RED CRAG. 
-195 
Fig. 121. 
Red Crag. London Clay. Chalk. 
It is chiefly in the county of Suffolk that it is found, rare¬ 
ly exceeding twenty feet in thickness, and sometimes over- 
lying another Pliocene deposit, the Coralline Crag, to be 
mentioned in the sequel. It has yielded—exclusive of 25 
species regarded by Mr. Wood as derivative-—256 species of 
mollusca, of which 65, or 25 per cent., are extinct. Thus, 
apart from its order of superposition, its greater antiquity 
than the Norwich and glacial beds, already described, is 
proved by the greater departure from the fauna of our seas. 
It may also be observed that in most of the deposits of this 
Red Crag, the northern forms of the Norwich Crag, and of 
such glacial formations as Bridlington, are less numerous, 
while those having a more southern aspect begin to make 
their appearance. Both the quartzose sand, of which it 
chiefly consists, and the included shells, are most commonly 
distinguished by a deep ferruginous or ochreous color, whence 
its name. The shells are often rolled, sometimes comminu¬ 
ted, and the beds have much the appearance of having been 
shifting sand-banks, like those now forming on the Dogger- 
bank, in the sea, sixty miles east of the coast of Northum¬ 
berland. Cross stratification is almost always present, the 
planes of the strata being sometimes directed towards one 
point of the compass, sometimes to the opposite, in beds im¬ 
mediately overlying. That such a structure is not decep¬ 
tive or due to any subsequent concretionary rearrangement 
of particles, or to mere bands of color produced by the iron, 
is proved by each bed being made up of flat pieces of shell 
which lie parallel to the planes of the smaller strata. 
It has long been suspected that the different patches of 
Red Crag are not all of the same age, although their chro¬ 
nological relation can not be decided by superposition. Sep¬ 
arate masses are characterized by shells specifically distinct 
or greatly varying in relative abundance, in a manner imply¬ 
ing that the deposits containing them were separated by in¬ 
tervals of time. At Butley, Tunstall, Sudbourn, and in the 
Red Crag of Chillesford, the mollusca appear to assume their 
most modern aspect when the climate was colder than when 
the earliest deposits of the same period were formed. At 
Butley, Nucula Cohholdim^ so common in the Norwich and 
certain glacial beds, is found, and Purpura tetragona (Fig. 
122) is very abundant. On the other hand, at Walton-on- 
