C— GEOLOGY 91 



pretation, namely that there were two glaciations involved. In the first 

 the ice movement was from North Wales and the Pennines towards the 

 south-east : in the second there was a similar, but less powerful North- 

 Welsh dispersion with some slight intermingling of Irish Sea material. 

 Simultaneous with this, however, in the east and in the Avon valley was 

 the Great Eastern glacier. 



By the end of the First Glacial epoch the general trend of the lowest 

 parts of the Severn seem to have been established as marginal channels 

 bordering the ice which lay thickest in the Salwarpe-Piddle Brook de- 

 pression. The first and the second glaciations were probably separated 

 by truly interglacial conditions (First Interglacial). 



The Second Glaciation came to an end in the Second or Great Inter- 

 glacial epoch which intervened between the deposition of the Older and 

 Newer Drifts. In the area under review we find at this stage that the 

 present directions of the rivers had been determined, and that the valleys 

 of those days can be recognised and their depths defined by the Kidder- 

 minster-Avon No. 4 Terrace, and perhaps by the ' High Terrace ' with 

 Hippopotamus in the Trent valley. There is, however, one exception to 

 this statement. I refer to the Iron Bridge gorge. This section of the 

 present river was non-existent at this time, and in its place was a high 

 watershed. The diversion of the Upper Severn across this waterparting 

 belongs to the story of the Newer Drifts. 



Newer Drifts. 

 (a) The Main Irish Sea Glaciation. 



This address has already become so long that I can only refer in the very 

 briefest way to the events that have occurred since the ' Great Interglacial.' 

 I have already mentioned that the Newer Drifts in the Midlands were the 

 product of the Alain Irish Sea Glacier, and I have attempted to define 

 its maximum extent on Fig. 2. This glacier belonged to the Third 

 Glaciation, 



The oncoming of this glacier seems to have coincided with the deepening 

 of the Severn valley below the Kidderminster Terrace level, in preparation, 

 as it were, for the great floods of sand and gravel that were fed into it as 

 soon as the ice crossed the old watershed near Iron Bridge and at the head 

 of the Worfe and Smestow valleys. These deposits are now the Main 

 Terrace, correlatives of which are the Second Terrace of the Avon and 

 probably the low terraces of the Trent and Tame. 



As the ice had been moving upstream in its invasion of the Dee and 

 Mersey basins, it must have impounded the drainage during the advance, 

 as we know it did later during the retreat ; but there seems to be no record 

 of an overflow into the Severn catchment during this growth stage. Pos- 

 sibly an overflow into the Trent may have existed during the advance ; 

 but the whole story of the effect of this glaciation on the east-flowing 

 drainage is at present very obscure, in fact, an interesting problem awaiting 

 investigation. 



As the ice melted back from the maximum position shown on Fig. 6, a 

 series of important drainage changes took place. First, at an early stage 



