106 J. V. EXSDEN — POST-TEETIAET 



high and low ground, being found high up on the top of the chalk 

 escarpment (see Fig. 3) and low down nearly at sea-level. 

 Beds of sand and gravel are sometimes interbedded with the 

 clay.* Sections of Boulder-clay may be seen at Haileybury, 

 Little Berkhamstead, Bayford, Buntingford, and at Bricket Wood, 

 near "Watford. 



Post- Glacial heds — Marine. — Under this head are classed a number 

 of small patches of brick-earth and loamy gi'avel, occupying slight 

 depressions in the Boulder-clay, and supposed to have resulted 

 from denudation of the Glacial beds during emergence from the 

 sea. They will presently be again referred to as denudation gravels. 

 Good examples may be seen in the kiln at Reed and in the brick- 

 yards near Barkway. 



Post- Glacial beds — Fluviatile. — "We should consider here all those 

 old valley-gravels, terrace-gravels, sands, and brick-earths which 

 owe their origin to existing rivers. (See Fig. 4.) Hertford, "Ware, 

 and Eye House stand upon flats formed of these old river-gravels. 



Fig. 4. — Section through the Valley of the Lea. 

 ■w s 



X. Postglacial. T. Upper Glacial. E. London Clay. 



There are likewise the more recent deposits of alluvium, consisting 

 of silt and peaty earth, bordering most of the streams, and occa- 

 sionally forming large spreads, as at the confluence of the Stort 

 and Lea. It is not always easy to distinguish some of the river- 

 gravels from the Glacial deposits, as they consist of the same 

 material re-arranged. For example, the old gravel-terraces north of 

 Essendon Hill, and in Hatfield Park, south of the house, and the 

 pebble-gravel and brown clay north of St. Albans, are doubtful 

 as to their age and origin. f In these Post-glacial fluviatile de- 

 posits land and fresh-water shells sometimes occur, together with 

 the bones and teeth of Mammalia ; while occasionally is found 

 here a flint implement, the earliest record of pre-historic man. 



Post- Glacial heds — Sub-aerial. — These beds, which have been 

 formed on land in most recent times, include surface soils and 

 vegetable mould. The characters of soils vary chiefly according 

 to contours, and their classification upon any other basis than their 

 origin is impossible. 



* ' Memoirs Geol. Survey,' Sheet 47, p. 59. 

 t ' Quart. Joum. Geol. Soc.,' vol. xxiv, p. 287. 



