22 The Geology of Cambridgeshire 



county boundary, the Greensand is about 120 feet thick, and 

 a fine cliff section, some 60 feet high, maybe seen on the north 

 side of the railway just west of the station. Here it rests 

 directly on Oxford Clay, and it is interesting to note that 

 within an inch of the junction the sand has no trace of clayey 

 material. The Greensand beds exposed are fine yellow sands, 

 much false bedded, but they contain no important phosphate 

 band. A little higher on the hill, however, an important 

 phosphate bed came to the surface and has been worked from 

 a large portion of the area known as Sandy Heath. 



The railway from Sandy to Potton skirts along the Lower 

 Greensand scarp which there exhibits its characteristic scenery 

 and flora. 



At Potton, about three-quarters of a mile south of the station, 

 a pit is still worked for phosphates but is 'preserved' with 

 great care and is well-nigh unapproachable. Under favourable 

 circumstances fine examples of derived phosphates pierced by 

 indigenous boring animals may be collected ; the commonest 

 are in the form of casts of Ammonites biplex, but lamelli- 

 branchs and reptilian bones are by no means rare. Erratic 

 pebbles are not very common. 



The phosphate bedJ'^fiain'Dy abdu) (t seven or eight feet 

 of very browr.,^ w i tn occasional carotenes, and the ex- 

 posure isj- pped by a W ash of Boulder-clay . 



,vUhin the county boundary the Greenland may be seen 

 resting on Ampthill Clay in the three pits* at Gamlmgay, but 

 no very interesting features can there be ma-, de out. 



At Great Gransden and also at Little G ransden exposures 

 showin^ good Carstone are also visible, a.fcd that at Great 

 Gransden is particularly interesting on account of the enormous 

 scale of its false bedding. The exposure is Capped by Boulder- 

 clay which rests on an absolutely even surface which obliquely 

 truncates the false-bedding planes. No tr^ce of Greensand 

 pebbles is here seen in tl. |> Boulde^iay. 



North-east of Great Gransden tlae Greensand is completely 

 buried by drift, and except for a . small stream-exposure at 



