C— GEOLOGY 



83 



6. The presence of a great lobe of ice down the Stour and down the 

 depression (referred to above), east of the present Severn gorge, 

 could supply a reason for the anomalous way in which the River 

 Severn selects the high Palaeozoic ground in preference to the wide 

 Triassic depression ; for the river might have taken this course 

 when it consisted of the marginal drainage off the west side of 

 the glacier. As such it might have cut a large marginal channel 

 which on account of its depth was permanently retained in later times. 



The River Terraces as Evidence of the Stages in the Erosion. 



If we are right in claiming a former far wider distribution of the drifts 

 than the areas where they now occur in force, the river valleys should 

 provide a great deal of evidence concerning the way in which their de- 

 struction has been brought about. In the present case this is certainly so ; 

 for we have in the Severn and its tributaries a wonderfully developed 

 system of river terraces and of deposits that originated under the rigorous 

 conditions of glacial climates, the so-called taele gravels and meh- water 

 flood gravels. A study of these has thrown much light on our problem. 



The farther we go from the plateau and from the drift-covered ground 

 on its north-west and eastern sides the greater the number of high level 

 terraces. In some cases the geographical distribution and the lithological 

 composition enable us to relate a terrace to a particular set of glacial 

 deposits. 



Table 



* Miss Tomlinson's nomenclature. * Height of top/height of base. 



* B, Bunter Pebbles ; W, Welsh ; F, Flints ; M, Malvernian ; S, Scottish and 

 Lake District. 



