NORWICH CRAG SERIES. 463 



of dissolution has been discussed by Mr. W. Whitaker, who has shown how 

 certain apparently eroded surfaces in the Red Crag are due to the dissolving action 

 of carbonated water in permeable beds, appearances formed after the deposition of 

 the upper beds by the dissolving away of the shells that they once contained.^ 



The introduction of the iron-oxide which colours the Crag appears for the most 

 part to be subsequent to the formation of the beds. In some places, Mr. Prestvvich 

 observes, the iron-oxide has given rise to large tabular beds of sandstone, and 

 concretionary masses of limonite. Crag at some distance beneath the surface, 

 as proved in well-borings, is usually blue or grey. 



In the section at Chillesford the Scrobicularia Crag rests upon 

 the Red Crag (of Butley) and is directly overlaid by the Chillesford 

 Beds. It was this section, first noticed by Prof. Prestwich, which 

 led to the relations of the Norwich (Fluvio-marine) Crag and Red 

 Crag being determined — the former being the equivalent of the 

 higher portion of the latter. (See sequel.) The accompanying 

 section (Fig. 77) shows the general relations of the beds in the 

 neigrhbourhood of Aldborou2fh.^ 



NORWICH CRAG SERIES. 



The term Norwich Crag was introduced by Lyell in rSjq. 



The beds consist of a variable group of sands, pebbly gravels, 

 and laminated clays, with occasional seams or patches of shells. 

 In Norfolk the beds rest on the Chalk and are exposed on the 

 borders of the valleys of the Bure, Yare, and Waveney, and their 

 tributaries. Near Norwich and in the Bure Valley the thickness is 

 about 30 feet ; but at Beccles as much as 128 feet, and at Saxmund- 

 ham 105 feet of Newer Pliocene strata have been determined.^ 



The following is a general summary of the divisions that may 

 locally be made : — 



4. Buff and red false-bedded sand and gravel, formed chiefly of flint-pebbles, 

 with also pebbles of quartz and ironstone nodules. Veins and seams 

 of laminated clay occur, and the gravel is occasionally cemented into a 

 conglomerate or "Iron-pan."* [This division has been called the 

 Pebbly Sands and Pebble Beds, and the Bure Valley Beds.] 



3. Laminated clay, with seams of sand and gravel ; or clay and sand in thin 

 layers rapidly alternating. [This division has been called the Chilles- 

 ford Clay.] 



2. White and brown sand with pebbly gravel and ironstone nodules. [This 

 division, including also No. I, has been separated into an Upper or 

 Chillesford Crag, and a Lower or F"luvio-marine Crag.] 



I. Bed of unworn and rolled flints, called the " Mammaliferous Stone-bed." 



In by far the larger number of sections shells are absent ; but they most 

 frequently occur in beds i and 2, to which the term Norwich Crag has usually 



^ Q. J. xxxiii. 122. 

 ^ Prestwich, Q.J. xxvii. 339, 342. 

 ^ Geol. Norwich (Geol. Surv.), pp. 89, 156. 



* The term Iron-pan is applied to any bed of gravel or sand cemented by iron- 

 oxide. 



