Enough for the two main ridges enclosing Gettysburg and its valley. We come now to that most striking feature of the landscape, notably on the side of Cemetery Ridge, but more or less characteristic of both sides of the valley. This is the group of hills standing off from Cemetery Ridge at either end, just as if, at some remote time, this ridge had formed a continuous chain, the summits of which had been cleanly shaved off at the centre, leaving these isolated clusters to show where the wasting forces had passed. From different points of view we may see one or both of them rising above the ridge like giant watch-towers set at the extremities of some high embattled wall. [Sidenote: Culp's Hill.] [Sidenote: Rock Creek.] Let us first take the northernmost cluster, formed of Wolf's, McAllister's, and Culp's hills. It is seen to be thrown back behind Cemetery Hill, to which Culp's Hill alone is slenderly attached by a low ridge, so making an elbow with it, or, in the military phrase, a refused line. Between Culp's Hill and Wolf's Hill flows Rock Creek, the shallow stream so often mentioned in connection with the battle, its course lying through a shaggy ravine.[5] The ravine and stream of Rock Creek threw Wolf's Hill somewhat out of the true line of defence, but the merest novice in military art sees at a glance why the possession of Culp's Hill was all-essential to the security of Cemetery Hill, since there is little use in shutting the front door if the back door is to be left standing open. [Sidenote: The Round Tops.] [Sidenote: The Devil's Den.] The same is just as true of the southernmost group, composed of Little and Great Round Tops, two exceedingly picturesque summits, standing up above the generally monotonous contours about them in strong relief. They also were wooded from base to summit, and they show, even more distinctly than the first group, where the crushing out or denuding forces have been at work, in shelves or crevices of broken ledge at the highest points, in ugly bowlders cropping out on the slopes, in miry gullies crawling at their feet, but most of all in the deformed heap of ripped-up ledges, topped with coppices and scattered trees, thrust out from Little Round Top and known as the Devil's Den.[6] When it is added that the way is open between the two Round Tops to the rear of Cemetery Ridge, the importance of holding them firmly becomes self-evident; and inasmuch as the greatest natural strength of this ridge lay at its extremities, or flanks, _so its weakness would result from a neglect to occupy those flanks_. [Sidenote: The Swale.] This line was assailable at one other point. As it approaches Little Round Top the ridge sinks away to the general level around it, or so as to break its continuity, thus leaving a gap more or less inviting the approach of an enemy. The whole extent of this crooked line, at which we have just glanced, is about two and a half miles. [Sidenote: Emmettsburg Road.]