TRIMMER ON SAND- GALLS IN THE NORFOLK CHALK. 



309 



The second of the two pits above referred to, worked in the solid 

 chalk, lies about 4 miles N. E. of Norwich, a little to the north of 

 Rackheath church. It is on the east side of a north and south 

 valley, which terminates northwards in the east and west valley of 

 the Bure. 



The surface of the chalk is marked by broad undulations, 3 or 4 

 yards asunder, and about a yard deep, the prevailing direction of 

 these undulations, drawn lengthwise, being north and south ; but 

 this direction is liable to exception. The chalk is worked to a 

 depth of from 12 to 18 feet below its own surface, the depth in- 

 creasing with the distance from the valley. Paramondras, which 

 are generally abundant in the chalk of this part of Norfolk, are 

 particularly so in this pit ; and I noticed one of them, having its 

 lower part fixed in the chalk, and its upper enveloped in the sand 

 of the crag, which here covers the chalk. 



The crag is from 10 to 18 feet thick ; but it thins off towards 

 the valley. The Pan rests immediately on the chalk ; and above 

 the pan are beds of yellow and white sand, alternating with bands, 



2 or 3 inches thick, of gravel and laminated clay. In the spring 

 of 1844, in the progress of the workings, a bed of crag-shells was 

 exposed, about 3 feet long and 2 feet thick, at the height of about 



3 feet above the surface of the chalk ; and another thin layer of 

 crag shells may be seen in the bank of an adjoining road. The 

 above lower crag strata conform to the irregularities in the surface 

 of the chalk, both where it sinks towards the valley and where it 

 rises above the ordinary level. The valley therefore appears to 

 have been partially excavated in the chalk, before the supra- 

 cretaceous beds were deposited. The upper beds consist of less 

 regularly stratified sand and gravel. 



In the beginning of May, 1843, a space, abcd (see Fig. 7.), 

 bordering on the edge of the valley, 19 yards by 14, was cleared 

 of the sand and gravel, which were here from 10 to 12 feet 

 thick. The surface of the chalk thus exposed was traversed 

 by numerous shallow and nearly parallel furrows, which had a 



Fig. 7. 

 ( Ground Plan of the cleared Surface of Chalk at Blackheath.) 



North. 



r.-. E 



West. 



'•V 



, 1 



Sg §f Hf 1= ^J \k < :«* 1 ^ « '* * ►* < 



iH £§ 1^ jf 



c 



• ; -^"-Y-Vv : -': T£:~,: '^'-iv » 



East. 



South. 

 The shaded parts represent the ridge, and the blank ports the furrow. 



x 3 



