GEOLOGY OF HERTFORDSHIRE. 155 



universal in all parts of England. The fine state of subdivision 

 in which rivers deposit sediment where they overflow their banks, 

 is almost as favourable to the growth of crops as that natural 

 admixture of rocks of different characters and composition which 

 is the chief feature of alluvial deposits. It has even been noticed 

 that the alluvium carried down by rivers from rocks which are by 

 nature unproductive, often forms a most fertile soil. 



The remainder of this district, extending along the valley of tlie 

 Stort to the boundary of the county at Sawbridgeworth, is chiefly 

 under arable cultivation, this neighbourhood being far better suited 

 for corn than the western part of this area, since the London Clay 

 is covered with the same chalky boulder-clay which covers such a 

 wide extent and forms some of the best corn-land of Essex. 



It has before been said that where the Woolwich and Reading 

 beds come to the surface, the soil is, in many cases, improved by 

 the admixture, the upper portions consisting of stiff clay and the 

 lower parts of sand and pebble-beds. But, upon the outcrop of 

 these beds, there is, around Hatfield and North Mimms, a wet clay 

 full of rounded flint-pebbles, marked on Young's map as poor 

 gravel, and described as the worst soil in the county. Young, him- 

 self the holder of a farm at North Mimms, said of this soil :'^' "I 

 know not what epithet to give it : sterility falls short of the idea — 

 a hungry vitriolic gravel. I occupied for nine years the jaws of a 

 wolf. A nabob's fortune would sink in the attempt to raise good 

 crops in such a country." Mr. Clutterbuck, also, says of this same 

 district:! "The soil of the upper levels of the clay district, marked 

 by the rounded flint-pebbles embedded in sand, is wet and un- 

 kindly, not capable of bearing grass of any value, and ungrateful 

 under the most liberal treatment as arable land. This gravel, with 

 its characteristic blue pebble, is transported, in many cases, below 

 the higher levels, where the sterility of the soil is in proportion 

 to the thickness of the bed." This barren district is said to be 

 improved by chalking. 



The draining, so necessary over most of this London-Clay district, 

 is assisted by the gentle undulations of the surface, and by the 

 numerous water-courses. The frequent gravel-beds furnish a water- 

 supply for many hamlets, and cause many surface-springs at the 

 junction of the underlying clay. It has been pointed out, by Mr. 

 Clutterbuck, I that, as the streams of this district pass over the 

 outcrops of the sands of the Woolwich and Reading Series, they 

 frequently sink by swallow-holes into the chalk, thus materially 

 assisting in the prevention of floods. This is especially to be 

 observed near Bushey and Aldenham ; and it has even been 

 necessary to prevent the waste of water, in the water-courses 

 leading from the reservoir at Elstree, by artificially stopping these 

 swallow-holes. It has been suggested that the drainage of the 

 clay districts, as well as the replenishment of the deep-seated 



* ' Journ. Eoy. Agric. Soc.,' vol. xxv, p. 271. 



t "Agricultural Notes on Hertfordshire," 'Journ. Rov. Agric. Soc.,' vol. xxv, 

 p. 306. + lb., p. 306. 



