34 The Geology of Cambridgeshire 



Upper Chalk or Chalk with Flints. 



The next zone of the Chalk is that of Holaster planus, and 

 this is now referred to as the lowest division of the Upper 

 Chalk. It includes the well-known Chalk Rock, and also 

 certain other beds of nodular white chalk both above and 

 below that horizon. 



The beds below the Chalk Rock are soft and white. Flints 

 are very common at certain levels, but there are often wide 

 intervals between the layers. Many of the flints contain 

 sponges. Marl layers also occur and are especially numerous 

 in the passage beds to the Chalk Rock. 



The Chalk Rock consists of a variable number of hard 

 semi-crystalline bands of creamy chalk which have layers of 

 green phosphatized nodules and rubbly chalk between them. 

 The upper surface of the rock is sharply defined, and may 

 in places be a surface of contemporaneous erosion. This 

 surface has long been taken as the base of the Upper Chalk, 

 but Mr Jukes-Browne in comparing the rich fauna of the Chalk 

 Rock with that of higher beds, finds that no separation of 

 stages is here possible, so he includes the whole of the Planus 

 zone in the Upper Chalk. 



The beds above the Chalk Rock are generally soft and 

 earthy, but harder bands occur and there are some marly 

 layers. Beds of flints are not numerous, but tabular flints 

 following joints are fairly common. 



Exposures are more abundant than in the underlying 

 beds, but all are at some distance from Cambridge. 



The lowest beds may be well seen in a pit by the roadside 

 just east of Great Chesterford, and there the zone fossil and 

 many Micrasters and Inoceramus may be collected. Of other 

 localities we may mention the little pit near Linton Work- 

 honise, also Lark's Hall, a pit east of Six Mile Bottom, and 

 lastly Dullingham. 



The Chalk Rock is even more sought after, and old pits 

 are common all along its outcrop. It is comparatively rich 



