LANDSLIPS. 597 



On the Durham coast at Sunderland, and between Hartlepool and Ripon, 

 similar subsidences have occurred ; these are due to the dissolution of the 

 Magnesian Limestone, which is riddled with caverns and cavities. In one case a 

 pit was formed from 80 to 100 feet deep and 200 feet in diameter.' In some parts 

 of Yorkshire cavities occur, which are due to dissolution of the Corallian limestone. - 



In many Oolitic and other limestones, gullies are formed by the enlargement of 

 joints and faults, and clay and other materials are washed in from above (see 

 p. 346) ; moreover, the dissolution of calcareous rocks sometimes produces joggles 

 or apparent faults which do not affect the underlying strata. 



Other cases of subterranean denudation occur in the sudden sinkings of ground 

 where " Pipes," " Earth-pots," or " Sand-galls," have been formed in the Chalk 

 (see p. 422), and the strata after a time "cave in." ^ In this way the Meres of 

 West Norfolk may have originated. They occur in a country where Chalk is 

 covered with sancl and Boulder Clay, and granting that a huge pipe had been 

 formed in the Chalk, then tlie washing in of Boulder Clay may have "tamped" the 

 hollow, in much the same manner as some of our ponds are rendered water-tight ; 

 while the Meres themselves depend for their supply of water partly on the dn-ect 

 rainfall and partly on the level of saturation of the Chalk. Mannington Mere, 

 near Aylsham, is said to have been formed in this way by subsidence.* 



Natural conical pits due to dissolution of the Chalk occur at AfTpuddle and 

 Piddletown Heaths in Dorsetshire, and in many other places.^ 



Landslips or ' Founders ' are due to the action of rain and 

 streams, and sometimes of tlie sea in undermining strata and 

 causing the superincumbent beds to slip over those subjacent. 

 They occur along escarpments and cliffs, especially where the 

 strata dip towards the valley or sea as the case may be, and where 

 porous beds are based on clays. The action of rain in loosening 

 the porous beds, and that of frost in producing vertical cracks 

 parallel to the line of cliff or escarpment, greatly tend to promote 

 landslips. Earthquakes may also set them in motion. (See p. 512.) 



" The founder most usually occurs where the following triple combination of 

 strata affords the most favourable circumstances : I. a superstratum of porous 

 rock ; 2. an interstratum of loose sand ; and 3. an argillaceous substratum 

 impervious to water. In such an arrangement of beds, the rain penetrating 

 through the porous rock, and arrested by the impervious clay, must be impoiaided, 

 as it were, in the intermediate sand, often nearly converting it into a mass of 

 quicksand. Where the slope of adjoining escarpments exposes this quicksand on 

 the surface, copious land-springs will gush forth, and carry away in different seasons 

 greater or less quantities of the loose material though which they flow ; and thus, 

 in process of time, the superincumbent rock will become partially undermined."^ 



Some of the most striking Landslips occur along the south coast of Devon 

 and the coast of Dorset between Sidmouth and Lyme Regis. There the Chalk 

 and Upper Greensand stretch over the denuded edges of the Lower Lias, Rhsetic 

 beds, and Red Marl, which are of a clayey nature. The Cretaceous beds in places 

 dip slightly towards the sea, and numerous springs are given out at the junction of 

 the Greensand with the impervious strata ; portions of the lower sandy beds of the 

 Greensand moreover would be actually removed by springs. Therefore we have 

 every condition favourable to the production of landslips, as previously mentioned, 



1 Rev. J. S. Tute, G. Mag. 1868, p. 178 ; A. G. Cameron, Brit. Assoc. 1881 ; 

 G. A. Lebour, G. Mag 1885, p. 513. 



2 C. F. Strangways, Explan. Sheets 95 S.W. and S.E. (Geol. Surv.), p. 19. 

 ' See Prestwich, Q. J. xi. 64 ; Trimmer, Q. J. x. 231. 



* Trans. Norfolk Nat. Soc. iii. 452, 642 ; F. J. Bennett, Geol. Attleborough 

 (Geol. Surv.), p. 17. 



^ O. Fisher, Q. J. xv. 187 ; see also G. Mag. 1865, p. loi ; J. Evans. G. Mag. 

 1868, p. 443 ; T. V. Holmes, P. Geol. Assoc, ix. 105 



^ VV. D, Conybeare and W. Dawson, Memoir and Views of Landslips, 1840. 



