STRUCTURAL GEOLOGY. 37 



through our district about east 30 north to west 30 south, that is 

 to say, approximately along the diagonal of the area shown on the 

 maps. 



This trough is steeper on the south-east side than the north- 

 west, the gentle dips of the northern limb being largely replaced 

 by faults. It is seldom easy to detect faults or small folds in the 

 soft Tertiary beds, and it is only by studying the levels of the 

 top of the Chalk as proved by borings that they become plain. 



Two faults occur in our area. The more important had not 

 been discovered when the special map was made; its course 

 lies from near Deptford to Wimbledon and Raynes Park Stations, 

 but it appears to die out towards Tol worth. At the Deptford 

 end it is replaced by a parallel fault to the south-east passing 

 through St. John's Station and Greenwich and across the River 

 to Beckton Gas Works. The down -throw of both faults is to 

 the north-west, that of the first being about 150 ft. at Raynes 

 Park; the second has long been known, since it brings the 

 Thanet Sand against the Chalk in the cutting at St. John's, part 

 of its course being shown on the map (Sheet 4). 



Three other faults are shown on this sheet, but should be 

 deleted. That between Woolwich and Erith has been pointed 

 out to many generations of students. There are several little 

 outcrops of chalk at the surface to the south of it ; at Crossness 

 to the north the Chalk is 140 ft. below the surface. The change 

 of level might be due either to a northerly dip or, if the beds 

 were horizontal, to a fault; the latter explanation was chosen 

 because a mass of pebbly material containing fossils occurs to 

 the north of and at a lower level than the outcrops of Chalk. 

 This material was taken to be Blackheath Beds in situ and is 

 shown as such on the map. Well sections, and other evidence 

 obtained since the original publication of the map in 1873, prove, 

 however, that the Chalk dips steadily north with no break; an 

 excavation near Abbey Wood Station showed that the pebbly 

 material rests directly on the Chalk, and has slipped from the 

 edge of the plateau above. The two ' faults ' indicated near 

 Ruxley are also nothing more than superficial landslips. 



Although faults of significant size appear to be confined to 

 the southern margin of the main trough, subsidiary folding 

 occurs throughout the area, and is of practical importance, 

 since it affects the surface topography and the water supply. 

 Mr. Wills's map of the sub-Tertiary Chalk surface 1 covers a 

 large part of our area. The complicated structure indicated 

 by the contours can probably be explained as the result of the 

 interaction of two sets of folds parallel and transverse to the 

 main axis. Thus in the south-east a synclinal fold runs parallel 

 to the trough through Mitcham, the Crystal Palace, and Shooters 

 Hill; a small transverse anticline running through Thornton. 

 Heath and Streatham deflects this fold, while another, which 



1 ' Records of London Wells ' (Mem. Geol. Surv.), 1913, Plate II. 



