16 THE aEOLOGY OF TYTHERINGTON AND GROVESEND. 



limestone paste. The upper beds are less coarse than those 

 below, but are of the same general character. Somewhat 

 farther on in the cutting these beds show well-marked 

 striated slickensides. 



At the 6|-mile post yellow and reddish calcareous marls 

 begin to be wedged in hetiveen the brecciated beds, the 

 brecciated beds above containing larger limestone fragments 

 and less milky quartz pebbles than those below. This 

 wedge of marly strata thickens towards the foot-bridge ; 

 but within it is a band of argillaceous limestone, which 

 thins out towards the bridge. Mr. Meredith's section does 

 not show this arrangement of the Triassic beds. 



The dovetailing of the marl with the brecciated beds is 

 an exceedingly interesting feature. It was noticed and 

 figured by Sir Henry De la Beche (" Survey Memoirs," 

 vol. i., pp. 241, 242, 249), in the Mendips and elsewhere. 

 But nowhere can it be seen more clearly than at the Thorn- 

 bury end of the Tytherington section. 



The Basement Beds of the Keuper in the Tytherington 

 section show very clearly the variable nature of the deposit 

 formed in the Keuper lake, under differing conditions in a 

 gradually sinking area. 



It should be noted that in Shillard's Lane, just to the 

 west of the Tytherington district, at Alveston, and along 

 the Mountain Limestone ridge leading to Almondsbury Hill, 

 Lias is mapped by Mr. Sanders and the Survey as resting 

 directly on the Palseozoic rocks. These beds there overlap 

 the Trias. The Palseozoics on which they rest were dry 

 land throughout the Keuper period. 



5. Modifications of the Map. 



The map published with this paper is based on and is on 

 the same scale as Mr. Sanders' map, with some slight modi- 



