GEOLOGY 



Chalk, and alter the character of the surface-soil. There are better 

 examples of such ' pipes ' in the Harefield chalk-pits just across our 

 county boundary, in Middlesex, than anywhere in Hertfordshire, but 

 fine examples have at various times been seen in the cuttings of the 

 three main railway lines which pass through the county ; nearly all are, 

 however, now grassed over. 



While the Lower and Middle Chalk frequently have but a thin 

 covering of surface-soil, chiefly owing to their outcrop being usually in 

 escarpments formed by comparatively recent denudation and having a 

 steep slope, the Upper Chalk seldom comes to the surface except quite 

 in the valleys where it and the superficial deposits upon it are subject to 

 the erosive action of our existing rivers. While ' clay-with-flints ' pre- 

 dominates on the west in the Colne district there are also, on high as 

 well as low ground, thick beds of gravel and sand, formed either by 

 glacial or river action. But on the east in the Lea district the Chalk 

 is almost entirely covered with boulder-clay, except where the rivers 

 have cut through this clay, exposing beneath it the glacial gravels and 

 sometimes the Chalk. 



The close of the Cretaceous epoch must have been marked by 

 considerable changes in the distribution of land and sea. Great Britain 

 during the deposition of the Chalk was but an archipelago, the islands 

 of which it was composed existing only west of a line running north and 

 south from the extreme north of England to Somerset and Devon. East 

 of this line all was sea, deepening eastwards ; west of it our present 

 mountains in Scotland, Ireland, the English Lake district, and North 

 and South Wales, with the highest land in Devon, were islands of small 

 size, except in Scotland ; there was open sea to the south, extending 

 over the north of France, but north of Scotland there was land, where 

 is now a deep sea. It was not, however, from the denudation of this 

 northern continent that the Chalk was formed ; it was built up by the 

 animals which lived in the Cretaceous sea animals most of which were 

 of microscopic size. The larger fossils which we now find in the Chalk, 

 numerous though they are in some places, formed but a minute fraction 

 of the number of living creatures which teemed in those deep seas 

 or sported on the surface. The great mass of the Chalk consists of 

 Foraminifera (Globerigina, etc.), and other microscopic Rhizopoda, or 

 rather of their calcareous shells or siliceous external skeletons, either 

 whole or reduced to fragments. It was minute creatures such as these 

 which built up nearly the whole of the groundwork of our county, 

 living and dying until their remains accumulated to a thickness of at 

 least 800 feet. 



The land then rose, the western archipelago becoming a continent, 

 and the sea covering only the midland, eastern, and southern counties of 

 England as far west as Devon, and of course including in its depths the 

 whole of Hertfordshire. 



The break between the Secondary or Mesozoic period and the 

 Tertiary or older Cainozoic period is the most important of any in 



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