GEOLOGY OF PART OF SOUTH DURHAM. 



197 



Fig. 20. — Denudation of Brockwell coal, George Pit, Etherley. 



the most extensive one being that following the course of the 

 valley of the Team, at a considerable depth below the present 

 surface. In the instance figured the "wash" is formed of silt, 

 sand, and clay, with gravel and boulders : the sand and clay are 

 forced several feet into the planes of bedding of the seam, and 

 the boulders are all more or less worn. 



In connection with these comparatively recent deposits may 

 be mentioned a very curious, eroded surface of the Magnesian 

 Limestone, that occurs beneath a drift clay, in a quarry south of 

 Morton Tinmouth. The Magnesian Limestone there, like most 

 of that rock seen in this part of the county, belongs to the lower 

 portion of the series, and is yellow and thin-bedded with marly 

 partings. Covering it for a thickness of eight or ten feet is a 

 deposit of clay with boulders, such as is usually classed as boul- 

 der clay. The surface of the limestone, as seen in section, 

 is worn into deep and irregularly shaped cavities, with even 

 more irregularly shaped prominences between. Fig. 21 repre- 

 sents a portion of the surface as seen by one of us in April, 1867. 

 The lower beds of limestone are hard, with well marked planes 

 of stratification: the upper portion is soft and not so well bedded. 



It is difficult to explain this eroded rock surface simply by 

 mechanical action. The cavities are not like the "pot-holes" 

 and other hollows formed by running water or by the action of 

 waves on coast-lines, nor yet like any water or weather-wasted 

 surface with which we have acquaintance. Its origin is probably 

 in a great measure chemical — similar to that of " sand-pipes" in 

 the chalk, and in other calcareous rocks. 



