1868.] HUGHES — HERTFORDSHIRE GRAVELS. 285 



The gravel of the Upper Plain consists chiefly of pebbles ; of these, 

 about fifty per cent, are of quartz, about ten per cent, of quartzite, 

 about five per cent, various (such as jasper and a conglomerate of 

 quartz pebbles in quartzite), and the rest flint. Sometimes the re- 

 lative proportions of quartz and flint change places ; but, as a general 

 rule, if we take pebbles the size of an egg, we shall find a larger per- 

 centage of flint ; and if we select those of the size of a pea, a larger 

 percentage of quartz. There are just enough subangular flints and 

 large partly worn pieces of quartz &c. to show that this gravel 

 derives its pebbly character from the waste of older pebble beds, with 

 which the unworn fragments got mixed, and not that they were all 

 worn together into pebbles along the shingly shore of the Higher- 

 Plain Gravel-sea. 



From their great extent, persistent character, and uniform level, 

 I think these gravels of the Higher Plain must be a marine deposit ; 

 but without a careful examination of the old coast-line, and of their 

 behaviour as they approach the Crag country, I should not like to 

 give any opinion as to their age. 



They may be well examined at Queen Hoo Hall, Bright's-Hill 

 Wood and elsewhere near Bramfield, at Hertford Heath, Brick-' 

 enden Green, Bayford, Berkhampstead, Essenden, and behind the 

 kiln at Hatfield Park*. 



4. Gravel of the Lower Plain. — The gravels, &c., of the Lower 

 Plain vary far more in their arrangement and composition than those 

 of the Higher Plain. Generally, they may be said to consist of about 

 fifty per cent, of pebbles, of which about ten per cent, are of quartz, 

 ten per cent, of quartzite, and about thirty per cent, of flint, derived 

 from the Tertiaries or Higher-Plain Gravel. The remaining fifty per 

 cent, are chiefly subangular flints with a few bits of ironstone, a few 

 fossils from the Lias and Oolite, and a few of the black partly formed 

 pebbles that occur at the base of the London clay. These subangular 

 flints look as if they had been broken and weathered by surface- 

 action, and are such as might be derived from any exposed flinty 

 soil. Occasionally we find flints hardly rolled or broken at all, as if 

 they had been derived directly from the chalk, or from the Boulder- 

 clay, which often preserves them in that state, and had only suffered 

 such decomposition of the surface as would result from their lying 

 in the porous gravel. There is frequently a great quantity of false- 

 bedded sand ; and about the middle of the deposit we often, indeed 

 generally, find a bed of brown loam and clay, passing sometimes into 

 Boulder-clay, with, as usual, drifted Oolitic fossils, rolled and scratched 

 lumps of chalk, &c. This may be examined in the railway- cuttings 

 north of Hatfield, and in a pit on the hillside, east of the oil-mill 

 south-west of Hertford. It can be traced all along the hillside 

 from that to Hatfield ; but there is no other good section here. Mr. 

 Baker, of Bayfordbury, informed me that, in sinking a well at the 

 east end of his house, they passed through 13 feet of springy gravel, 

 20 feet of dark -blue clay, and 10 feet of loose gravel and sand 

 before they reached the chalk. These middle beds can be seen also 

 * See Geological-Survey Memoir on sheet 7, p. 22. 



