We made a large group as we stood upon the hill, and it was not long before we attracted the enemy's attention. A puff of white smoke from a knoll on the right of the Sharpsburg road was followed by the screaming of a shell over our heads. McClellan directed that all but one or two should retire behind the ridge, while he continued the reconnoissance, walking slowly to the right. I think Fitz-John Porter was the only general officer who was retained as a companion in this walk. I noted with satisfaction the cool and business-like air with which McClellan made his examination under fire. The Confederate artillery was answered by a battery of ours, and a lively cannonade ensued on both sides, though without any noticeable effect. The enemy's position was revealed, and he was evidently in force on both sides of the turnpike in front of Sharpsburg, covered by the undulations of the rolling ground which hid his infantry from our sight. The examination of the enemy's position and the discussion of it continued till near the close of the day. Orders were then given for the Ninth Corps to move to the left, keeping off the road, which was occupied by other troops. We moved through fields and farm lands, an hour's march in the dusk of evening, going into bivouac about a mile south of the Sharpsburg bridge, and in rear of the hills bordering the Antietam. The village of Sharpsburg is in the midst of a plateau which is almost enclosed by the Potomac River and the Antietam. The Potomac bounds it on the south and west, and the Antietam on the east. The plateau in general outline may be considered a parallelogram, four miles in length from north to south, and two and a half miles in width inside the bends of the river. The northern side of this terrain appears the narrowest, for here the river curves sharply away to the west, nearly doubling the width of the field above and below the bend. From the village the ground descends in all directions, though a continuous ridge runs northward, on which is the Hagerstown turnpike. The Boonsboro turnpike enters the village from the northeast, crossing the Antietam on a stone bridge, and continuing through Sharpsburg to the southwest, reaches Shepherdstown by the ford of the Potomac already mentioned. The Hagerstown turnpike enters the town from the north, passing the Dunker Church a mile out, and goes nearly due south, crossing the Antietam at its mouth, and continuing down the Potomac toward Harper's Ferry.