 As McClellan 's Army slowly advanced , Lee took up a defensive position in Sharpsburg , Maryland , along Antietam Creek . The 21st , leaving two companies behind at South Mountain for guard detail , now numbered only 150 men . On September 16 , the IX Corps took up a position on the left flank of the Union army , near a 125 feet ( 38 m ) stone bridge soon to be known as Burnside 's Bridge . 
