114 THK UNDULATIONS OK THE CHALK IN ESSEX. 



of the Chalk occurs, one, two, three, or more, hundred feet above or 

 below the sea-level, the ciphers being omitted for the sake of distinct- 

 ness, and the plus and minus signs respectively indicating height 

 above and depth below the Ordnance Datum. The zero^ of course, 

 implies that the Chalk at or near that line is just at sea-level. The 

 straighter lines are faults whose existence is imperceptible on the 

 surface of homogeneous clay, even where not concealed by drift, but 

 which are sufficiently established by their effect on the Chalk contour- 

 lines. 



The Chalk outcrops from beneath the Tertiary sands along lines 

 running from Sudbury to Bishop's Stortford, and from East Tilbury to 

 near Wennington. To the north and south respectively of these 

 lines of outcrop occur isolated patches of the Tertiary beds, only the 

 more important of which can be shown on so small a map. 



The Chalk is not everywhere at the surface in the spaces de- 

 nuded of their original Tertiary covering, for (ilacial and Post- 

 Glacial drifts mask both Tertiary and Cretaceous areas, and much 

 of the ground that is shown as Chalk in the map consists of these 

 gravels and clays, extending to depths of sometimes more than loo 

 feet. 



I have considered it impracticable to attempt to make out any 

 undulations in the Chalk beyond the Tertiary boundary, for the 

 simple reason that where the eroding forces of the Glacial sea laid 

 bare the Chalk, they dealt with it as erratically as with the Tertiary 

 beds, cutting it into deep and shallow, at the will of the changing 

 currents. Instead, therefore, of an undulating plane whose position 

 may be calculated with a fair approximation to accuracy, as is the 

 case under the Tertiary area, we cannot safely pronounce on the 

 position of the Chalk under the Drift fifty yards beyond where it is 

 seen, or proved by boring. Mr. Whitaker has shown us'^ how in the 

 Cam valley, between flanks of Chalk, the alluvium, barely 300 

 yards wide, conceals a drift-filled fissure of great depth, showing that 

 calculations from exposures in such an area are liable to be 

 completely erroneous. When East Anglia becomes the scene of 

 numerous collieries, perhaps we shall learn from the undulations 

 of the under-surface of the Chalk that which we cannot gather from 

 the open surface, which was lost in the Glacial period. Accordingly 

 I have left that region untouched, and dealt only with the area where 

 the (ilacial erosion has not succeeded in reaching the Chalk. 



2 Essex Nat. \o1. iii pp. 140-142, i88y ; (Juan. Jouni. Geol. Soc. vol. xliv. pp. 333-340 [1890). 



