74 MIDDLE ALBIAN STRATIGRAPHY 



Now, Inoceramus sulcatus is characteristic of and restricted to the orbignyi and 

 cristatum Subzones and yet the ammonites recorded from the nodule bed at 1424 

 feet (434-03 m.) depth include material of varicosum Subzone age. 



A re-examination of the material stated to have come from this nodule bed shows 

 that it includes ammonites from various Upper Gault horizons. The specimen of 

 Prohysteroceras (GSM. AT 3787) is indeed correctly identified. It is, however, a form 

 of the varicosum Subzone preserved in an identical manner to those of the basal 

 varicosum nodule bed in the Leighton Buzzard area indicating a southerly extension 

 of that bed. As it was found below the lowest recorded occurrence of /. sulcatus at 

 1400 feet (42672 m.) depth, the only possible explanation is that it must have fallen 

 from the side of the hole together with the other phosphatic fragments during the 

 collapse of the hole reported by Dewey (in Treacher & Dewey 1925 ; 450). 



Material acquired later from Templeman is preserved in the Palaeontology Depart- 

 ment of the Institute of Geological Sciences and shows that the Lower Gault under- 

 lain by mammillatum Zone sediments was in fact traversed by this boring. These 

 specimens, unfortunately, have no depth measurements recorded against them, but 

 include Euhoplites cf. opalinus (GSM. AT 4800) indicating the lautus Zone ; Euhoplites 

 of the meandrinus group (GSM. AT 4799) indicating the upper part of the loricatus 

 Zone ; and spathi Subzone Hoplites (H.) spp. (GSM. AT 4801-4) preserved in pebbly 

 gritty greyish phosphate. 



Unfortunately, this boring is now stratigraphically suspect, but if one disregards 

 the so-called phosphatic nodule bed at 1424 feet depth then it is possible to reinterpret 

 the lower part of the hole. The last record of Inoceramus sulcatus was at 1400 feet 

 (42672 m.) depth about 27 feet (8-23 m.) above the base of the Gault. This figure of 

 27 feet is not an unreasonable one for the Lower Gault when one considers the 

 geographical position of the boring. The highest record of /. sulcatus is at 1358 feet 

 (413-91 m.) boring depth which indicates that the combined thickness of the cristatum 

 and orbignyi Subzone sediments is at least 42 feet (12-80 m.) thick. This is a thick, 

 but not impossibly thick, sequence. 



(b) The area of the Kent Coalfield 



Despite the large number of borings and various colliery shafts which penetrated 

 through the Gault in the search for Coal Measures in Kent, only a small fraction has 

 yielded information on the stratigraphy of the Gault. Financial costs dictated that 

 boring through the Mesozoic rocks should be as rapid as possible and the sequence 

 was often chipped. However, at the following seven localities shown on text-fig. 31, 

 useful information has come to light and it is apparent that eodentatus and lyelli 

 Subzones sediments are of widespread occurrence. 



Chislet Colliery 



In the downcast shaft of the Chislet Colliery situated 3020 yds N. 54 30'E. of the 

 North Shaft (TR 232657) an exposure of approximately 12 feet (3-65 m.) was seen 

 of Lower Gault resting on the basal conglomerate of mammillatum Zone age which in 

 turn rests unconformably on Coal Measures (Casey 1961a ; 535). A phosphatised 



