.76 DR. E. HULL ON THE PHYSICAL GEOLOGY OP 



which shows that the original relative levels of the saddle and the 

 plateau have been absolutely reversed. 



In brief, therefore, we infer that when the river began to erode 

 its channel in the region of the Cumberland Plateau, this tract was 

 relatively lower than that to the south of its present course. Ey the 

 process of denudation these relations have been reversed ; but the 

 river, having once begun to wear down its channel, continued to 

 deepen it as the land rose ; so that, having once selected its course, 

 itnever afterwards left it. 



If it be permitted to compare small things with great, we may 

 say that the process of valley-erosion as applicable to the Tennessee 

 is somewhat analogous to that which took place in the South-east of 

 England during later Tertiary times, in consequence of which streams, 

 such as the Medway and the Ouse, pass into the sea by channels 

 traversing the escarpments of the Chalk and Lower Greensand. The 

 high grounds forming the sources of these streams in the centre of 

 the Wealden area represent the ridge of the Unaka and Blue Moun- 

 tains ; the plain of the Weald Clay represents the Valley of East 

 Tennessee, and the escarpments of the Greensand and Chalk the 

 Cumberland Plateau. How these channels were formed, together with 

 the adjoining escarpments, has been ably explained by Messrs. Poster 

 and Topley in their joint paper on the " Denudation of the Weald " *, 

 and further illustrated by Sir Andrew Ramsay f. The principles of 

 interpretation which have been adopted in the one case are applicable 

 in the other, though on a larger scale, and need not be repeated J. 



The effects of denudation here described were doubtless accelerated 

 during the " Pluvial" or " Charoplain " Period, corresponding to the 

 later stages of the Glacial Period. This region was, it is true, far 

 to the south of the limits of the great ice-sheet of North America, 

 as shown by Mr. T. C. Chamberlain § ; but the evidences of extra- 

 ordinarily copious rainfall and of the former erosive and transporting 

 action of the rivers over the regions lying along the margin of the 

 great ice-sheet are abundantly evident, and are fully recognized by 

 American geologists. Along the eastern side of the Alleghanies 

 the representative of this epoch is the " Columbia Pormation " de- 

 scribed by Mr. W. J. M'Gee || ; and to a similar stage is probably 

 referable the remarkable deposit of red loam by which the surface 

 of the country in the valleys of the Tennessee and Sequaehee is over- 

 spread to a depth of many feet or even yards. The effects of extensive 

 aqueous erosion, and the consequent deposition of sediment in the 

 valleys beyond the reach of existing streams, are everywhere mani- 

 fest in this part of America. 



* Quart. Journ. Geol. Soc. vol. xxi. 



t ' Phjs. Geol. & Geogr. of Great Britain,' 



\ It is right to observe that Professor Safford and Mr. J. P. Leslie account for 

 tlie preservation of the Cumberland Plateau by faulting, which has relatively 

 lowered the Carboniferous strata ; but the well-defined escarpment with which 

 the strata ci-op out along the Valley of East Tennessee near Chattanooga seems 

 to me to show that such a cause is insufficient. 



§ ' Seventh Annual Keport U.S. Geol. Survey,' p. 155. 



jl Ibid., " Taxonomy of the ' Columbia Formation,' " p. 611, &c. 



