68 



Wood's name of Middle Glacial Gravel. It is mapped 

 on the survey map as glacial gravel [pink]. It is cut 

 off sharply at the brow, never descending near us below 

 the 200 feet contour line into the valley; it lies on a surface 

 which had evidently been levelled by marine action, and 

 through it streams have cut gullies which previously had 

 no existence. Its composition is of flint, tertiary pebbles, 

 and so great an abundance of Northern rocks as to warrant 

 the conclusion that it is the marine wash of materials trans- 

 ported by glacial action from a distance. This layer is 

 much thinned out on its Southern limit, and if it (or the 

 Boulder clay) extended Southward across the valley of the 

 Thames, the quantity was insufficient to resist that denu- 

 dation which has cleared so much from the country on the 

 North of the Thames and left any remaining in situ. 



I have examined the higher land around Shooters Hill 

 and the table-land from Blackheath to Erith especially, 

 and although I think that the glacial gravel or clay may 

 have reached so far, for quartz and quartzite -pebbles are 

 not uncommon, yet as most of the table-land lies below the 

 200 feet contour, these diminished relics are not distinctly 

 separable from those which might have been deposited by 

 the Thames, which commenced to spread erratics derived 

 from the glacial beds, from about the above level down- 

 wards to its present bed. 



With the glacial gravel it is almost impossible to 

 avoid an immediate coupling of the chalky Boulder clay, 

 which lies upon it to so large an extent, and which con- 

 stitutes the latest widespread deposit of Middlesex and 

 Essex, covering .the land like a "pall." Like the glacial 

 gravel it lies at its Southern boundary between 200 and 

 400 feet contours.* 



This Boulder clay is not so homogenous a material as 

 to preserve its constituent materials through ages without 

 change, consequently we find that its surface has been 



* Compare remarks on a thick sheet of Boulder clay on the Weald 

 clay at Shipbourne near the same level, &c., further on. 



