Archive for May, 2010

The Battle of Vicksburg and Geology

May 29th, 2010

Union cannon in original positions for the Siege of Vicksburg, Mississippi (1863).

VICKSBURG, MISSISSIPPI — The Wooster Geologists southern USA team spent the better part of the day at the site of the Civil War Battle of Vicksburg (May 18-July 4, 1863).  As is the case with virtually every battle, the local geology played a prominent role here.  Union forces wanted complete control of the Mississippi River to maintain communications with the northwest, to split the Confederacy into two large parcels, and to deny the South the use of the river for transport.  Vicksburg held the key, as President Lincoln said, to the Mississippi and maybe the success of the Union war strategy.  General U.S. Grant had an innovative (and expensive) plan to attack the fortress city from the land side to the east.  To do that he faced a series of fortified bluffs which protected the city’s flanks.  Several direct Union assaults on these bluffs failed, so a long siege of Vicksburg began until it surrendered for want of supplies and low morale.

Part of the battlefield in the bluffs just east of Vicksburg. Looking from the Union lines to the Confederate positions.

The immediate geological issues are derived from the Mississippi River and its ancestor.  At the end of the Pleistocene the glacial meltwaters flowing south through this area were tremendous, producing a vast braided stream complex.  Sediment from these channels was picked up by the wind and deposited in parts of the Mississippi Valley as thick layers of loess.  Loess is an unusual sediment because it is highly uniform in composition (silt-size subangular particles and clays) and it has a very high angle of repose (meaning it erodes into very steep slopes — cliffs, really.)  As the later Mississippi River meandered through its valley, it cut a series of bluffs at its easternmost extent at Vicksburg.  The city thus has a port on the river surrounded by high bluffs well suited for artillery to protect all approaches.

A loess cliff exposed on the side of a bluff in the city of Vicksburg.

Since loess sticks together so well, it is useful for digging entrenchments and caves for protection from artillery and rifle fire.  Many people in Vicksburg lived in loess caves during the siege to protect themselves from Union cannon fire.

Cannon on the Union side aimed towards a Confederate position in the Vicksburg bluffs.

Cannon on the Union line aimed towards a Confederate position in the Vicksburg bluffs.

We can’t say that geology controlled the Battle of Vicksburg — there are numerous and decisive factors of human courage, persistence and innovation — but we can conclude that both sides had to adapt to the geological circumstances in both military and civil ways.

Vicksburg National Cemetery. We never want to forget the cost of war. That the ages of most of the soldiers etched on the tombstones is that of present college students is especially poignant to us.

Since this is our last post from the Alabama and Mississippi summer 2010 geological team, we would like to thank our excellent guides Jon Bryan (Northwest Florida State College), Peter Harries (University of South Florida), and most especially George Phillips of the Mississippi Museum of Natural Science.  George spent days with us, giving us access to sites and people we could never have dreamed of meeting on our own.  Even more important, he is an excellent paleontologist with encyclopedic knowledge of Mississippi fossils, both invertebrate and vertebrate.  Through the generosity of George and many others, we have material for many future Cretaceous-Tertiary paleontological projects.  This trip has been an excellent example of the collaborative nature of geology.

A paleontological meeting at the Owl Creek Formation

May 28th, 2010

RIPLEY, MISSISSIPPI — On our last full field day we met a team from the American Museum of Natural History (led by paleontologist Neil Landman) and converged on the famous Late Cretaceous Owl Creek Formation exposures near Ripley in northern Mississippi.  This site has been studied since 1810 and has produced extraordinary fossils, especially ammonites with pearly layers of aragonite still preserved in their shells.  As fun as the geology was, it was even more entertaining to see the mix of southern and New York accents and mannerisms on the outcrop!

The gray unit in the bottom half of the cliff is the Owl Creek Formation (Late Cretaceous); the brown and orange sands above are the Clayton Formation (Lower Tertiary). Yet another example of the K/T boundary on this trip.

A mix of geologists from England, Ohio, Michigan, Mississippi, Kansas and New York at the Owl Creek Formation section near Ripley, Mississippi.

A more recent history

May 28th, 2010

BALDWYN, MISSISSIPPI — When possible on these geological field trips we explore the local culture and history of the region in which we are temporary guests.  This morning we visited the small Civil War battlefield of Brice’s Crossroads (June 10, 1864) in Lee County, Mississippi.  It lies between our field sites at Blue Springs in the south and Owl Creek to the north.  The center of the battlefield is marked by two cannon and a stone monument which memorializes both the Union and Confederate dead.  A Confederate cemetery is nearby.

At the time of the battle, Union commander General William Tecumseh Sherman was conducting his famous March to the Sea through Georgia and other southern states.  (One of his soldiers was Corporal Julian Adolphus Wilson of the 57th Illinois Infantry — my grandfather’s grandfather.)  Confederate General Nathan Bedford Forrest and his cavalry threatened Sherman’s supply lines, so Union General Samuel Sturgis was sent into northern Mississippi to stop him.  With superior tactics, Forrest decisively defeated Sturgis at Brice’s Crossroads, forcing a long retreat.  It was a rare Confederate victory in that time and place, but Forrest was ultimately distracted from his goal of cutting Sherman’s communications.

Graves of some of the Confederate dead from the Brice’s Crossroads battle.

Bryozoan Paradise at the K/T Boundary

May 27th, 2010

NEW ALBANY, MISSISSIPPI — One of the main advantages of being a geologist in a liberal arts program is the diversity of experiences our students and faculty have.  While some Wooster geologists are enjoying a “soft rock” adventure in the Cretaceous-Tertiary sediments in the Deep South, others are exploring “hard rock” quarries in the North.  Later this summer we may have simultaneous posts from Alaska, Iceland, Utah and Israel.

Today the southern expedition was very successful in its task to find bryozoans just below and just above the Cretaceous-Tertiary boundary.  Paul Taylor is a happy man.

Numerous bryozoans (the twig-like fossils) in the uppermost Cretaceous Prairie Bluff Formation east of New Albany, Union County, Mississippi.

The Cretaceous-Tertiary boundary east of New Albany, Union County, Mississippi. The uppermost Cretaceous is the brown clay, and the lowermost Tertiary is the orange sand at Megan's painted fingertip.

Which came first?

May 27th, 2010

NEW ALBANY, MISSISSIPPI — The Cretaceous oyster above was collected from the Coon Creek Beds of the Ripley Formation (Upper Cretaceous) near Blue Springs, Mississippi.  The holes are borings called Entobia which were produced by clionaid sponges which built a network of connected chambers inside the shell so that they could carry out their filter-feeding with some safety from grazing predators.  The branching white fossil is a cyclostome bryozoan, probably Voigtopora thurni.  Which was present first on the shell, the borings or the bryozoan?  Is there evidence that they were living at the same time?  The largest holes are about two millimeters in diameter.

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.

Fixing your search images

May 26th, 2010

NEW ALBANY, MISSISSIPPI — The kind of science our paleontological field team is doing ultimately depends on unpredictable discoveries.  We came to this part of the world based on the recorded experiences of generations of geologists who assembled maps of rock types, calculated stratigraphic ages, and made long lists of fossils they found.  From this body of knowledge we could estimate our chances of finding certain kinds of fossils in certain places.  Nevertheless, as with those pioneering scientists, we ultimately have to find things on our own.

Scouring the ground for fossils in the Nixon Sand Facies of the Prairie Bluff Formation (Upper Cretaceous) in Pontotoc County, Mississippi.

The old adage that “you find what you’re looking for” has some truth in exploratory paleontology.  You have to know what your target fossils look like before you can collect them.  This means recognizing them despite their orientations in the sediment or their preservation.  We develop a “search image” over time for each particular types of fossil.  Paul Taylor, for example, can pull bryozoans off the ground right under my nose because he has trained a set of search images for decades.  On this trip we have all learned what to expect when we crawl across the Prairie Bluff or Clayton formations.  It is an honor to spend a day plucking little treasures from the ground and adding them to the store of human knowledge.

A Cretaceous oyster encrusted in the top left of the shell with a bryozoan and drilled by a predatory snail in the center, with a coin showing The Great Emancipator for scale (Troy Beds, Ripley Formation, Pontotoc County).

Geologists a bit weathered after a week of southern sun

May 26th, 2010

A week’s worth of fieldwork done.  All is going well.  We have learned so much since we arrived last week so fresh and clean.  Mark Wilson, Caroline Sogot, Megan Innis and Paul Taylor.  Two more days of fieldwork to go.  Bags and bags of fossils already collected.  Photo taken at Rockin’ G Ranch, Pontotoc County, Mississippi.

An abundance of Cretaceous shark teeth

May 25th, 2010

Shark teeth found in the Upper Cretaceous Prairie Bluff Formation in Starkville, Mississippi.

STARKVILLE, MISSISSIPPI — OK, Andrew Retzler, please identify these teeth as best as you can in the comments below!  For everyone else, Andrew will be leaving with me in little more than a week for Israel where he will be collecting Late Cretaceous shark’s teeth as part of his Independent Study project.  He already proved very adept at sorting out a set of Israeli fossil shark’s teeth I had collected last year, so we’re giving him some more practice before his fieldwork.  It is possible we will have collected enough teeth by the end of this trip that he will be able to use them for comparisons.  Megan has proven especially good at finding teeth and other shark bits.

We meet the Cretaceous-Tertiary boundary in Mississippi

May 25th, 2010

STARKVILLE, MISSISSIPPI — George Phillips took us to a series of Starkville outcrops today straddling the Cretaceous-Tertiary boundary.  The boundary here is gradational and generally marked by a color change from gray in the upper Prairie Bluff Formation to light brown in the lower Clayton Formation.  Since we want to collect fossils just below and just above the boundary, these localities were ideal for us.

Megan Innis and George Phillips at the Cretaceous-Tertiary boundary in Starkville, Mississippi.

We were able to collect many encrusters and borings above and below this fateful event horizon.  I was most impressed by the diversity of encrusting foraminiferans on shells and phosphatic pebbles on both sides of the K/T, apparently showing little effects of the extinction.  A long time ago I did some systematic and paleoecological work with this group, so I may return to them to test these observations.

We also noted the proliferation of tiny oysters (especially Pycnodonte pulaskiensis) in the Clayton sediments immediately above the extinction horizon.  These are part of the initial survival and recovery fauna and thus keys to the future repopulation of this shallow marine ecosystem.

Small oysters in the lowermost Clayton Formation (Paleocene) in Starkville, Mississippi.

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