4:18 TRANSACTIONS OP SECTION C. 



dropped 8,000 feet within the limits of Oligocene time. It is improbable that 

 the numerous faults now traceable operated with concerted gentleness, or 

 invited the Oligocene sea to lap in by imperceptible gradations from the west. 



AhruptTKSs of certain Geographical Changes. — Biver-capture. 



There is a totally different class of terrestrial phenomena which lends itself 

 also to speculation, or to that imaginative faculty, proper to our Section, which 

 enables the geologist to reconstruct. The deluge that appeared to affect the 

 world, as known to Chaldeean sages, has long been regarded as confined to a 

 limited valley of western Asia. But geographers have taught us to speak 

 lightly of river-diversion and river-capture, and to treat them as frequent occur- 

 rences in the history of existing lands. It is intei'esting to inquire what this 

 process on a large scale may involve. 



The draining of the Ragunda lake in Sweden ^' in 1796, by the rapid cutting 

 of a ravine 100 feet deep in a soft barrier, shows how many of our Glacial lakes, 

 dammed by morainic matter, may have excavated their outlet gorges and run 

 dry in the course of a few hours. The history of the temporary lake behind 

 the Gohna landslip, so brilliantly studied by our Vice-President, Sir Thomas 

 Holland,'*'' provided a lesson both in hill-destruction and catastrophic flooding. 

 The diversion of the Colorado River, however, in 1905, into the sluice leading 

 to the Salton Sink gives us a definite illustration of river-capture. The ' New 

 River ' thus produced in the depression to the north-west of Calexico cut a 

 valley seventy feet deep through the agricultural land that it was meant to 

 serve, and worked the head of this valley backward at the rate of a third of a 

 mile a day. 



One of the most remarkable instances of river-diversion in the European 

 record is that of the waters from the north side of the central Alps. At the 

 close of the Pliocene period, the chain had already become grooved by the sub- 

 sequent valley of the Rhone, and this river had been shifted, by earth-movement 

 in the Juras, south-westward towards its present course at Geneva. The north 

 slopes of the St. Gothard mass and the Bernese Alps, supplying the torrents of 

 the Reuss-Aar-Saane system, still, however, drained across the hummocky land 

 near Bale and sent their waters over to the Doubs. The great Rhine-trough 

 drained southward, and its streams formed tributaries of the Alpine flow near 

 Bale. 



The Mainz basin, however, which was infilled by Lower Pliocene alluvium, 

 became tapped by the head of a river that had long run northward from the 

 Hunsriick-Tamius range. This river is the Rhine that we know north of 

 Coblenz, and its alluvium was then spread out where the sea now stretches 

 between Holland and the Yorkshire coast. Its mature valley is still traceable '^' 

 above the present stream-cut in the hills. This river could have no direct 

 influence on the course of the drainage from the Alps. But the bulging of 

 the land at the north end of the Juras still continued. As the text-books 

 remark with some complacency, the Burgundian gate was closed, and the river 

 that had previously crossed westward was diverted northward to the Rhine- 

 trough. 



Can we exactly picture what this means ? The whole Reuss-Aar-Saane 

 system ' on some particular day began to flow northward along the far older 

 tectonic trough, carving away the infilling of detritus, washing back tree stems 

 that were floating quietly from the Lake of Mainz on their way to the Mediter- 

 ranean, and finding, when it reached that lake, a notch sufficiently low for its 

 escape across the Hunsriick-Taunus range. An enormous body of water was 

 thus added to that which had formed in Pliocene times a mature valley across 

 these hills.' '■ The addition of the drainage of Graubiinden, including the 



^° See especially H. W. Ahlmann, ' Ragundasjons Geomorfologi,' Sveriges 

 Geol. Undersoh, 1915 ; also Ahlmann, Carlzon, and Sandegren, ' Quaternary 

 History of the Ragunda Region, Jamtland,' Geol. Foren. Fbrhandl., vol. xxxiv. 

 (1912), p. 343. 



"• Records Geol. Surv. India, vol. xxvii. (1894), p. 55, and Nature, vol. 1., 

 p. 501. 



" W. M. Davis, Die erkldrende Bcschreibung der Landformen (1912), p. 106. 



'- G. A. J. Cole, The Growth of Europe (1914), p. 109. 



