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The Geology of Cheddar Cliffs. 



By Dr. William T. Ord, F.G.S., Chairman of the Geological Section. 



(An Address delivered in Cheddar Gorge at the excursion of the Society 

 on May 29th, 1912). 



1\T EMBERS of this Society have on two previous occasions 

 visited the Mendips to study their geological structure, 

 namely, in June, 1909, and in June, 1910,* but this is the first 

 occasion when Cheddar, and its magnificent cliffs, have received 

 a visit. The occasion being of some importance, it was thought 

 that a sketch of the origin and formation of this stupendous work 

 of Nature would probably interest many who knew nothing of 

 geology. 



The height of the cliffs, shown in plate IV., is 600ft., and 

 the rock of which they are formed is a variety of limestone, known 

 to geologists as the Carboniferous limestone, because it was found 

 in the great period of geological time, which also gave us the 

 Coal Measures. The Mendip Hills consist of five great elongated 

 masses of Carboniferous Limestone, arranged en echelon, and 

 running from E.S.E. to W.N.W. The total thickness of this 

 limestone in the Mendip area is 2,000ft., of which only 600ft. 

 are exposed in Cheddar Gorge at the point of observation. Now, 

 limestones differ from other rocks that have been deposited as 

 sediments from mud, or sand, or shingle, in that they have been 

 built up almost entirely by living organisms. It is a stupendous 

 fact that these 2,000ft. of solid limestone rock have been formed 

 particle by particle by the growth, decay, and death of lowly forms 

 of life, which all had this peculiarity, that they formed or lived in, 

 calcareous shells or shelters. To understand how this took place, 

 one must picture this part of England, in that remote period, 

 covered by a moderately deep sea, and enjoying a tropical climate 

 much as is now found in the Pacific. In this sea lived countless 

 myriads of corals, shellfish, sponges, and zoophytes ; these were 

 similar, though of different species, to those now existing in 

 tropical seas. All these creatures absorb lime from the sea, and 

 build it into the various forms which compose their shelters or 

 their homes, and these, sinking to the ocean bed, as their owners 

 died, during a long period of time slowly built up the huge thick- 

 ness of limestone which we see at Cheddar. This building up 

 was partly helped by the corals, which all the time were briskly 

 constructing vast coral reefs, perpetually added to by succeeding 

 generations of corals, as the older ones sank with the ocean bed. 

 For. as this process went on, the sea bottom very slowly sank, 

 so that after some 2,000ft. of sediment of lime fragments had 

 rjeen deposited, the sea depth was probably about the same as 

 at the commencement of the process. But now a change took 



* Proceedings Vol. I., p. 51, 67 ; Vol. II., p. 43. 



