PROCEEDINGS OP GEOLOGICAL SOCIETIES. 



455 



November 7, 1860. 



1. "On the Denudation of Soft Strata." By the Rev. 0 Pisher, M.A., 



r.G.s. 



The author first described the general features of the north-eastern portion 

 of Essex, with tiie table-lands of gravel, clay valleys, and tidal rivers. The 

 present configuration of the district cannot be due, in the author's opinion, to 

 the action of such causes as we see now in operation on the coast, combined 

 with a slow elevation of the land. As a rule, the sea waves cannot excavate 

 long narrow islets in horizontal and homogeneous beds, such as the gravel and 

 clay of the district under notice, but give rise to long, approximately straight 

 lines of cliff. The rounded sides of the Essex valleys seem to show they were 

 not formed by wave-action; nor are there any evidence of shingle-beds at the 

 foot of the hills. Mr. Eisber believes that the surface of this district, and that 

 of many other districts composed of yielding strata, must have been formed by 

 a superincumbent mass of water drained oil' from a flat or slightly dome-shaped 

 area. Slight depressions, cracks, or lines of readily yielding materials would 

 determine the drainage-streams as the water rctrcated"^ ; and these clianuels would 

 be more or less scoured out according to the velocity of the water. Where the 

 gravel covering of such a district was cut through, the clay beneath would be 

 channelled with a narrower valley; and where the gravel was wholly lemovedj 

 the valleys would be wider and the intermediate high ground rounded instead 

 of being flat to])ped, just as is represented in those parts of the district where 

 the clay composes the surface. Similar ap])earances may be seen on a small 

 scale in the mud of a tidal river. Tidal action, however, is not, according to 

 the author, calculated to excavate narrow valleys in horizontal beds. 



Mr. Eisher suggests that the land must have been elevated by a sudden 

 movement sufl&cient to have caused a rush of water from the raised portions to 

 seek a lower level, — either the land being raised high and dry at once, or the 

 sea-bottom raised to a higher level, though still remaining beneath water. Such 

 an elevation might be repeated again and again, with intervals of submergence; 

 and such conditions appear to hcive obtained in Norfolk as well as in Essex. 



The author states that, in his opinion, escarpments, such as are so common 

 among the secondary and tertiary beds, are rarely old cliffs, and their often 

 rounded forms must be due to agencies similar to those which have produced 

 the valleys of Essex. In some deep gorges of the Chalk near Dorchester the 

 author has seen flints and great blocks of Tertiary puddingstone so arranged as 

 to leave little doubt of their having been left by violent currents of water The 

 position of the Marlborough " Weathers" is also attributed by the author to 

 torrential action. 



Brick-earth is in part referred by Mr. Eisher to the deposition of sediment 

 from turbid waters ; but also in great part to the unlading of icebergs. 



With regard to the manner in which the uprising of the land, which brought 

 about these aqueous cataclysms, has been effected — whether by one slow and 

 continued movement, or by one or more sudden movements, or by a mixed 

 succession of these, the author argued that a slow and gradual elevation is not 

 in accordance with the contour of the existing surface of our softer strata ; that 

 the elevation of the land previous to the period of the great-mammalian fauna, 

 when its present contour was mainly given, was not gradual ; and that, after 

 subsequent depressions, there have been sudden depressions since that period. 



Lastly, it was pointed out that sudden vertical movements of the surface on 

 a grand scale are of as probable occurrence as those lesser movements with 

 which we are historically acquainted, because, both in the case of strata pre- 



* Compare with IsLr. Frere's remarks in the " Axchaeologia :" 1797. Er>, Geol. 



