MARINE GEOLOGICAL EXCURSION. Ixi. 



St. Ealdhelm's (commonly called St. Allan's) Head and Etwnit Hill. St. 

 Ealdhelm's Head (353 ft.) is a prominent point showing a fine geological section. 

 We now perceive some of the effects of the Kimmeridge anticlinal, since 

 Kimmeridge Clay makes its appearance, though much obscured by fallen blocks 

 of Portland Kock, which also run out seawards and help to increase the turmoil 

 of the " Race." Above the Portland Sands there is an unusual development of 

 the Portland chert -beds, and it is this circumstance which seems to have 

 determined the salience of the most southerly and conspicuous headland in the 

 Isle of Purbeck. St. Ealdhelm's Chapel is just on the edge of the Lower Purbecks, 

 which there constitute the platform. Turning the corner and steering north for a 

 while, we note the long line of Emmit Hill on the right ; this is a fine example of 

 an escarpment face, where the Kimmeridge Clay at the base is succeeded by 

 Portland Sands, and these by Portland Eocks up to the summit platform. It is 

 here that the Portlandian escarpment, so characteristic of the Kimmeridge basin, 

 commences to wind inland, and, after producing the eminences of Hound-tout 

 and Swyre Head, passes in a noble sweep through Smedmore Hill and Tyneham 

 Cap into the final precipice of Gadcliff. 



Chapman' 1 's Pool and JZncombe. 'Bot'h these places represent deep excavations 

 in the Kimmeridge Clay, which here attains an exceptional thickness. The 

 Upper Kimmeridge of these parts is reckoned to be nearly 700ft. thick, and is 

 characterised by paper-shales, bituminous shales, and cement-stone beds, which 

 latter in their continuation seawards form the dangerous Kimmeridge ledges. 

 The cliffs about Chapman's Pool are noted for Upper Kimmeridge fossils, such as 

 Lucina minuscula, Discina latissima, and, above all, Ammonites liplex (Olco- 

 stephanus pallasianus). Our late President obtained many excellent fossils from 

 these beds of Kimmeridge Clay, owing to his connection with Smedmore. 



The Kimmeridge Coast. This may be described as extending from Chapman's 

 Pool on the east to Brandy Bay on the west, a distance of nearly 5 miles. The 

 stratigraphical features of this basin are interesting. The coastal cliffs are not 

 very lofty, though constantly disturbed by a series of small faults, which tend to 

 counteract the prevailing easterly dip until the inlet of Kimmeridge Bay is 

 reached. Here there occurs a kind of dome or anticlinal axis, the general effect 

 of which is to bring up lower beds of the Kimmeridge series. Thus beds of 

 undoubted Lower Kimmeridge occur in Hobarrow Bay. West of Kimmeridge 

 Bay the beds dip very strongly to the N.W., whilst the downthrow of the 

 numerous small faults is to the east, thus counteracting the effect of the dip, 

 which, however, is much steeper than on the south-east side of the axis. Owing 

 to the high angle of dip the exposure of Kimmeridge Clay is shorter on the 

 Brandy Bay side ; under Tyneham Cap the K.C. attains an elevation of 500ft., 

 with a dip of 23 N.N.W., before disappearing beneath the Portlandian 

 escarpment. The Broad Bench is a salient platform, just above sea-level, made 

 up by stone beds in the K.C. It should be noted that Kimmeridge Coal occurs at 

 Clavell's Hard in the form of a bed of block-coal over 2ft. thick, succeeded by 

 10 or 12ft. of bituminous shales. Traces of this coal may also be seen on the 



