Vol. 6$.] THE ORIGIN OF CERTAIN CANON-LIKE VALLEYS. 477 



III. The Cheshire Plain and the Gorge at Ironbridge. 



It cannot be doubted that the advance of an ice-sheet from the 

 Irish Sea over the Cheshire plain would obstruct and eventually 

 impound its existing drainage. 



PL XXXI, a contour-map of the southern part of that region, 

 shows also portions of the drainage-basins of the Trent on the east, 

 and of the Lower Severn on the south, the three areas being separate 

 and clearly defined. 



The Cheshire plain, a Drift-covered Triassic region, lying for the 

 most part below the 300-foot contour, with a gradual slope north- 

 ward to sea-level along the basins of the Weaver and the Dee, is 

 surrounded by high Palaeozoic land; on the west by the Welsh 

 mountains, on the south by the highlands of Shropshire, and on the 

 east by the Pennine Hills ; but the girdle is broken towards the 

 south-east by a gap of no great width, between Market Drayton 

 and Xewport, excavated out of Triassic strata. 



This gap, however, lies in the path of the ice-stream from the 

 Irish Sea which accumulated so many erratic boulders round 

 Wolverhampton, and upon the flanks of Cannock Chase ; exposed, 

 therefore, to direct attack, it may have suffered considerable erosion 

 during the Glacial Period. Even if the Cheshire plain had then 

 been more continuously surrounded by high land than it is now, it 

 seems probable that the Triassic region around Newport stood 

 somewhat lower than the Palaeozoic isthmus through which the 

 gorge has been cut. 



The drainage of the southern part of the Cheshire plain in pre- 

 Glacial times was probably northwards, towards Chester or Crewe. 

 The Vyrnwy and the Severn, for example, flow at first to the 

 north-north-east, 1 and, unless otherwise prevented, they must have 

 continued to occupy channels eroded through the Triassic rocks, 

 taking a more direct as well as an easier course towards lower 

 ground and the sea, the fall being greater in that direction then 

 than it is now. 



The gorge at Ironbridge seems, so to speak, an after-thought. It 

 has no natural connexion with the Cheshire plain on the one hand, or 

 with the basin of the Lower Severn on the other, although it unites 

 them artificially, as the Panama Canal may unite the Pacific Ocean 

 with the Caribbean Sea, each cutting transversely across a narrow 

 isthmus of high land. 2 



There is no evidence to show that the Ironbridge gap represents 

 an isolated remnant of a river-valley older than the excavation of 

 the low-lying plains which it connects. It forms for 2 or 3 

 miles a narrow and nearly rectilinear canon, very recent in 

 appearance, having no resemblance to the type of valley originating 



1 The Severn may formerly have taken the more direct course from Mont- 

 gomery to Shrewsbury. (See map. PI. XXXI.) 



2 Prof. P. F. Kendall remarks, as to the overflow-channels of the Cleveland 

 district, that they are entirely independent of the natural drainage, Quart. 

 Journ. Geol. Soc. vol. lviii (1902) p. 483. 



