OVERVIEW: For three days in July, the 1st. to the 3rd., in the year 1863, two great armies clashed in the struggle which was to forge a new nation, a truly United States of America. The place of that battle is were we are to travel, seeking to understand how the geologic setting was in fact the underlying factor which lead to major military decisions and outcomes. We will visit sites that will illustrate the geologic history and ultimately the military history of the battle of the Gettysburg campaign. While making our tour you may refer to the two location maps; one topographic in detail the other a simplified topo/geologic. The formations and structure of the rocks are simplistic when compared to the rest of the Piedmont province and yet set the stage for historic events and outcomes that were to alter the course of history for a young nation coming to grips with itself.
GEOLOGIC SETTING:The Gettysburg Battle Park is set in and around the small township of Gettysburg in Adams county Pennsylvania, in the southwest corner of the Newark-Gettysburg basin, one of several Triassic basins extending from the Connecticut Valley in the north to the Wadesboro area in southern North Carolina. ( ref. map 1.) The Gettysburg portion of the basin is a half-graben, down faulted on the western edge in contact with South Mountain ,the northern most part of the Blue Ridge Province. The floor of the basin is underlaid with gently dipping redbeds of Triassic age sandstones, and shales that have been intruded by Triassic/Jurassic age diabase sills and dikes. (1) It is the more resistant diabase intrusions which form the topographic features that standout in higher relief throughout the battlefield, and thus played a major role in the evolution of the three day battle. Simply put, it was these high ground features for which the two armies contested.
FIELD TRIP SITES::GEOLOGY MAP AND STOPS ON FIELD TRIP
STOP # 1: McPHERSON'S RIDGE AND THE RAILROAD CUT
Setting: The higher relief of this ridge is due in part to resistive sandstones of the Gettysburg Formation (Upper Triassic) which we will examine within the railroad cut. Looking west we see the higher topography of South Mountain through which passes the Cashtown Road along the strike of the Marsh Creek Fault, a right-lateral (dextral) fault , in the Catoctin Formation (Precambrian greenstone).
Battle: Advanced elements of Lee's and Meade's armies first made contact here. Units of A.P. Hill's corps advancing down Cashtown road toward Gettysburg encountered dismounted cavalry of Gen. John Buford along Willoughby's Run just to the west of us. What started out as a small engagement soon developed into a full fire-fight with both sides sending in more units. The Union forces were finally driven from the ridge, through the town and up the slopes of Cemetery Hill, but were not driven from the high ground on day one; a mistake which would ultimately prove to play a deciding factor in the outcome of the battle. Gen. John F. Reynolds was mortally wounded in McPherson's Woods; he was one of the best corps commanders in Meade's army. Gen. Abner Doubleday, who fired the first shot from Fort Sumpter (1861), also played a major role in the first days battle.
Geology: We will enter the railroad cut just to the east and north of the bridge along the road leading to Oak Ridge and its monument. As we walk west along the cut we are passing into younger strata of red medium to fine-grained sandstones and shales of of the Gettysburg Formation, Triassic in age. Note the texture and colors of the formation. The rocks here are dipping toward the northwest at about 20 degrees. Further west along the cut is the contact with the Heidlersberg Member of the Gettysburg Formation, containing red, green, and gray shale and argillite with minor gray to white sandstone. (3)
STOP # 2: SEMINARY RIDGE AND THE RAILROAD CUT
Setting: Walking toward the town of Gettysburg we pass out of the McPherson's Ridge railroad cut and begin to enter the Seminary Ridge railroad cut.
Battle: The Lutheran Seminary and its main building stand on this high ground. It was from the cupola that both Gen. John Buford and later Gen. Lee observed the views of the surrounding battlefield on the first day ; July 1, 1863.
Geology: From the service road to the diabase dike intrusion is some 400 plus feet which will give us exposures of the Triassic sandstone and shale. Paleomagnetics puts the Gettysburg area at a 10 degree north latitude during the Triassic; which was hot and tropical for that location ( 75 degrees Fahrenheit and some 50 inches of rainfall). (2) The contact or bake zone begins at about 300 plus feet with red shale being converted to the grayish-red argillite, then into the hornfels at the contact with the diabase dike. The dike is about 100 feet in diameter, and is known as the Rossville Diabase with a composition of calcic plagioclase from anorthite to bytownite (Ab 30 An 70). (1) A new line in the railroad has opened up more of the rock at the time this guide was being prepared and has removed the site of the diabase contact at the western end of the dike; however, digging may reveal the eastern contact point up the railroad line. Participants MAY collect at this site ONLY since we are now out of the park and on property of the college .
STOP # 3: SEMINARY RIDGE AND THE VIRGINIA MONUMENT
Setting: We are now at the point from which Gen. Lee and others viewed the fatal attack known as Pickett's Charge. Imagine thousands of men three or more rows deep waiting for the command to step-off and cross the open field that you see before you, and to do so under heavy fire from the Union forces waiting for them on Cemetery Ridge.
Battle: Throughout the three day battle Gen. Lee's forces were arranged along a five mile front from Culps Hill in the northeast to Round Top in the south. Day 2 of the battle found the Union forces dug in along a three mile front on the high ground of Culps Hill, Cemetery Ridge, and Little Round Top. Day 2 of the battle was concentrated at the left of the Union line toward the Devil's Den and Round Top area.
Geology: Seminary Ridge is formed by an intrusive arm of the main diabase body of the Rossville composition. The evolution of the magmas leading to the diabase intrusions are believed to have been associated with the rifting of Pangeae and represent an assimilation process as the melts rose toward the surface. The process is summarized as follows: (1) generation of magma in the mantle as an olivine tholeiite ( basaltic), (2) crystallization and interaction with mantle during ascent leading to the Rossville diabase having higher calcic plagioclase content with little or no assimilation with crust, (3) crystallization and interaction with crust during ascent leading to the York Haven diabase having a lower calcic plagioclase content (4) intrusion to present site. (1)
STOP # 4: THE PEACH ORCHARD
Setting: We are now in the small valley separating Seminary and Cemetery Ridge along the Emmitsburg Road.
Battle: Elements of Meade's army under the command of Gen. Daniel E. Sickles had pushed out from the Cemetery Ridge to this point creating a dangerous position for the Union forces which left the two Round Tops undefended. Gen. Lee intended to take advantage of this maneuver and attack with force on the Union left, but the attack was delayed till 3:30 p.m. Heavy fighting took place here ,in the Wheat Field and Devil's Den pushing back the Union line to the two Round Tops and Cemetery Ridge.
Geology: The valley floor is composed of the Gettysburg Formation which provides the red soils, typical of weathered Triassic redbeds. The Gettysburg Formation rests upon the New Oxford Formation, which is similar in composition. The source of the sediment for these strata is interpreted as a granitic or high-grade metamorphic terrane as indicated by the amounts of feldspar in the sandstones,and conglomerates within the basin. The feldspar decreases stratigraphically upward away from the southern outcrop edge, and the texture is from coarse basal strata to a finer grained sediment within the younger strata, indicating that the source region was to the south. (3)
STOP # 5: BRIDGE OVER PLUM RUN
Setting: We are now at the base of the Round Top (Big Round Top) at the bridge crossing Plum Run. Just up stream of us is the Devil's Den , which we will visit next.
Battle: Elements of Gen. Longstreet's Corps had pushed out on the Union left and were making their way up the slope of Round Top in an attempt to out flank the Union line. Big Round Top was wooded on the day of the battle and presented major problems for bring-up artillery which would have commanded the flanking operation. As it was impossible to fell enough of the woods to provide a field of fire, the attack moved forward toward Little Round Top.
Geology: We are now on the diabase sheet of the York Haven composition; having a lower calcic plagioclase content than that of the Rossville diabase. The bridge over Plum Run is of interest in that it contains representative rocks quarried from Adams county area, and has several features to note: diabase blocks showing tool marks; desiccation patterns in sandstone; ripple marks both symmetrical and asymmetrical; clastic dikes; rain drop casts; dinosaur skin (?); and reptilian foot prints. Explore each side of the bridge and see if your observations confirm these findings. (1)
STOP #6: DEVIL'S DEN
Setting: We are now in a small valley through which flows Plum Run, with Devil's Den to the west and Little Round Top to the east. Take care as you walk in this area.
Battle: Looking up from our position at the base of the Little Round Top you can get a feel for the importance of taking and holding that position. Devils Den was finally taken by units of Gen. Longstreet's Corps on the second day, July 2, 1863, and served as a position form which sharpshooters were able to assault the Union forces on the Little Round Top. Meade was forced to withdraw units on Culp's Hill to the north to strengthen the left flank. Gen. Edward Johnson of Ewell's Corps was within striking distance of Meade's reserve artillery, and Meade's line of retreat along the Baltimore Pike, but failed to move forward and thus let victory slip away. Gen. Longstreet withdrew at the end of the day having lost some 5,000 men.
Geology: Around you are massive boulders of the diabase sill, York Haven composition, illustrating the effects of differential weathering. The joint patterns play a major role in this process, allowing the decomposition of the diabase to increase the spacing between boulders. Scaly-like patterns may be observed on some of the rock surfaces indicating micro-fractures in the diabase. Also, on the surfaces of some of the boulders you may see raised ridges that may be mineralogically different from the main body of the diabase , containing concentrations of ferro-silicates (hornblende and biotite). Perhaps during later stages of intrusion, iron bearing solutions entered the micro-fractures which were forming during the cooling process. (2)
STOP # 7: LITTLE ROUND TOP
Setting: From our position on Little Round Top we have a commanding view of much of the battlefield; similar perhaps to that of Gen. G. K. Warren.
Battle: Little Round Top is at the end of the "fishhook" or battle line which Gen. Meade's army occupied, and almost proved to be his undoing. As day 2 of the battle progressed Little Round Top was undefended by Union forces, having only a signal corps at the summit. It was the foresight of Gen. Gouverneur K. Warren, Meade's chief engineer, that saved the Union's left flank on day 2. Seeing the high ground undefended, Gen. Warren placed units on the summit and along the southern slope. It was here that the 20th Maine made history by "refusing the line" under the command of Col. Lawrence Chamberlain and preventing the left flank of the Union line to be turned by the advancing units of Gen. Longstreet's corps.
Geology: The diabase outcrop extending from Big Round Top to Culp's Hill is locally known as the Gettysburg Sill with an average thickness of about 1800 feet and dips to the northwest at about 20 - 30 degrees. The outcrop here at Little Round Top is of a coarser texture than that exposed at the railroad cut (stop 1 and 2) ranging from intermediate to finely phaneritic crystals of plagioclase and pyroxene minerals of the York Haven composition. (2) Examples of exfoliation and spheroidal weathering of the diabase may be found here as they were in Devil's Den. Micro-fractures and joint sets running parallel to the surface of the exposed rock due to the expansion of the bedrock as overburden is removed by erosion allows infiltrating water to penetrate and set-up conditions for frost action that results in the "peeling" of the outer surface of the rock.
STOP #8: THE HIGH WATER MARK OF THE C.S.A.
Setting: You are now standing at the center of the Union defensive line, looking out across the valley toward Seminary Ridge and the army of Gen. R. E. Lee.
Battle: Since Gen. Meade had a shorter front he was able to move troops within his defense line, allowing him to adjust to needs as attacks came on either flank. It was anticipated that Gen. Lee would strike at the center of the front on day 3 of the battle since both flanks had been assaulted on day 2. It may be said that artillery played the major role on day 3. Col. Alexander , chief of Confederate artillery, had 150 field cannons lay down a bombardment on the Union center that lasted for nearly two hours. Gen. Henry Hunt , chief of U.S. artillery, returned fire with 80 union guns. Many of the shells were fired too high, and thus caused casualties in the ranks massed behind the lines. At 3 p.m. Gen. Longstreet's assault began to advance, lead by Gen. George Pickett's divisions, some 15,000 troops. For nearly a mile the gray lines marched as on parade, union guns ripping into their ranks, but on they came. At the wall the exchange of gun fire and cannonade was point-blank and yet over the wall came Gen. L. Armistead leading the way with his hat atop his sword. A battery of cannon was captured, but Gen. Armistead was mortally wounded, and died at that spot, just yards from his fallen friend Gen. Winfield S. Hancock, who was in command that day of the union center.
Geology: At this point in the "fishhook" of the Gettysburg sill we are about 570 feet in elevation above sea level, with Big Round Top at 785 feet , Little Round Top at 650 feet. The town of Gettysburg is about 500 feet in elevation. Although the Union forces along their front were unable to dig in due to the exposed bedrock, walls of diabase played a major role during the battle. The Confederate forces had to advance up hill, which made it all that more difficult to win the battle on day 3.
STOP # 9: THE MARYLAND MONUMENT
Perhaps no other monument on the battlefield dedicated to the struggle that was to determine the course of history for our country is better expressed than that of the monument for those who fought on both sides of the Civil War.
In closing may we recall the words spoken by President Lincoln on Nov. 19, 1863:
Fourscore and seven years ago our fathers brought forth on this continent a new nation, conceived in liberty, and dedicated to the proposition that all men are created equal. Now we are engaged in a great civil war, testing whether that nation, or any nation so conceived and so dedicated can long endure. We are met on a great battlefield of that war. We have come to dedicate a portion of that field, as a final resting place for those who here gave their lives that that nation might live. It is altogether fitting and proper that we should do this. But in a larger sense we cannot dedicate, we cannot consecrate, we cannot hallow this ground. The brave men, living and dead, who struggled here, have consecrated it far above our poor power to add or detract. The world will little note, nor long remember, what we say here. It is for us the living, rather, to be dedicated here to the unfinished work which they who fought here have thus far so nobly advanced. It is rather for us to be dedicated to the great task remaining before us - - that from these honored dead we take increased devotion to that cause for which they gave the last full measure of devotion -- that we here highly resolve that these dead shall not have died in vain -- that this nation, under God, shall have a new birth of freedom -- and that government of the people, by the people, and for the people, shall not perish from the earth.
My thanks to those who helped make this guide possible: Mr. Jim O'Connor for resources that only Jim seems to have. The Pennsylvania Geologic Survey for providing the booklets on Gettysburg. Gettysburg College for permission to collect samples at the railroad cut.
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