Geology and Physiography 5 



were proved; the latter to a depth of 660 ft. below o.d. Eastward, how- 

 ever, at Culford and near Harwich, Cretaceous rocks rest directly upon 

 much older strata assumed to be Lower Palaeozoic; the Jurassic system has 

 disappeared. It would be of considerable interest to know the depth and 

 nature of tliis Palaeozoic floor. To this end, Bullard, Kerr-Grant, and 

 Gaskell have applied the refraction seismic method with some success, 

 and have obtained the results which are summarised briefly in Fig. 2. 



Fig. 2. 



Depth of the Palaeozoic Floor in feet below O.D. Observations at further stations 

 show that the valley to the west is less deep than is shown by the contouring above. 



The stations shown above are as follows : 



These results are provisional and may be slightly different from the values 

 fmally published. The experimental error, due to inaccuracies of measure- 

 ment and difficulties of interpretation, is of the order of 50-100 ft. The 

 form of the contours round the margins of the map is based to some extent 

 on a number of seismic stations and bores which lie outside the area. 



