler are excited about slickenfibres along a fault surface in the Bald Eagle Formation.

Close-up view of the slicken-fibres.

Close-up view of the slicken-fibres.

Next, we went to the Bear Valley Strip Mine. The beautifully exposed folds and giant iron concretions are simply breathtaking!

Bear Valley Strip Mine. Notice the person in the yellow jacket (center of the picture) for scale.

Bear Valley Strip Mine. Notice the person in the black jacket (center of the picture) for scale.

Finally, we stopped to sample some (very orange) acid mine drainage at a nearby pump slope.

Palmer Shonk and Becky Alcorn standing beside a river full of "yellow boy," an iron hydroxide phase that is precipitating from acid mine drainage.

Palmer Shonk and Becky Alcorn standing beside a river full of "yellow boy," an iron hydroxide phase that is precipitating from acid mine drainage.

Become a fan of the College of Wooster Geology Department page on Facebook and see more photos from this trip!

Tree Ring Dating of Revolutionary War (and some later) Buildings in PA and OH

June 7th, 2009

crew3This summer the Wooster Tree Ring Lab is funded by the College of Wooster’s Center for Entrepreneurship to date buildings of historical interest using tree rings. The crew in this endeavor is shown above and includes – Dr. Greg Wiles, students Kelly Aughenbaugh and Colin Mennett, and Nick Weisenberg who is a timber-framer and works in building restoration. The demand for our service has been excellent and is concentrated in the Pittsburgh area, locally in Kidron, Ohio and now in the Cincinnati area. We can use our archive of tree-ring data and lots of hard work by our staff to tell homeowners, folks at historical sites, historical archaeologists etc. the year timber was cut prior to building the structure. The tools of the trade are shown below. We will tell you a bit about the results of our work soon.

Below: The gear pile includes standard powertools and generators along with specialized dry wood bits. Nick illustrates the proper coring technique. This historic building is Woodville PA – the oak beams that he is coring were cut in the fall or winter of 1785.