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 GEOLOGY Irregular hollows of the old estuary however remained and held great sheets of water, some of them in the direct course of the rivers, others isolated from them, and only connected by narrow artificial channels called ' gatways.' These are the Broads which form such picturesque features in East Norfolk. They vary in size from small pools or ' pulks ' to sheets like that of Wroxham covering 92 acres, and Hickling 578 acres. All appear to be slowly filling up and contracting by the mud brought down by the rivers, by peaty accumulations, and by the growth of reeds and rushes along their borders. The tide now flows up the Waveney as far as Shipmeadow Lock (27 miles), up the Yare to Norwich (29 miles), and up the Bure to Wroxham (25 miles). Higher up the streams are navigable by means of locks. The artificial hindrances to the flow of the streams have, in times of long continued and heavy rain, Jed to disastrous floods, such as that which occurred at Norwich and elsewhere in the county in 1878. Comparatively little erosion now appears to be taking place along the river valleys. The rivers have reached their base-level, and the tributary streams alone are able to deepen their courses. As Mr. Reid has remarked the waste of the coast-line has undoubtedly curtailed the drainage areas of some of the rivers, especially that of the Bure, and consequently the waters have become more sluggish.^ The boundary between Norfolk and Suffolk is marked by the courses of the Waveney and Little Ouse ; and it is a remarkable fact that both rise near together at South Lopham, in a sandy tract intersected by dykes. A 'causeway' about 86 feet above sea-level now forms the division between the two drainage areas. If the rivers now appear to exercise but little influence on the waste of the land, this is not the case with the sea. The waste of some portions of the Norfolk coast has attracted a good deal of attention, but there are compensations in the growth of land on the Fen margins, in the heaping up of blown sand which has been banked up against the old cliff south of Winterton, and in the growth at Yarmouth. The clifi^s elsewhere between Happisburgh and Weybourne waste away at a rate estimated at from two to three yards a year, so that two or three miles may have been lost since Roman times. Since the Norman period, indeed, several villages, Shipden, Wimpwell, and great part of Eccles-next-the-Sea, have been washed away. The waste is accelerated by landslips which occur along the coast, the irregular accumulation of contorted loam and clay with basin-shaped hollows of sand and gravel being peculiarly susceptible to the influence of springs which loosen and undermine the strata. Mud streams are also a noticeable feature along the coast ; while the wind, as before-mentioned, lends its aid in the destruction. Then the sea washes away the tumbled material and the coast is ready for 1890, pp. 62, 73. 27
 * Reid, Geology of Cromer, p. 131 ; J. H. Blake, 'Geology of Yarmouth,' etc. (G«/. Survey),