Handbook to the natural history of Cambridgeshire / edited by J.E. Marr and A.E. Shipley.

Date:
1904
Catalogue details

Licence: In copyright

Credit: Handbook to the natural history of Cambridgeshire / edited by J.E. Marr and A.E. Shipley. Source: Wellcome Collection.

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    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 tlie 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 orcoprolites 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 hiplex. 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
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