in East Anglia in Tertiary Times. 



203 



of Saxmundham, but overlap occurs farther northwards, and in 

 Norfolk the London Clay rests directly upon the Chalk. The same 

 point is brought out by the distribution of the isopachytes of the bed. 

 Like those of the Lower London Tertiaries, they are convex to 

 a central axis (now north-east of that of the isopachytes of the 

 Reading Beds), but their gradient is much steeper, and their form 

 therefore better marked. It remains only to say that the London 

 Clay near the point of deflexion does not reveal more evidence of shore 

 action than that on the north and west. The thinning of the bed is 

 due to erosion, and the surface is inclined to be irregular. It seems 

 probable that slight uplift was taking place in the central area during 

 deposition, and sagging and sedimentation on each side of it, but it 

 was not until post-London Clay times that the movement became 

 very marked. 



<. — w.s.w. 



near 

 Feet. Chipping Ongar 

 400 H 

 300 



Stour Orwell 



Deben 



Orford 



Lowestoft 

 near j Yarmouth 



FIG. 2. — Section (to scale) across the axis of unrest in South-East Suffolk. 

 The line of section is taken parallel to the Eocene boundaries. 

 L.C. = London Clay ; D. + C. = Drift and Crag; L.L.T. = Lower London 

 Tertiaries ; O.D. = Ordnance datum. 



The unconformity of the Crag (considered as a whole) upon the Chalk 

 and older Tertiary beds is worthy of note. At Mundesley and other 

 places in North-East Norfolk, it rests upon the lunata zone (to retain 

 an old name) ; in the neighbourhood of Norwich, upon the mucronata 

 zone; in South Norfolk (Tharston, etc.), upon a very low horizon of 

 the latter zone ; and in East Suffolk possibly upon a high horizon of 

 quadratus Chalk. In South-East Suffolk, however, the Crag boundary 

 (see Fig. 1 ) osculates with that of the Eocene beds, or even passes a little 

 Avay within it. In this area the Pliocene beds are seen resting upon 

 Thanet Beds, Reading Beds, and, most frequently^, London Clay, but 

 not upon the Chalk. On passing into West Suffolk we find the Crag 

 again resting upon bare Chalk of the cor-anguinum zone, e.g. at Stoke- 

 by-Clare. 



One of two things has therefore happened. Either the Crag 

 boundary was once more or less parallel to that of the Eocene beds, 

 and retreated as a result of erosion to its present position in pre- 

 TJpper Glacial times (pre-Glacial of the area under consideration), or 

 the Crag was never deposited over part of the critical area. The latter 

 case would yield evidence of the anticlinal form of the central area in 

 Pliocene times; the former, of differential uplift in the area, resulting 

 in greater erosion of the Crag, since the general level of the country in 

 South-East Suffolk is as high as on the north-east and south-west, and 



