EARLY VS. WALLACE ALONG THE RIVER, THE RAILROAD, AND THE ARABY RIDGES: MILITARY GEOGRAPHY AND GEOLOGY OF THE CIVIL WAR BATTLE OF MONOCACY, FREDERICK COUNTY, MARYLAND—9 JULY 1864

On 16 June 1864 Confederate Gen. Robert E. Lee tasked Gen. Jubal Early to clear the Shenandoah Valley of Union forces, then move NE to threaten Washington, D.C. and draw troops from the besieged city of Petersburg, VA. Early and his 15,000-man army arrived at Frederick, MD, on 9 July to find their route to the capital blocked by a 6,000-man force of Union militia and regulars commanded by Gen. Lew Wallace, positioned mainly on the E-side of the Monocacy River. Wallace’s thin line of defense started directly E of Frederick at the N-end and extended south a distance of 4 mi. Assuming Early’s objective was Washington, Wallace concentrated his strongest fighting units toward the S at the iron B&O railroad and wooden Georgetown Pike bridges over the Monocacy and on the Taylor farm to the SW. The stage was set for the battle of Monocacy, “the battle that saved Washington.”

The Monocacy battlefield is situated in the Piedmont Physiographic Province on the E flank of the Frederick Valley synclinorium. Bedrock consists of the Lower and Middle Cambrian Araby Formation (dark gray, silty sandstone) and overlying Upper Cambrian Frederick Formation (dark gray to black, shaly lime mudstone). At the battlefield the two formations are tightly folded into narrow anticlinal ridges (Araby) and synclinal valleys (Frederick). Brooks Hill at the SW edge of the battlefield has a local relief of 250 ft. The local topography is the combined result of the Cambrian stratigraphy and erosion by the Monocacy River. A significant fluvial feature is a meander of the Monocacy on the W-side of Brooks Hill, where a point bar on the SE and sediment deposition at the mouth of Ballengers Creek on the NW create the Worthington-McKinney ford.

Wallace’s well-situated forces successfully held off Confederate attacks until late afternoon, when Gen. John Brown Gordon was able to ford the river and break the Union left flank. This forced Wallace to conduct a hasty withdrawal to the E and NE toward Baltimore. The spirited Union resistance along the Monocacy delayed Early’s advance on Washington for a day and probably saved the Union capital from temporary capture. When the Confederates arrived in front of Fort Stevens NW of Washington on 11 July, they faced newly reinforced Union defenses. After a day of skirmishing, Early ordered a retreat, crossing the Potomac back into Virginia on the 13^th.