Before dawn on December 11, Union soldiers and engineers began to launch and fasten together pontoon boats to bridge the Rappahannock River. The Confederate sharpshooters waited until it was light, and when the fog lifted, they opened fire on the men building the tandem pontoon bridges. Burnside ordered a brigade to cross the river in loose pontoon boats to roust the snipers. By evening, the Union soldiers had cleared the town. Burnside moved his army across the bridges on December 12, and on the 13th, attacked Lee's well defended positions on Marye's Heights. Burnside's attack was turned back with 12,600 casualties. Lee's repulse of the Union army at Fredericksburg redeemed his failure at Antietam.