The Antietam is a deep creek, with few fords at an ordinary stage of water, and the principal roads cross it upon stone bridges. Of these there were three within the field of battle; the upper one in front of Keedysville, the middle one upon the Boonsboro turnpike, and the lower one on the Sharpsburg and Rohrersville road, since known as Burnside's bridge. McClellan's staff was better supplied with officers of engineers than the staff of most of our separate armies, and Captain Duane, his chief engineer, systematized the work of gathering topographical information. This was communicated to the general officers in connection with the orders which were given them. In this way we were instructed that the only fords of the Antietam passable at that time were one between the two upper bridges named, and another about half a mile below Burnside's bridge, in a deep bend of the stream. We found, however, during the engagement of the 17th, another practicable crossing for infantry a short distance above the bridge. This was not a ford in common use, but in the low stage of water at the time it was made available for a small force. It was about noon of the 15th of September that Lee placed the forces which he had in hand across the turnpike in front of Sharpsburg. D. H. Hill's division was on the north of the road, and on the south of it Longstreet's own old division (now under General D. R. Jones), Hood's division, and Evans's independent brigade. Stuart's cavalry and the reserve artillery were also present. The rest of the army was with Jackson at Harper's Ferry, or co-operating with him in the neighborhood of Maryland Heights. Out of forty-four brigades, Lee could put but fourteen or fifteen in line that day to oppose McClellan. He was very strong in artillery, however, and his cannon looked grimly over the hill-crests behind which his infantry were lying. Cutts's and Jones's battalions of the reserve artillery were ordered to report to Hill for the protection of the left of the Confederate line, and gave him in all the sixty or seventy guns which he speaks of in his report, and which have puzzled several writers who have described the battle. Whenever our troops showed themselves as they marched into position, they were saluted from shotted cannon, and the numerous batteries that were developed on the long line of hills before us no doubt did much to impress McClellan with the belief that he had the great bulk of Lee's army before him.