.66 SECTIONAL ADDRESSES? 



base of the formation. These lithological changes suggest that during the 

 deposition of the Rhaetic the surface was inclined from west to east. 



In the Vale of Glamorgan, however, only the lower zones of the Lower 

 Lias are now preserved. In the absence, in the ascending succession, of 

 definite evidence of conditions indicating the proximity of a shore line 

 or of shallowing of the area, it can hardly be supposed that no higher 

 Liassic strata were deposited. Taking into account the variation in thick- 

 ness of the Lias, as proved at the outcrop and in various borings, I believe 

 that anything between 600 feet and 1 ,000 feet of that formation maj^ have 

 been deposited in the region of the Bristol Channel. 



It is true that the thickness of the formation in Somerset and South 

 Gloucestershire is usually less than the lower of these figures, but such 

 evidence as exists goes to show that apart from local irregularities the 

 thickness increases westward of these places as well as southward and 

 northward, and in North Gloucestershire the formation attains a thickness 

 of nearly 1,400 feet. The amount of deposit accumulated at any place was 

 probably controlled in the main by the amount of subsidence, and not so 

 much by interruptions of sedimentation due to upheaval. The unequal 

 subsidence during the deposition of the formation has, however, to be 

 borne in mind in estimating the deformation that these rocks have suffered 

 in subsequent geological periods. 



Although the thickness of the Lias west of the Cotswolds exceeds 

 1,000 feet, it appears to fall off rapidly eastwards, and borings in the 

 London area, at Ware in Hertfordshire and Culford in Suffolk, all proved 

 the absence of the formation on the Palaeozoic floor. Whether the 

 formation reached its maximum thickness in the neighbourhood of its 

 present outcrop, or continued to increase farther to the west, it is impossible 

 to say, but we find that at Frees Heath nearly 70 miles west of the base of 

 the formation, the Lower Lias, which is there succeeded by Middle Lias, 

 has been proved by boring to exceed 400 feet. 



In view of this considerable development of the Lower Lias at Frees 

 Heath, and the presumption of some additional thickness of Middle and 

 Upper Lias having been deposited, and in view also of the great develop- 

 ment in North Gloucestershire within a few miles of the Falaeozoic rocks, 

 and the occurrence of the formation in the north-east of Ireland, it would 

 be rash to assert that the Lias did not extend over the Falaeozoic area of 

 Wales and the Welsh Borders ; the formation may, indeed, have attained 

 a thickness of several hundred feet over that area. If one could be sure 

 that the Falaeozoic region remained relatively stable during the deposition 

 of the Lias, one would feel little doubt of the extension of the formation 

 over it in force, but if Lamplugh was correct in assuming a zone of 

 maximum thickness at or near the present outcrop the subsidence of the 

 crust in that area might have been complementary to an upward warping 

 to the west. In that event the Lias may not only have thinned away very 

 rapidly towards the Falaeozoic upland, but even deposits formed at one 

 stage may have been removed from the area during a later stage of the 

 Liassic period. 



Of the Jurassic formations that intervene between the Lias and the 

 Corallian, the predominant member is the Oxford Clay, which was 

 probably laid down under much the same physical conditions as the Lias. 



I 



