An Upper Cretaceous paleodrainage system on the Coastal Plain unconformity of Alabama-Georgia

ABSTRACT Rocks of the Upper Cretaceous Tuscaloosa Formation (Cenomanian) and Eutaw Formation (Santonian) in southwestern Georgia and southeastern Alabama record an interval of fluvial and nearshore marine deposition. In the vicinity of Columbus, Georgia, basal units of the Tuscaloosa Formation consist of a residual paleosol built on crystalline rocks of the Appalachian Piedmont covered by conglomeratic sandstones deposited in braided stream systems flowing across the mid-Cenomanian Coastal Plain unconformity. The unconformity, which separates Cretaceous detrital rocks from underlying metamorphic rocks and residual paleosols built on those metamorphic rocks, lies primarily within the Tuscaloosa Formation in this region and is marked at the modern surface by the geomorphic Fall Line. Mapping of the unconformity across the region reveals areas of significant paleorelief associated with a number of distinct paleovalleys incised into the mid-Cenomanian surface. The most distinct of these lie immediately east of the Alabama-Georgia state line, within 15 km of the modern Lower Chattahoochee River Valley. Spatially, these distinct paleovalleys lie immediately north of a Santonian estuarine environment recorded in the Eutaw Formation, disconformably above the Tuscaloosa Formation. Collectively, paleo-valleys in the mid-Cenomanian surface, the fluvial nature of the Tuscaloosa Formation in southwestern Georgia and southeastern Alabama, and the estuarine environment in the younger Eutaw Formation suggest a persistent (~10 m.y.) paleodrainage system that may be a forerunner to the modern Chattahoochee River.

Over the Chattahoochee, May–July 1864

This chapter assesses how, in the closing weeks of May of 1864, William T. Sherman encountered a problem that had perplexed all successful commanders: how to complete a devastating pursuit. In particular, he needed to figure out how to prevent the escape of an enemy that might seek refuge in another position as formidable as the one previously vacated. With the Crossing of the Chattahoochee River, virtually without loss, Sherman brought this phase of the campaign to an end with a stunning feat for which he has received insufficient credit. There is one cardinal rule for river crossings, as for amphibious landings—cross or land where the enemy is not. In some ways, Sherman’s achievement is even more impressive than Ulysses S. Grant’s Crossing of the James the previous month, because of the close proximity of the enemy determined to resist his forays and superior in cavalry. Sherman out-thought and out-maneuvered Joseph E. Johnston. And for once, in this series of intricate movements, his cavalry played a creditable role.

Crossing the Chattahoochee

Demonstrating increasing tenacity as he neared Atlanta, Johnston fell back from Kennesaw Mountain only a few miles to a prepared position at Smyrna Station. He held here a short while before falling back to his last position north of the last river protecting the city, constructing the Chattahoochee River Line. It possessed somewhat innovative features designed by artillery chief Francis Shoup and thus could be termed the Shoup Line. It was one of the few Confederate defences of the campaign constructed largely by slaves and impressed free blacks. Despite its strength, Johnston evacuated the Chattahoochee River Line on July 9 when Sherman crossed troops over the river miles upstream. Then Johnston concentrated on improving the Atlanta City Line, designed and constructed by Capt. Lemuel P. Grant for the past several months, as Sherman rested his troops for a full scale crossing of the Chattahoochee River.

Flanking the Kennesaw Line

For nearly a week after the failed assault of June 27 the Federals continued to search for a way to break the Kennesaw Mountain Line. Fourteenth Corps troops lodged on Cheatham’s Hill sniped at the Confederate position only a few yards away and began digging a mine to blow up the angle in the opposing line. More promising was Sherman’s continued effort to flank the Confederate left wing. Meanwhile life in the trenches became stressful for soldiers in both armies. By July 2, Sherman was ready to break away temporarily from his railroad supply line and conduct a massive flanking march toward the Chattahoochee River. When Johnston got wind of this move, he evacuated the Kennesaw Mountain Line that night.

The Mountain Line, the Gilgal Church Line, and the Mud Creek Line

Terrain and effective Confederate fortifications continued to slow Federal operations during the 2nd and 3rd weeks of June between the Etowah River and the Chattahoochee River. Johnston’s engineers sited three lines of trenches blocking Sherman’s advance. The Mountain Line encompassed Lost Mountain, Pine Mountain, and Brush Mountain but was too extended for the number of his troops. Johnston contracted this position a bit to create the Gilgal Church Line but this position had a sharp angle which was vulnerable to Union artillery fire. He then contracted once again, bringing his left wing back to Mud Creek. But the position still had a sharp angle in the center of the line, so Johnston planned to fall back to Kennesaw Mountain. The Federals dealt with all three lines by closing in, intensely skirmishing, and opening enfilading artillery fire on the angle to compel their enemy to creep back. They employed something like siege approaches to accomplish this slow advance.

Jonesboro

On August 26, Sherman launched his final offensive in the Atlanta campaign by leading 6 corps in a wide flanking march to the west and south of the city, breaking contact with his railroad. He left one corps behind to protect the railhead north of the Chattahoochee River. By the time Sherman’s column reached the first railroad many miles southwest of Atlanta and began to tear it up, Hood knew where he was and shifted troops to the last railroad that lay due south of the city near Jonesboro. Here was fought the last battle of the campaign on August 31 and September 1. The Confederates attacked the first day and were repulsed. The Federals attacked the second day and drove the Confederates away. Light fieldworks helped the Federals the first day but they overcame Confederate fieldworks the second day. Hood evacuated Atlanta on the night of September 1-2 and Federal troops occupied it on September 2.

Tactics, Terrain, and Trenches

Sherman consistently urged his subordinates to take the tactical offensive and Johnston consistently took the defensive. Hood initially took an aggressively offensive stance but soon substituted it with a cautious defensive mode of operation. As far as the terrain was concerned, it favoured Sherman’s offensive minded operations during the early phases of the Atlanta campaign, from Dalton to the Etowah River, but south of that stream the terrain favoured the Confederates and their defensive operations. South of the Chattahoochee River, there was no terrain advantage for the Confederates. Both sides came to learn how to dig very effective field fortifications but the Federals used their works both offensively and defensively while the Confederates almost always used theirs in merely a defensive manner. Effective use of fieldworks to hold Confederate forces in place while moving mobile columns to outflank that position was an important key to Union success in the campaign.