If the warpings were considerable, much more decisive changes in drainage would result. Suppose the drainage of a given region to be represented by the streams in Fig. 163. If there is uplift along the axis 1–2, that part of _ac_ above the axis of uplift would be ponded, or at least have its velocity checked, while the flow of some of the tributaries of _d_ would be accelerated, and might work back and capture the other stream (Fig. 164). Crustal warping was one of the conditions under which the Tennessee achieved its present anomalous course, and its history[67] is illustrative of the complex changes which drainage suffers when warping affects the area where the rock structures are of unequal resistance. At the close of the Cretaceous cycle of erosion, when the Appalachian Mountains had been reduced to a peneplain, the waters falling in the area now drained by the upper course of the Tennessee flowed south-south-west to the Gulf in a stream (the Appalachian River, _a_, Fig. 165) the lower part of which had the general position of the Coosa and the Alabama. To the west of the Appalachian River, shorter streams flowed west and southwest into the Mississippi embayment (Fig. 165) by courses which are not now definitely known. The succeeding cycle of erosion was inaugurated by uplift and deformation of the peneplain. The axis of greatest elevation (_AB_, Fig. 166) was nearly parallel to the Appalachian River, and the effect of the differential uplift was to impose a greater task on this river (_a_, Fig. 166), which flowed along the axis of uplift, than upon the rivers which flowed westward and southwestward to the Mississippi embayment. The result was that the strongest of the southwesterly flowing streams worked its head back into the drainage basin of the Appalachian River, and captured, one by one, the head-waters of its westerly tributaries, establishing some such drainage relations as are shown in Fig. 166. Still later, after the land area of the region had been considerably extended by the withdrawal of the sea, the Appalachian River itself was reached by the invading stream, and its waters carried away to the Mississippi Bay by a course the lower part of which is thought to have corresponded approximately with the course of the present Black River (_b_, Fig. 167). [Illustration: +Fig.+ 165.—Shows the general position of the main drainage lines in the southern Appalachians at the close of the Cretaceous cycle of erosion. The lower part of stream _b_ is made to follow the course of a portion of the present Tennessee.] [Illustration: +Fig.+ 166.—Shows the general position of the main drainage lines in the southern Appalachians, after the capture of the westerly tributaries of the Appalachian River by stream _b_. Compare Fig. 165.] [Illustration: +Fig.+ 167.—A stage later than that shown in Fig. 166. The sea is represented as having withdrawn from a considerable area which was submerged at earlier stages (Figs. 165, 166).]