78 SECTIONAL ADDRESSES 



It is significant that its deposits occur on the watersheds, and at the 

 same time reach into the valleys. 



The southern limit of the Main Irish Sea drifts is shown on Fig. 2. It 

 is generally marked by a great concentration of boulders. In some places 

 they are so numerous that all the garden walls in a hamlet may be built of 

 Scottish and Lake District granites. Most of the line between Bridgnorth 

 and Walsall on Fig. 2 is to appear on the forthcoming Dudley sheet of the 

 Geological Survey. I have the Director's permission and Mr. White- 

 head's consent to publish it. It was traced partly by myself, but chiefly 

 by Mr. Whitehead, to whom I am indebted for much help in this and other 

 matters. The continuation east of Walsall is based on earlier work by 

 the late F. W. Martin, H. G. Mantle and others. As shown by the alter- 

 native lines on Fig. 2, there is considerable doubt here as to the exact 

 limits. The same is true in the Trent and Shenstone valleys. 



It is now generally accepted that we can divide our British Glacial 

 deposits into ' Older ' and ' Newer Drifts.' The Newer can be recognised 

 by reason of the freshness and unaltered state of their surface features 

 which exhibit clearly original forms like kames, asar, kettle-moraine, 

 moraine-lakes, and so on. Their obvious influence on, and relation to, 

 the present drainage is another characteristic feature. On both these 

 counts, the Irish Sea Drifts of this north-western area must be regarded 

 as part of the Newer Drifts. Outside the line marking their limits the 

 rest of the Midlands belongs to the realm of the Older series, and has 

 remained extra-glacial since the time of the deposition of the latter. As 

 might be expected under these circumstances, the Irish Sea Drifts have 

 not undergone the extensive denudation to which the rest of the region has 

 been subjected. 



It is fortunate that the Irish Sea Glacier brought with it a great influx 

 of Scottish and Lake District erratics, for such vast numbers of them 

 found their way into the Severn via the Worfe and Smestow as to give a 

 characteristic lithology to the Main and Worcester Terraces, by which they 

 can be recognised with certainty as later than the terraces which belong to 

 the time of the ' Older Drifts.' In the latter such rocks are conspicuous 

 by their absence. 



The other two districts belong to the domain of the ' Older Drifts.' 

 We may consider next the eastern area. Here the most characteristic 

 drift is the Chalky Boulder Clay and its associated flinty gravels and sands. 

 Though there are other drifts present, these deposits are proved by super- 

 position to be the most recent. As is well known, the Chalky Boulder 

 Clay was the product of a mighty ice sheet to which Harmer gave the title 

 of the Great Eastern Glacier. In addition to chalk and flints, it always 

 contains much Jurassic material which varies with the outcrops over which 

 the ice had passed. The distribution of boulders of Charnwood and 

 Leicestershire igneous rocks and of Jurassic erratics shown on Fig. 3 

 allows us to trace the trend of its movements. 



The limits of the Great Eastern Glacier are indicated on Fig. 2. In 

 the main area, which is that lying east of the Tame and lower Anker, and 

 round Nuneaton, Coventry and Rugby, the drifts are the westward con- 



