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 ii
