1868.] HUGHES HERTFORDSHIRE GRAVELS. 283 



among the Cheddar Cliffs furnish similar and even more convincing 

 evidences, not only of the action of water, but of water so charged 

 with solid matter as to be capable of grinding rock-surfaces. In 

 these and other limestone caverns, the roof is often hollowed in such 

 a way as to indicate a powerful and upwardly directed grinding 

 agenc5% such as is now only possessed by sea-waves. 



From the amount of knowledge hitherto collected, it would seem 

 impossible to form a consistent scheme of the later oscillations to 

 which the shores of the Bristol Channel have been subjected. The 

 statements concerning Roman and other relics found under silt and 

 peat near Bridge water, and further towards the north, contained in 

 the ' Transactions of the Somersetshire Archaeological Society'*, can- 

 not be consistently generalized by supposing any great changes in 

 the relative levels of land and sea to have occurred in historical 

 times ; but, with a little allowance for inaccuracy, they can be in- 

 cluded in a probable explanation by supposing that the natural or 

 artificial embankments of the sea have, in different places and at 

 different times, been breached, so as to lay the lower land behind 

 under water for periods of greater or less duration, and that the 

 extent of dry land has been increased by the growth of peat in 

 marshes and a process of natural silting up, which causes an appa- 

 rent retreat of the sea without any real change of level. A good 

 section of the internal arrangement of the mud-flats was very lately 

 (1866) furnished by digging a well, in presence of the author, near 

 Mr. Candy's house, Lower Clevedon. Order descending : — 



feet. 



Blue clay, with a few stones 10 



Peat, with alder-branches and a few bones of deer (?). 2| 

 Fine light-coloured sand, thickness unknown. 



In the immediate neighbourhood, at the base of the limestone es- 

 carpment, the detrital covering of the hills seems to run under the 

 sand. 



5. On the Two Plains of Hertfordshire and their Gravels. By 

 T. M*=K. Hughes, Esq., M.A., E.G.S., of the Geological Survey of 

 England. 



Contents. 



1. Introduction. 



2. Physical Geography. 



3. Gravel of the Upper Plain. 



4. Gravel of the Lower Plain. 



5. Eiver- Gravels. 



6. Summary. 



7. Conclusion. 

 Postscript. 



1. Introduction. — The observations on which the following paper is 

 founded were mostly made in the spring months of 1865 and 1866, 

 while engaged in carrying on the Geological Survey of part of the 

 neighbourhood of Hertford, and are now published with the per- 

 mission of the Director-General of the Survey. 



* See references to some of these statements in Mr. Poole's article, Quart. 

 Jo urn. Geol. Soc. vol. xx. p. 120. 



