1870.] 



METEE — PORTSMOTTTH LOWER TERXIARIES. 



83 



the clay escarpment ; but their relation to the principal gravel-bed 

 has not been clearly shown. 



Passing from the gravels to the deposits next in age, we come to 

 various beds of silt and silty clay underlying the most recent mud- 

 deposits of the harbour. These beds have been exposed in clear 

 vertical sections many hundred feet in length. In all of these 

 sections the underlying Tertiary deposits are shown to have been 

 cut away to a smooth, if not always to a level surface, the depth 

 to which they have been eroded varying from a few feet above low- 

 water level, as along part of the general Section (fig. 1), to that of 

 from 20 to 30 feet beneath low water in the north-west portion of 

 the area. 



The surface of the mud over the whole area stands at from 6 to 

 7 feet above low-water level of the ordinary spring tide. 



The following section (fig. 4) exhibits the principal features seen 

 in these deposits. 



Fig, 4. — Ideal Section showing the Relative Positions of the Gravel- 

 and Mud-deposits. 



X. Gravel. 



A. Old mud-deposit, with stumps and roots of trees. 



B. Recent mud-deposits. 



C. Shingle. 



In this section the gravel-bed, marked x, and the older and 

 newer mud-deposits, A and B, are shown at their relative levels in 

 relation to the present high and low water. It is not improbable 

 that this section, which represents only a very small portion of the 

 great mud-flat between Portsmouth and the foot of the chalk 

 escarpment of Portsdown, may serve to illustrate the general con- 

 dition of the surface-deposits of the harbour. 



There can be little doubt that the gravel-bed (ce) was at one time 

 continuous over a great portion, if not the whole, of the surface of 

 the harbour now covered by mud or water. At what time or in 

 what manner it was denuded I shall not stay to consider ; it is suf- 

 ficient to know that the denuding agent, whatever it may have been, 

 has cut down to the underlying sands and clays of the Eocene- 

 beds, which present, in all the sections I have examined, a cleanly 

 swept surface beneath the mud. 



The older and newer mud-deposits, A and B, shown in the 

 mud-section (fig. 4), were probably formed under very similar con- 

 ditions. The bed A is first seen in the sections at about 300 feet 

 from the low gravel-capped escarpment, and spreads out northward 

 and westward until cut off^, as it were, by the deep water along the 



g2 



