45^ PLIOCENE. 



Under this grouping also may be included the ' Forest Bed ' of Norfolk (familiarly 

 known as " Pre-Glacial," but sometimes grouped as Pleistocene), because that 

 deposit clearly underlies the oldest Boulder Clay in the country, and is more 

 intimately connected with the later deposits of the Crag than with the overlying 

 Glacial accumulations. 



It would, however, be wrong to group all so-called Pre-Glacial deposits as 

 Pliocene, as the former ambiguous term (introduced by John Phillips in 1853) is 

 sometimes conveniently used for deposits that underlie Boulder Clay, and that can 

 be definitely referred neither to the Pliocene nor to the Glacial period. 



Some of the pebbly gravels which cap the Tertiary hills in ' 

 Hertfordshire, Surrey, Berkshire, and elsewhere, may possibly be 

 of Pliocene age, but this view depends, to some extent, upon the 

 position of the boundary-line taken between Pliocene and Glacial 

 deposits.^ In some localities also there are deposits of white clay, 

 sand, and chert, the age of which is exceedingly doubtful. (See 

 p. 444.) 



The occurrence of Crag deposits at Lenham in Kent (600 feet 

 above sea-level), and at St. Erth in Cornwall, suggests that a 

 great part of the east and south of England was submerged 

 during Pliocene times.' 



The Pliocene strata contain a large number of living species of 

 Mollusca, and they yield some species, such as Vohita Latnhcrti, 

 Panopaa Faujasii, Pictunculus glycimeris, etc., which are found also 

 in Miocene deposits. 



The Mammalia include the Machcerodus, Trogontheriiim, Elephas 

 vieridkmalis (and other species). Hippopotamus, Mastodoji, and many 

 other forms, some of which are Recent species. No evidence of 

 Man's existence during the Pliocene period has been obtained in 

 this country.^ 



The climate of the Pliocene period appears to have undergone 

 a gradual refrigeration. The warmer Miocene period was suc- 

 ceeded by a milder climate during the deposition of the Coralline 

 Crag, whose Molluscan fauna is like that of the Mediterranean ; 

 while in the Red and Norwich Crags the southern forms diminish 

 in numbers, and we find an increase of northern and arctic 

 Mollusca. The higher stages of the Cromer Forest Bed Series 

 indicate more markedly the approach of the still colder climate, 

 which culminated in our Glacial period.'* 



CRAG. 



The deposits of shelly sand and gravel known as ' Crag,' which 

 occur on the eastern borders of Norfolk, Suffolk, and Essex, have 



^ Bristow, Geol. parts Berks and Hants (Geol. Surv.), p. 43 ; T. McK. Hughes, 

 Q. J. xxiv. 285 ; Whitaker, Guide to Geol. London, ed. 4, p. 58 ; Prestwich, 

 G. Mag. 1881, p. 466 ; Ussher, Post-Tertiary Geol. Cornwall, p. 10, G. Mag. 

 1879, p. 102. 



2 C. Reid, Nature, Aug. 12, 1886, p. 342 ; see also P. F. Kendall and R. G. 

 Bell, Q. J. xlii. 207. 



3 See A. Bell, G. Mag. 1886, p. 71. 



* See E. Forbes, Mem. Geol. Survey, i. 336. 



