THE GAULT AND UPPER GREENSAND 39 



nodules frequently contain large Ammonites and other 

 fossils. Next follow the Sandstone and Rag beds, about 

 50 feet of sandstone with alternating layers of rag. The 

 sandstones are grey in colour, weathering buff or reddish- 

 brown, tinged more or less green by grains of glauconite. 

 Near the top of these strata is the Freestone bed, a thick 

 bed of a close-grained sandstone, weathering a yellowish 

 grey, which forms a good building stone. Most of the 

 churches and old manor and farm houses in the southern 

 half of the Island are built of this stone. Then forming 

 the top of the series are 24 feet of chert beds, bands of 

 a hard flinty rock called chert alternating with siliceous 

 sandstone, the sandstone containing large concretions of 

 rag in the same line of bedding. The chert beds are very 

 hard, and where the strata are horizontal, as above the 

 Undercliff, project like a cornice at the top of the cliff. 

 Perhaps the finest piece of the Upper Greensand is Gore 

 Cliff above Niton lighthouse, a great vertical wall with 

 the cornice of dark chert strata overhanging at the top. 

 The thickness in the Undercliff, including the Passage 

 Beds, is from 130 to 160 ft. 



The Upper Greensand may be studied at Compton Bay, 

 and at the Culvers ; and along the shore west of Ventnor 

 the lower cliff by the sea consists largely of masses of 

 fallen Upper Greensand, many of which show the chert 

 strata well. In numerous walls in the south of the Island 

 may be seen stone from the various strata sandstone, 

 blue limestone or rag, and also the chert. 



Let us think what was happening when these beds were 

 being formed. The sandstone is much finer than that 

 of the Lower Greensand ; and we have limestones now, 

 marine, not freshwater as in the Wealden. Marine lime- 

 stones are formed by remains of sea creatures living at 

 some depth in clear water. And now we come to a new 

 material, chert. It is not unlike flint, and flint is one of 

 the mineral forms of silica. Chert may be called an 



