=460. Physical Features in the East.=—The Alleghany Mountains and the Mississippi River divided the Confederacy into three somewhat distinct parts. The eastern portion, which lies between the Alleghanies and the Atlantic, is characterized by a succession of rivers that rise in the mountains and flow in a southeasterly direction nearly parallel with the Potomac and James. To advance through Virginia to Richmond, it was possible for the Union forces to go by water to the mouth of the James and then ascend along the river, or to cross the Rappahannock, the Rapidan, the York, and the Chickahominy, besides a large number of smaller streams which were sometimes swollen to the volume of navigable rivers. Much of the intervening region, moreover, was swampy, and at times almost impassable. East of the Alleghanies, the subordinate range known as the Blue Ridge provides a fertile, intervening valley, through which the Shenandoah flows northward into the Potomac at Harper’s Ferry. This Valley of Virginia, however, near the northern end, is subdivided by a low range of mountains in such a way as to enable an army driven down one side to retreat up the other. Across the Blue Ridge at Manassas Gap, a railroad connects the Valley with the eastern portion of Virginia and makes it possible to transfer troops rapidly from one side to the other. At Manassas, this railroad crosses an important Southern line which runs from Washington to Richmond and Lynchburg, and to the far South. This crossing, therefore, was of the first strategic importance in the war, and was naturally the first point of collision.