The turnpike from Boonsboro to Sharpsburg continues southward a couple of miles, crossing the Potomac to Shepherdstown, which lies on the Virginia side of the river. A bridge which formerly carried the road over the stream had been burned; but not far below the ruined piers was a ford, which was a pretty good one in the present stage of water. Shepherdstown was the natural place of junction for Lee and Jackson; but for Lee to have marched there at once would have exposed Jackson to attack from the northern side of the Potomac. The precious stores and supplies captured at Harper's Ferry must be got to a place of safety, and this was likely to delay Jackson a day or two. Lee therefore ordered McLaws to obstruct Franklin's movement as much as he could, whilst he himself concentrated the rest of Longstreet's corps at Sharpsburg, behind the Antietam. If McClellan's force should prove overwhelming, the past experience of the Confederate general encouraged him to believe that our advance would not be so enterprising that he could not make a safe retreat into Virginia. He resolved therefore to halt at Sharpsburg, which offered an excellent field for a defensive battle, leaving himself free to resume his aggressive campaign or to retreat into Virginia according to the result. McClellan had ordered Richardson's division of the Second Corps to support the cavalry in the advance, and Hooker's corps followed Richardson. [Footnote: Hooker's Report, Official Records, vol. xix. pt. i. p. 216.] It would seem most natural that the whole of Sumner's wing should take the advance on the 15th, though the breaking up of organizations was so much a habit with McClellan that perhaps it should not be surprising that one of Sumner's divisions was thus separated from the rest, and that Burnside's right wing was also divided. [Footnote: We must not forget the fact, however, that the order dividing the army into wings was suspended on that morning, and that this gives to the incident the air of an intentional reduction of the wing commanders to the control of a single corps. Official Records, vol. xix. pt. ii. p. 297.] The Ninth Corps was ordered to follow the old Sharpsburg road through Fox's Gap, our line of march being thus parallel to the others till we should reach the road from Boonsboro to Sharpsburg.