Taylor and Movant — Sand-pipes in the Clialk. 121 



the lower stratum of gravel is very much consolidated and extremely 

 hard, owing to the water having carried down portions of oxide of 

 iron from the upper to the lower parts, which it has thus cemented 

 together. "When the re-deposited Chalk is overlain by beds of clay 

 or brick-earth, sand-pipes are rarer in occurrence, and smaller in 

 size. The rarity is due to the difficulty of percolation, and the small 

 size to the slight percentage of acid having already combined with, 

 and dissolved, the calcareous matter mixed with the brick -earths or 

 clays. The tortuous or winding sand-pipes usually occur where 

 the overl\-ing strata overlap each other's edges, so that the action of 

 the water has been guided by its line of drainage. 



The pebble beds of the Lower Drift are largely developed in the 

 neighbourhood of Xorwich. and where these overlie the re-deposited 

 Chalk, sand-pipes are very common. We have remarked on the 

 cementing power of the percolating water, and several instances are 

 on record where hollows occur in these gravels, just above the sand- 

 pipes, as at Horstead, Brandon, etc. In every case these hollows- or 

 natural caverns, have been caused by the falling of portions of 

 the overlying gravels into the hollow pipes dissolved out of the 

 chalk. This circumstance utterly precludes the possibility of sand- 

 pipes having been eroded by mechanical action. As has already 

 been remarked by Sir CTiarles Lyell and 3Jr. Prestwich, in every 

 case the sand-pipes are filled up by material falling in from above, 

 or carried there by the water in its process of dissolution. The age 

 of these sand-pipes, or rather of their filling-up. can s-ometimes be 

 told. For instance, last !May there was exposed in a marl-pit at 

 Coltishall, about six miles from Xorwich. a fine sand-pipe, running 

 down the face of the section, and having a diameter of nearly forty 

 feet. The pebble beds overlie the chalk, and we could plainly see 

 where the whole of the strata overlying the pipe had been let bodily 

 down to a depth of above twenty feet from tiieir original position. 

 !N"ot withstand in g this, the contour of the surface was not affected in 

 the slightest degree, so that this sand-pipe must have been formed, 

 and the pebble beds let into it. before the existing configuration of 

 the surface was produced. Dtiring the progress of the sewerage works, 

 several pumping- engines have been working night and day. lifting 

 about 17.000,000 gallons of water in each twenty -four hours. The 

 wells for two miles round Xorwich have mostly been drained, although 

 the river, which flows through the middle of the drained area, has been 

 unaffected. The wells sunk in the solid undisttirbed Chalk, were 

 those first affected ; the wells situated in the re-deposited Chalk, or 

 '■'chalk-marl." being so in a less degree. TTells on the higher 

 grounds, sunk through the sand and gravel, have been untouched, 

 especially if they had a bottom of briek-earth. In practical well 

 sinking it has been found necessary to bore through the re-deposited 

 Chalk until the xmderlying hard chalk, into which it gradually 

 passes, was reached, when water is fotmd in abtmdance. The thick- 

 ness of this marl varies from ten to thimr or forty feet. 



The most important point in the sewerage works, as bearing in- 

 directly upon the excavation of the proposed Cliannel Tunnel, is the 



