Yol. 52.] PLIOCENE DEPOSITS OF HOLLAND. 769' 



while recognizing the Chillesford Clay as distinct, thinks that it may 

 be equivalent to the Weybourn and Belaugh Crag (referred to 

 below), but the extraordinary abundance of Tellina balihica in the 

 latter, and its entire absence from the former, seem to me strongly 

 opposed to that view. At Belaugh and Weybourn more than half 

 the specimens present are those of this species. The Chillesford 

 Clay is not often fossiliferous, but where it is, although other species 

 of Tellina commonly occur, this form has never been found. 



I am far from thinking that every example of laminated clay in 

 East Anglia should be referred to this horizon. At the same time 

 there are a number of sections of what Mr. Woodward calls ' good 

 Chillesford Clay,' as to which no doubt has ever been expressed, 

 which were originally regarded as such by Sir J. Prestwich, and 

 which were so mapped 25 years ago by Mr. Wood and myself, and 

 since then by difTerent officers of H.M. Geological Survey. These 

 exposures show that the Chillesford Beds are in places nearly 20 feet 

 in thickness, and that they maintain for nearly 70 miles their 

 distinctive character. They seem to me not only to form a definite 

 geological horizon, but to mark a decided change in the geological 

 condition of the Pliocene basin. 



The abrupt change, first from the sands of the Norwich Crag, 

 full of drifted and comminuted shells, to the finely laminated 

 micaceous clays of the Chillesford Beds which rest on them, and then 

 to the newer and overlying pebbly gravels is, I think, capable of 

 explanation. 



Beds of clay and mud can only originate in quiet water, either at 

 a depth too great to be affected by the movement of tides and 

 currents, or in a position sheltered from their influence. They are 

 especially characteristic of the tidal estuaries of flat countries, being 

 deposited in slack water on the top of the tide, not so much in the 

 channels in which the current flows as upon banks on either side of 

 it, which are alternately covered and uncovered by water. On these 

 the mud quietly settles in the form of films and thin laminae. The 

 character of the sediment may vary with the season. During the 

 floods of winter the water is often turbid with the coarser matter 

 which it contains, while in dry weather the suspended material is 

 of a finer character. It is, moreover, in the higher reaches of the 

 estuary, where the scour is least, that banks of clay principally 

 accumulate ; these are eventually raised above the level of the 

 water, and form marshes bordering the stream. Towards the mouth 

 of the valley such beds become intermittent, and the mud is inter- 

 stratified with sand and gravel. 1 



1 Shallow-water deposits of clay and silt are not, however, entirely confined 

 to estuaries. Eeference should perhaps be made to the alluvium which fringes 

 the eastern margin of Lincolnshire, connecting the estuarine beds of the Humber 

 with those of the Wash, but the destruction of the Holderness coast, which 

 has been so long, and is still, going on, makes it probable that the left bank 

 of the Humber formerly extended fartber south than it does at present, so 

 that to some extent the Lincolnshire alluvium may be of estuarine origin. 

 Scrobicularia piperata, a species common in muddy estuaries, is characteristic 

 of these beds. 



