78 SECTIONAL ADDRESSES 



for the deposits of Welsh and Northern Drift often lie side by side. It 

 is now generally believed that the Welsh and Irish Sea ice-sheets were 

 practically contemporaneous, although the northern ice seems to have 

 arrived first and weakened and retreated first ; thus, on the Welsh Borders 

 it was overridden by the Arenig Ice which carried Welsh erratics as far 

 as Wolverhampton and Birmingham — indeed, beyond the limits pre- 

 viously reached by the northern ice. That this phenomenon was but 

 evidence of the give-and-take of glaciation has, however, been demon- 

 strated by the officers of the Geological Survey, who have traced out 

 the junction between the two ice-sheets, marked by morainic belts, 

 kettles, meres and peat-bogs, along a line approximately through Wrexham 

 and Ellesmere towards Shrewsbury. On the shores of Cardigan Bay, 

 North Wales, and Lancashire, and also in Anglesey there are, however, 

 two beds of boulder clay, sometimes separated by sands and gravels. 

 In some places the boulder clays differ in composition, but in others 

 they resemble one another closely. Also, Dr. Bernard Smith noted near 

 Ellesmere, in the Cheshire Plain, the presence of a boulder clay lying 

 above mounds of sands and gravels like those that overlie the boulder 

 clay of the Newport district. The two coastal boulder clays have not 

 yet been traced inland, but general opinion is that only the lower extends 

 to Newport and Buildwas (the main Irish Sea glaciation), while it is the 

 upper clay which seals the caves at Cae-gwyn and Ffynnon Beuno. In 

 the mountain-district of North Wales, the evidence of two glaciations 

 has not been clearly worked out, but several investigators have observed 

 features referable to two ice-advances with retreat-stages between. In 

 fine, the Lower Boulder Clay of Lancashire, Cheshire and Wales would 

 appear to be correlatable with the Early Scottish glaciation of the Lake 

 District, and the Upper Boulder Clay with the Lake District Maximum 

 and Hessle Boulder Clay. 



The Severn Drainage. 

 A study of the overflow-channels of the extra-glacial Lake Lapworth 

 (with its earlier stages. Lakes Newport and Buildwas) at Ironbridge, 

 which were formed during the retreat of the northern ice, has enabled 

 Dr. L. J. Wills in a masterly paper to connect the terraces of the river 

 Severn below the Ironbridge Gorge with the glacial phenomena of the 

 Cheshire basin above it. He regards the ' Main Terrace ' of the Severn 

 as corresponding to his Stage II and subsequent stages of the ice-retreat, 

 as deduced from glacial lake-levels and terrace-gradients. Another 

 connecting-link with the drifts and river-deposits of the Midland area is 

 provided by the overflow channel at Gnosall, which discharges into Church 

 Eaton Brook and so into the Trent Basin. Mr. E. E. L. Dixon informs me 

 that with this channel is connected a valley-train of sand and gravel that 

 occurs at a low level, and is therefore later than the old Trent river- terrace. 



The Avon-Stour Area. 

 The absence or rarity of mammalian remains and implements in the 

 terraces of the Lower Severn has rendered very difficult Dr. Wills 's efforts 

 to correlate these deposits with those of the river Thames. Meanwhile, 



