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 GEOLOGY The age of the older embankments is unknown, but Mr. Spurrell says there are none below Purfleet so old as the Roman period ; ' and the fact that the marshes so protected are now in many places below the level of high-tide is attributed by Mr. Whitaker to the drying and con- traction of the alluvial deposits since they were preserved from inundation.* The drainage off the alluvial tracts before they were embanked led to the formation of irregular islands, which have retained their outlines since they were protected. Some like Canvey and Foulness are wholly alluvial and recent. Others like Osea and Northey in the Blackwater valley, are formed partly of London Clay and partly of Gravel or Allu- vium. In the Crouch valley there are ancient islets of London Clay no longer surrounded by water, but enclosed by marshland. Bricks arc made from the alluvial clay on Bridgemarsh Island south of Latch- ingdon. Although the tidal waters of the Thames ascend as far as Richmond and Teddington, nevertheless its estuary is sometimes taken to mean that portion east of a line drawn between Southend and Sheerness, onwards to the bank known as the Kentish Knock. The area off Essex is composed of broad banks and shoals of sand and mud with shells, trending north- east and divided by channels known as 'deeps' which are connected with the Thames, Crouch and Blackwater. Some of the isolated banks as well as the broad foreshore known as the Maplin Sands are dry at low- water. The Maplin Sands, though shifting sands, are mostly firm when uncovered, although there are muddy tracts near the shore. The condi- tion of the estuary is regarded by Capt. T. H. Tizard as due to the action of the sea in casting up banks, and of the tidal flow in cutting channels through the banks thus formed.* Changes in the depths of channels and banks are continually in progress, and their forms are liable to more abrupt alterations during gales. The presence of peaty beds in the Alluvium indicates that the land has subsided some twenty feet or more, and consequently that the tidal waters are enabled to extend higher up than was formerly the case. As the Rev. O. Fisher has remarked, ' The tidal rivers are evidently nothing more than a continuation of the valleys beneath the sea-level.' ' The estuarine character of the waters is favourable to the cultivation of oysters, and the Colne, Crouch and Blackwater form famous grounds for the Colchester and Burnham ' natives.' Nevertheless there has been much silting up of the tidal creeks, and Maldon in old times was a more important port than it is now. Much waste is going on along the coast between Harwich and Southend, waste which has in places been greatly accelerated by land- slips. The greater losses have taken place between the mouth of the 1 See C. Vancouver, General new of the Agriculture of Enex (1795) 5 and Spurrell, 'Early Sites nd Embankments on the Margins of the Thames Estuary,' Artb. Jcnr*., vol. xliii. p. 269. vol. xlix. p. 67. 21
 * GuiJe to Geol. LonJm, ed. 5. (1889), p. 78.
 * Nature, April 10, 1890, p. 539; see also J. B. Redman, ' The River Thames,' Pne. Intl. C. S.,
 * Quart. Journ. Geol. Soc., vol. xvii. p. I.