Lee and McClellan's armies at Sharpsburg were arrayed for battle roughly along Antietam Creek. The landscape was that of rolling farmland and groves of trees, and Lee was in a strong defensive position, but no where near impregnable. His army was along an approximately four mile long low ridge overlooking the valley containing the creek. Both of his flanks were anchored along the sides of a bend in the Potomac River. Although this meant that he could not be flanked, it also meant that if defeated it would be very difficult to make an orderly retreat. Lee's position could be broken down into three parts, distinguished by recognizable landmarks. On the left was Jackson's Corps around Dunker Church and the Cornfield. In the center was D. H. Hill's division along the Sunken Road, a road bordered by fences which formed a natural trench. On the right was Longstreet's Corps, and the landmark there was what was later called Burnside's Bridge over Antietam Creek. By the time Lee was attacked on the 17th, all of his scattered troops had arrived except for A. P. Hill. His division was on it's way from Harper's Ferry, and would arrive sometime that day.