THE R1DGWAY FAULT. 61 



organic remains which it contains offer nothing of special interest ; 

 the lower strata pass into beds of hard and rocky material, con- 

 taining some grains of green ^ilicate of iron, and throughout which 

 the flints of the upper beds become very rare. Messrs. Buckland 

 and de la Beche described the great Ridgway Fault as one of the 

 most curious and instructive they had ever seen, in consequence of 

 the variety of sections afforded along its course. They found 

 abundant evidence of slickensides in some of the sections examined 

 by them. The following description is taken in their own words 

 from the paper previously mentioned : " The Fault emerges from 

 the Chalk formation at Moignes Down Farm on the north side of 

 the Circus of Moignes Down, and brings the truncated lower ends 

 of strata of Portland stone against truncated upper ends of strata 

 of chalk, both dipping to the north. Here a valley of denudation 

 runs exactly along the line of fault, having its north side composed 

 of chalk and its south side entirely of Portland stone. The strata 

 have been raised on both sides of this fault, but raised unequally, 

 whence it results that on the north side the chalk rises to the fault, 

 whilst on the south side the Portland stone dips towards it, as if 

 plunging beneath the chalk ; whereas the Portland stone has been 

 elevated from its original position relatively, though not absolutely 

 much higher than the chalk. Yet, owing to the effect of denuda- 

 tion, no other results are visible than those of an ordinary valley 

 of denudation of horizontal chalk." Thus the authors proved this 

 to be an example of an upcast fault a rarer phenomenon than the 

 more ordinary faults with a downthrow. In this case it is the 

 strata on the south side which have been elevated relatively, and 

 which would otherwise have dipped beneath the chalk. The fault 

 extends about 15 miles in a due east and west line from Moignes 

 Down Farm to Abbotsbury, where it bends round towards the 

 north. "We have just noticed the section at Moignes Down. At 

 Button valley Portland sand forms the south side of the fault, and 

 greensand surmounted by chalk the north. At Upwey Portland 

 stone covered by Purbeck beds forms the south side and horizontal 

 chalk the north side. Near Portishani the fault deviates to the 



