Union Gen. Ambrose E. Burnside's corps of 12,000 men had been trying to cross a 12-foot-wide bridge over Antietam Creek since 9:30 a.m. About 450 Georgian sharpshooters took up positions behind trees and boulders on a steep wooded bluff some 100 feet high and overlooking the Lower Bridge . Greatly outnumbered, the Confederates drove back several Union advances toward the bridge. Finally, at 1 p.m. the Federals crossed the 125-foot-long bridge (now known as Burnside Bridge ) and, after a 2-hour delay to rest and replenish ammunition, continued their advance toward Sharpsburg .