Lower Greensand 21 



The cement is oxide or sulphide of iron, but except near 

 the surface the sand is generally quite friable. Near the 

 surface percolating water has largely re-distributed the cement- 

 ing material, and depositing it along certain beds and joints 

 has brought about the formation of the hard Carstones which 

 give the rock its local name. The irregular occurrence of the 

 same process has given rise to the celebrated 'boxstones' of 

 these beds. 



One of the strangest features of our Lower Greensand is 

 the presence of beds of rolled phosphatic nodules or coprolites 

 among it. These have long attracted the attention of geologists, 

 and, being at one time of considerable economic importance, 

 were also available for careful study. The beds of phosphate 

 come in at various horizons and are rarely continuous over 

 wide areas. They form conglomerates, and their nodules, 

 many of which are rolled casts of recognizable fossils, bear in 

 themselves evidence of being derived from the various Upper 

 Jurassic Clays. A few are of later date, but Mr Teall declares 

 that in certain places more than 50 per cent, were once portions 

 of Ammonites blplex. Whatever the source, the phosphate 

 content of the nodules seems to be practically invariable. 

 Along with the nodules are many pebbles of such rocks as 

 vein quartz, chert, lydian stone ; and boulders of gneiss, 

 granite, mica schist, and trilobite-bearing Carboniferous lime- 

 stone have been observed. 



In most places the Lower Greensand is unfossiliferous, but 

 wherever phosphate beds have been worked, a few, and in 

 some places a great many, indigenous fossils have turned up. 

 Fragments of coniferous wood are generally abundant. The 

 present exposures of Lower Greensand in Cambridge are poor 

 in the extreme. The outcrop over most of its course is com- 

 pletely buried by superficial deposits such as Boulder-clay, 

 gravel and peat, and north of Cambridge the thickness de- 

 veloped is too small to exert any distinctive influence on the 

 character of soil or scenery. 



At Sandy, in Bedfordshire, some three miles beyond the 



