At daylight on Monday morning, after the battle at South Mountain, General Richardson's division of the Second Corps moved down the mountain side through Boonesboro' to Keedysville. It was found that General Lee was massing his troops on the west bank of the Antietam, and planting his batteries on the hills north of Sharpsburg. General Richardson deployed his troops. Captain Tidball and Captain Pettit ran their batteries up on the hills near Porterstown, and commenced a cannonade which lasted till night. General Hooker's, General Mansfield's, General Burnside's, General Sumner's, and General Porter's troops arrived during the night. On the morning of Tuesday, the 16th, General McClellan reconnoitered the position which Lee had chosen. The forenoon passed before the corps were in position to make an attack. General McClellan's plan was to attack the enemy's left with Hooker's and Mansfield's corps, supported by Sumner's; and, as soon as matters looked favorably there, to move Burnside across the lower bridge, and attack Lee's right, south of Sharpsburg. If either of these flank movements were successful, then he would move upon the center with all the forces at his disposal. About two o'clock in the afternoon, Hooker crossed the Antietam by the upper bridge and by the ford near Pray's Mill. The Rebel pickets were in the cornfields on Mr. Hoffman's farm, and their first line in the strip of woods east of Miller's cornfield. The Pennsylvania Reserves were in advance. There was a sharp skirmish and a brisk cannonade which lasted till dark. The Rebels were forced back. They retreated almost to Poffenberger's house. General Hooker advanced, planted his guns on the hill near Hoffman's, and threw out his pickets. His men lay down to sleep in the fields and amid the rustling corn. Mansfield crossed during the night. He went up from the stream but a short distance, halting nearly a mile in rear of Hooker. Sumner's corps remained east of the stream, near Pray's Mill. Porter was posted on the east side near General McClellan's head-quarters, while Burnside passed down through Porterstown and came into position on the farm of Mr. Rohrbach at the base of Elk Ridge, near the lower bridge. An auspicious hour had passed by never to return. Lee had only Longstreet, D. H. Hill, and two divisions of Jackson's corps on the ground on Tuesday, the 16th. Jackson arrived on the morning of the 16th, after a hard night-march from Harper's Ferry. His troops were exhausted. They were not in condition to fight on Tuesday; but by the delay of General McClellan they obtained rest and strength. McLaw's, Anderson's, Walker's, and A. P. Hill's divisions had not arrived even when the great contest began on the 17th.[61] [Footnote 61: Pollard, Vol. II. p. 125.]