Retreat of the Late Pliocene and Lower Pleistocene Crag sea
Table 3. Wroxham Crag Formation: clast lithological data (8- 16mm).
Sources Rose et al. (2000,2001)
including red and brown quartzites, with small percentages of Carboniferous and Rhaxella chert and
traces of Spilsby Sandstone, Greensand chert and volcanic rocks. In the Mundesley Member, the
proportion of far-travelled rocks declines again, to less than 15%.
The relatively high proportions of white or colourless quartzose lithologies and the Rhaxella chert
in the Dobbs Member indicate important contributions by the Thames and Ancaster rivers, with a
minor input from the Bytham River (Fig. 3b). Although there is an increase in the range of lithologies
in the How Hill Member, the same catchments are involved. However, the increase in coloured
quartzites indicates that the input from the Bytham River had increased, to be come the major
contribution. The lithologies of the Dobbs and How Hill Members link them with the 'Sudbury
Formation'.
The reduced proportion of quartzose and other far-travelled rocks in the Mundesley Member is in
keeping with the changes already outlined in the Thames catchment, reflecting the loss of the
headwaters (Fig. 3c), though the reduction might be relative, due to an increased input of flint from
enhanced local erosion (Rose et al., 2001).
Thus the sequence shows an early dominance by the Thames being usurped by the Bytham and
Ancaster Rivers. The Thames, however, continued to contribute, probably debouching increasingly
further away from Norfolk as it migrated southwards (Fig. 3).
IS
Essex Naturalist (New Series) 18 (2001)