Middle Cham 33 



The Melbourn Rock is a band of very hard greenish-yellow 

 chalk full of even harder nodules of white chalk which is often 

 recrystallized. It is very persistent, and though never more 

 than ten feet thick is sufficient to determine a feature along 

 the Chalk escarpment. 



The rest of the zone consists of ordinary white chalk with 

 very occasional bands of flint nodules. It has numerous marly 

 partings and is well stratified throughout. Rhynchonella 

 Cuvieri is generally abundant and swarms in certain layers. 

 The thickness assigned is sixty to seventy feet. 



The best exposure of Melbourn Rock is in the lime pit at 

 Melbourn. There it has its usual character and rests on the 

 Belemnite marls. A similar exposure, much overgrown, exists 

 at Steeple Hill, Shelford. Fossils are always rare. 



The beds immediately above the Melbourn Rock are well 

 seen in the old pit and antiquarian excavations on the south- 

 east side of the road above the great Cherryhinton pit, in 

 which Rhynchonella and Inoceramus labiatus swarm. Other 

 localities worth noting are the road cutting about a hundred 

 yards north-east of the Babraham cross roads, and a small pit 

 on the east side of the road from Stapleford to the Gog 

 Magog Hills. The first of these is rich in Rhynchonellae, but 

 the second shows only bedded chalk with Ostrea vesicularis. 



The second zone of the Middle Chalk is known as the zone 

 of Terebratulina gracilis, but the name is unfortunate, as the 

 original of T. gracilis seems only to occur in the Norwich 

 Chalk, while the proper name of the zone fossil is T. lata. 

 The exposures near Cambridge are poor in the extreme, but 

 being harder than its neighbours the zone determines a line of 

 steeper slopes, and so is easily traceable along the escarpment. 

 It is often termed the Wandlebury Chalk and is about one 

 hundred feet thick. Two quarries are worked in it at Royston, 

 and the old railway at Worsted Lodge provides a section. 

 Fossils are badly preserved and rare. Flints are not abun- 

 dant, but when present possess remarkable shapes and 

 generally lie vertically rather than horizontally. 



M. & S. 3 



