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PEOCEEDIXGS or THE GEOLOGICAL SOCIETY. [Feb. 26, 



2. Physical GeograxjJiy. — As we stand on the high ground near 

 Hertford Heath, Brickenden, Bayford, Essenden, or the upper part 

 of Hatfield Park, we can hardly fail to observe that all those flat- 

 topped hills, together with the high wooded country north-west of 

 Bramfield and north of St. Albans, form part of a great plain ex- 

 tending as far as the eye can reach in all directions, the boundary of 

 which is somewhere far beyond the district under consideration. 



Out of this highest plain, a great valley has been excavated, the 

 bottom of which itself forms another plain of very considerable 

 extent, upon which Bayfordbury, Hertingfordbury, Bengeo, Bram- 

 field, Cole Green, Welwyn Junction, and the lower part of Hatfield 

 Park stand. The boundary of this lower or valley-plain is well 

 defined. In the district under examination, it is bounded on the N. 

 by the hills which rise suddenly behind Bramfield, on the W. by 

 the hills which run from Welwyn to St. Albans, and on the S. by 

 the hills which extend from Hertford Heath to Hatfield Park. It 

 spreads out to the S. between St. Albans and Hatfield, and runs to 

 the E. and S.E. by Ware and Parndon, while it probably extends 

 round the base of the high ground behind Bramfield far to the N.E. 

 Out of this lower or valley-plain the small and comparatively new 

 valleys of the Rib, the Beane, the Mimram, and the Lea have been 

 excavated. 



The section (fig. 1) is drawn N. 15° W. and S. 15° E., through Hert- 

 ingfordbury, from Brickenden Green, about three miles south of 

 Hertford to Bright's-Hill Wood, about one mile north of Bramfield, 

 and is intended to show the relation of the two plains to one another, 

 and to the present river- valleys. 



Fig. 1. — Section from Brickenden Green, three miles south of Hert- 

 ford, to Bright's-Hill Wood, one mile north of Bramfield. 



a. Boulder-clay. 



b. Subangular gravel, sand, and loam of 

 Lower Plain, with boulder-clay under it, 

 in it, and on it. 



c. Pebble-gravel of Higher 

 Plain; large percentage of 

 quartz. 



d. Chalk and Tertiaries. 



3. Gravel of the Upper Plain. — Now let us examine the character- 

 istic Posttertiary deposits of these two plains, and see what light 

 this will throw on their relative age. 



The Boulder- clay rests on, never under or in, the gravels of the 

 Upper Plain ; but, as it overlaps the gravel, and lies equally on the 

 slopes of the hills and on the Lower Plain, its manner of occurrence 

 only proves that the gravels of the Higher Plain are older than any 

 Boulder-clay in that district. 



