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.

References:

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.

Nabataean water management in the northern Negev (circa 2nd Century BCE)

March 18th, 2012

MITZPE RAMON, ISRAEL–We had an earlier post about water management techniques by Iron Age peoples in the northern Negev. Today during our last period of fieldwork on this trip we ran into a complex Nabataean system in a valley a few kilometers north of Mitzpe Ramon. Nabataeans were an Arab people based in Jordan who spread in influence and settlement through this region from roughly the third century BCE to the third century CE. They are most remembered here for their water systems to support their small villages. The infrastructure they built is still used in many places by the Bedouin.

Today while exploring more Upper Cretaceous sites, we came across the cistern pictured at the top of this entry. It is a Nabataean structure because it is cut into solid rock (the Iron Age equivalents were mostly in clays) and it had a roof held up by the central pillar and interior walls. There are also steps cut into the rock for climbing in and out. The Nabataeans inherited the earlier Iron Age technology and improved on it by better water retention in the container, and reduced evaporative loss.

The cistern we just saw is pictured here from a distance. It is indicated by the tailings of rock debris produced in its construction. On the left hand side you can see a diagonal line of rock indicating part of the water catchment system. There is a similar line on the right, but it is very hard to see.

This even more distant image shows the cistern again as a cone of tailings in the upper left. The valley below is where the irrigated fields were. They are a bit complicated by a series of trenches dug across them recently. (This is an Israeli Army training ground.)

The low rock wall here held in soil for an irrigated field on the left side. The soil has been modified by the original farmers, who built it up with water-holding loess deposits. Some of these fields are still in occasional use by Bedouin who plant wheat in the ancient ground.

Iron Age water management in the northern Negev

March 16th, 2012

MITZPE RAMON, ISRAEL–This region has a very deep human history, and some of it is evident in subtle changes to the landscape itself. Throughout the northern Negev are simple stone structures that are sometimes called “Davidic forts” after King David of Israel. They are, though, a lot more mysterious and difficult to date. They are usually associated with cisterns and water systems, and so they may have indeed been guardposts of some kind. Who exactly made these buildings and the water infrastructure is unclear. All we can say is that they are Iron Age and show an early agricultural people who had skills in collecting and managing the scarce water resources of this area. We saw evidence of them today in the field north of Mitzpe Ramon. Above you see Yoav and Melissa walking by one of these open cisterns cut into Upper Cretaceous limestone (the Vroman Bank of the Ora Formation) and then dug below in shale and claystone.

The hillsides to the sides and above these cisterns have long ditches lined with slabs of limestone on their downward sides. These were designed to catch runoff water from the slopes above and direct it to the open cistern below. Some of these ditch-and-rock channels stretch for kilometers.

Here a ditch heads to a large cistern, recognizable immediately by the sediment tailings dug out of the hole. This system takes advantage of the heavy and infrequent rains in the northern Negev. Sheetflow and water in small natural channels is captured and sent along the gentle gradient to the cistern below. By keeping the water from flowing too fast these early engineers minimized erosion of their channels.

This is the second cistern we saw today. It is many meters deep and could have held a great deal of water year-round. Bedouin herders today still use some of these cisterns for their flocks.

The later Nabateans elaborated upon these innovations and made roofed cisterns to reduce evaporation (which is 2.5 meters of water per year in Mitzpe Ramon). Sometimes they dug their cisterns into solid limestone rather than shale so they could have a small top opening and large covered container below.

As I write this the wind howls and rare rain is falling on the Negev. These rock systems are still channeling water after 3000 years!

Wooster Geologists in the Wilderness of Zin

March 14th, 2012

MITZPE RAMON–Three times we cross Nahal Zin (or Wadi Zin) on our way to Makhtesh Gadol from Mitzpe Ramon. Nahal Zin is an intermittent stream, meaning it is dry most of the time, but during the rainy season can have a considerable flow, even to the point of flooding. I’ve always seen it bone dry. Nahal Zin is 120 kilometers long with impressive canyons in its upper region and meandering channels in its lower parts. It is the largest wadi that begins in the Negev.
The significance of Nahal Zin is that it is the defining feature of the “Wilderness of Zin” from biblical times. There is still some dispute about its location among biblical enthusiasts, but experts agree that it is essentially the northern portion of the Negev. The passions among the amateurs have much to do with the historicity of the Exodus events. This region was explored by T.E. Lawrence just before he became Lawrence of Arabia.
Google map image of Wadi Zin near Avdat. The red asterisks mark where it crosses Highway 40.

The Wilderness of Zin was on the southern border of Judah and is mentioned several times in the Hebrew Bible. Here are three:

“So they went up, and spied out the land from the wilderness of Zin to Rehob, to the entrance of Hamath.” (Numbers 13:21)

“The children of Israel, even the whole congregation, came into the wilderness of Zin in the first month: and the people abode in Kadesh; and Miriam died there, and was buried there.” (Numbers 20:1)

“The lot for the tribe of the children of Judah according to their families was to the border of Edom, even to the wilderness of Zin southward, at the uttermost part of the south.” (Joshua 15:1)

(Courtesy of Biblos.com)

During our work in the Negev we do not usually see much of biblical relevance, so living and working in the Wilderness of Zin reminds us of just how deep the human history here runs.

Wooster Geologist on the Crampton’s Gap Battlefield in northern Maryland

November 26th, 2011

In September 1862, Union forces under General George B. McClellan pursued General Robert E. Lee‘s Army of Northern Virginia through northwestern Maryland. Lee had invaded Maryland to demoralize the North ahead of the November elections, and to convince Europe that the Confederacy had legs and deserved recognition. A copy of Lee’s orders were lost (famously found by Union soldiers wrapping three cigars), alerting McClellan to his plans. The key to defeating Lee lay in capturing three passageways through South Mountain, one of which is known as Crampton’s Gap (shown above in this Google Earth image).
Crampton’s Gap as viewed from the southern side looking north. There were no structures here during the battle.

South Mountain is a north-south extension of the famous Blue Ridge into Maryland. It is a sharp ridge made of resistant metamorphic rocks, including gneisses, schists and quartzites. The slopes on either side are unusually steep and so passing from east to west over the mountain is best done through “gaps” made by eroding antecedent river systems. Water gaps are deepest and have streams currently flowing through them. (One is made by the Potomac River.) A wind gap was also made by river erosion, but the water was long ago snatched away by stream piracy. Crampton’s Gap (39° 24′ 36″ N, 77° 38′ 24″ W) is a wind gap less than 300 meters wide.

Quartzite exposed in Crampton’s Gap, probably from the Late Precambrian (?) Swift Run Formation Cambrian Antietam Formation (thanks, Callan).

On September 14, 1862, McClellan finally moved on Lee and attacked the three gaps through South Mountain to turn back Lee’s invasion. Crampton’s Gap was the southernmost part of what later became known as the Battle of South Mountain.
Union forces under Major General William B. Franklin, after a long preparation, attacked from the east a much smaller Confederate force at Crampton’s Gap. The Confederates resisted all day, taking advantage of the steep slopes and narrow pass with a battery of cannon. By the end of the day, though, the Union force broke through the Confederate lines, sending the remaining rebels down the western slopes. Strangely, Franklin failed to follow up on his victory, allowing the rebel troops to join Stonewall Jackson to capture the Union garrison and arsenal at Harpers Ferry. The overall battle was a Union victory as it blunted Lee’s invasion, forcing him to stand at Antietam and eventually retreat from Maryland. The resistant rocks of South Mountain protected his army long enough for him to frighten the Northern public, but those ancient wind gaps were his undoing.

How Fossils Saved Civilization: A National Fossil Day Talk

October 20th, 2011

WOOSTER, OHIO — National Fossil Day has now been in place for two years. Curiously enough, two Wooster alumnae, Erica Clites and Eva Lyon, have been critical organizers and promoters of this great event as Paleontology Interns with the National Park Service. It is sponsored by the NPS and the American Geosciences Institute (AGI). They even have an official National Fossil Day song! The College of Wooster is proud to be one of their academic partners on a list we hope will grow with the years.

As part of my contribution to National Fossil Day, I gave a talk to the Geology Club titled, “How Fossils Saved Civilization”. My title was inspired by “How the Irish Saved Civilization“, and like that book my tale had a bit of blarney in it. Nevertheless, I strongly believe that the proper understanding of fossils was one of the keys to the scientific revolutions of the 18th and 19th Centuries. Here’s to the beauty and wonder of fossils!

Wooster Geologists in Sweden

July 11th, 2011

STOCKHOLM, SWEDEN–I had not realized how much water is in the city of Stockholm. Almost a third of the city’s area is water because the center is built on 14 islands connected by bridges and ferries. “The Venice of the North” some call it.

Rachel, Nick and I are here for a very short visit. We’ll spend tomorrow in the Naturhistoriska Riksmuseet (Royal Museum of Natural History), so for now we just explored the neighborhood around our hotel. It is called Östermalm and is one of the older parts of Stockholm.

The Nordiska museet (Nordic Museum) has a glorious array of building stones, all from Sweden. The simple blocks are sandstones and fine-grained metamorphic rocks, and the carved pieces are limestones.

The local bedrock is 1750-1900 million years old, formed during the Svecofennian (Svecokarelian) orogeny. The outcrops I saw, like this example of “living stone” at the base of a building, are metavolcanics (metamorphosed volcanic rocks, usually basalt). Apparently the bedrock of Stockholm is an engineering geologist’s dream because of its stability, moisture repelling capabilities, and uniform strength — great for bridge abutments and subway tunnels.

I spent my Stockholm afternoon in the museums found in an easy walk around our hotel. I was impressed with the Viking runestones on display in the Historiska muséet (History Museum), and I was touched by this one. The runes are translated as: “Una/Unna had this stone raised in memory of her son Eysteinn who died in christening robes. May God help his soul.” They are carved in a glacial granite boulder, the kind of rock we saw scattered across the Estonian western islands. Note the dark xenoliths.

A Day in Tallinn, Estonia

July 10th, 2011

TALLINN, ESTONIA–Like our Wooster Geology colleagues in Iceland, we also have a nearly-final day in a city. Tallinn is the capital of Estonia, the medieval town square of which is shown above. We started here briefly at the airport, and will leave from the same place early  tomorrow morning. The only difference is that we have one more big city to go: Stockholm, Sweden.

Tallinn, or at least a significant settlement in this place, goes back to the 11th Century, and before that there are Bronze Age artifacts. After the Danes conquered it in the 13th Century, it became known as Reval until the Estonian War of Independence in 1918-1920 when the Estonians could finally give it their own name: Tallinn (or Tallinna). It was a member of the Hanseatic League, being an important trade link between northern Europe and Russia. (And so the merchants in the town square are dressed in the Medieval garb of “Old Hansa”.) This year it is a European Capital of Culture. Tallinn does not sit on a major river but takes advantage of Ordovician limestone heights to raise it above the coastal swamps and bogs.

We enjoyed a day off in the city under (as you might have predicted) sunny skies. Tomorrow is another travel day, and then back to work one more time in Stockholm!

The newly renovated Freedom Square in Tallinn. The memorial is for the Estonian War of Independence (1918-1920).

Rachel and Nick can be barely made out here on the other side of Freedom Square.

Suur Strait (Moon Sound, Moonzund)

July 9th, 2011

TALLINN, ESTONIA–The Wooster Geology team in Estonia successfully returned to the Estonian capital city of Tallinn today, which means we crossed by ferry the Suur Strait between the western Estonian islands (notably Muhu) and the Estonian mainland. This is an interesting strip of water with a complicated geological and human history.

There is an Estonian dream of building a bridge or digging a tunnel across the Suur Strait to eliminate the need for the ferry line and more efficiently connect Muhu and Saaremaa to the main part of Estonia. It will not be an easy task (and it is probably too expensive to ever be attempted), but it has led to considerable study of the strait and its oceanographic, biologic and geologic characteristics. The currents are complicated as they move between the Gulf of Riga to the south and the Baltic Sea proper in the north, and it freezes solid in the winter (when it is crossed by a 9 km long ice road). The strait hosts one of the most significant bird migration routes in northern Europe, and the marine fauna and flora here is still poorly surveyed.

The floor of the Suur Strait is highly variable from exposed Silurian limestone bedrock to thick mantles of glacial till. As you can deduce from the Google Earth image, some parts of the strait are very shallow, and the deepest regions are no more than 21 meters of water. Because of isostatic rebound, the region gets shallower about 2 mm per year as the land rises.

Suur Strait as viewed from northeastern Muhu (July 2007).

Historically, the Suur Strait has been the “backdoor” to the Gulf of Riga to the south. Any navy that controls the Baltic wants to keep that backdoor open for itself, but close it to enemies. This was especially the case during World War I when the Imperial German Navy sought to trap elements of the Imperial Russian Baltic Fleet in the Gulf on October 17, 1917, during Operation Albion. Most escaped north through the Suur Strait (known then in English as Moon Sound) following carefully dredged channels lined with mines. One Russian battleship, the Slava, was severely damaged and took on too much water to pass back through the shallow strait and was scuttled.

The Suur Strait was crossed by the Germans in 1941 as they invaded the western Estonian islands (Operation Beowulf), and again by the Russians when they re-invaded in 1944 (Moonzund Landing Operation).

We made it across on one of the car ferries which ply the Suur Strait between Kuivastu and Virtsu. Like the Russian warships of old, it also follows a dredged channel through the shallow and storied waters.

View from our ferry west across the Suur Strait towards Kuivastu on Muhu island. Last ferry ride of the trip!

Reference:

Saaremaa Fixed Link – Report of Preliminary Environmental Impact Assessment – Final Draft – 15.07.2005 (Google this and you can get a thorough report as a pdf.)

A visit to Kaali Crater for our last day on Saaremaa

July 8th, 2011

KURESSAARE, ESTONIA–A dramatic geological site on our last Saaremaa day: the meteorite craters at Kaali. We hiked around the largest crater (shown above) and then visited one of the smaller subsidiary craters nearby (shown below). The main Kaali crater is 110 meters in diameter and about 22 meters deep. The meteorite was between 20 to 80 metric tonnes and was traveling 10-20 km/s. It broke up into pieces 5-10 km above the ground before the multiple impacts. The date of this event is disputed. We have seen ranges in the literature from 4000 to 2700 years ago. Some archaeologists have evidence that an ecological catastrophe followed the impacts with massive wildfires and a drop in crop production for a century. Others think there is a connection between the Kaali event and Baltic mythology. (I think it is a delightful coincidence that the Estonian place name “Kaali” used for this fiery event is coincidentally the same name as the fearsome Hindu goddess.)

To our surprise, the Kaali Museum had a thorough display on the geology of Saaremaa, including this polished cross-section through Nick’s critical Wenlock/Ludlow section.

Our last stop was a virtually abandoned little harbor at Turja on the southeastern coast of the island. It was a nice place for lunch as we contemplated how our fieldwork did not include bears and wolverines (as with our Wooster colleagues in Alaska) or gale-force winds and thick fogs (as experienced by our Iceland friends). We were quite fortunate to gather such excellent geological data with so few such adventures!

Tomorrow we drive to Tallinn to spend a day and a half, and then to Stockholm for a day in the Natural History Museum looking at comparative Silurian material. On Wednesday of next week we fly home.

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