THE TERTIARY ERA: THE EOCENE

Ages must have passed while the ocean flowed over this part of the world, and the chalk mud, with its varied remains of living things, gradually accumulated at the bottom. At last a change came. Slowly the sea bed rose, till the chalk, now hardened by pressure, was raised into land above the sea level. As soon as this happened, sea waves and rain and rivers began to cut it down. There is evidence here of a wide gap in the succession of the strata. Higher chalk strata, which probably once existed, have been washed away, while the underlying strata have been planed off to an even surface more or less oblique to the bedding-planes. The highest zone of the chalk in the Island (that of Belemnitella macronata) varies greatly in thickness, from 150 ft. at the eastern end of the Island to 475 at the western. The latest investigations give reason to conclude that this is due to gentle synclines and anticlines, which have been planed smooth by the erosion which preceded the deposition of the next strata,—the Eocene.[11] At Alum Bay the eroded surface of the chalk may be seen with rolled flints lying upon it, and rounded hollows or pot-holes, the appearance being that of a foreshore worn in a horizontal ledge of rock, much like the Horse Ledge at Shanklin.

The land sank again, but not to anything like the depth of the great Chalk Sea. We now come to an era called the Tertiary. The whole geological history is divided into four great eras. The first is the Eozoic, or the age of the Archæan,—often called Pre-Cambrian—rocks; rocks largely volcanic, or greatly altered since their formation, showing only obscure traces of the life which no doubt existed. Then follow the Primary era, or, as it is generally called, the Palæozoic; the Secondary or Mesozoic; and the Tertiary or Kainozoic. Palæozoic is used rather than Primary, as this word is ambiguous, being also used for the crystalline rocks first formed by the solidification of the molten surface of the earth. But Secondary and Tertiary are still in constant use. These long ages, or eras, were of very unequal duration; yet they mark such changes in the life of animal and plant upon the earth that they form natural divisions. The Palæozoic was an immense period during which life abounded in the seas,—numberless species of mollusca, crustaceans, corals, fish are found,—and there were great forests, which have formed the coal measures, on land,—forests of strange primeval vegetation, but in which beautiful ferns, large and small, flourished in great numbers. The Secondary Era may be called the age of reptiles. To this era all the rocks we have so far studied belong. Now we come to the last era, the Tertiary, the age of the mammals. Instead of reptiles on land, in sea and air, we find a complete change. The earth is occupied by the mammalia; the air belongs to the birds such as we see to-day. The strange birds of the Oolitic and Cretaceous have passed away. Birds have taken their modern form. In some parts of the world strata are found transitional between the Secondary and Tertiary.

The Tertiary is divided into four divisions,—the Eocene, the Oligocene (once called Upper Eocene), the Miocene, and the Pliocene; which words signify,—Pliocene the more recent period, Miocene the less recent, Eocene the dawn of the recent.

In the Eocene we shall find marine deposits of a comparatively shallow sea, and beds deposited at the mouth of great rivers, where remains of sea creatures are mingled with those washed down from the land by the rivers. These strata run through the Isle of Wight from east to west, and we may study them at either end of the Island, in Whitecliff and Alum Bays. The strata are highly inclined, so that we can walk across them in a short walk. Some beds contain many fossils, but many of the shells are very brittle and crumbly; and we can only secure good specimens by cutting out a piece of the clay or sand containing them, and transferring them carefully to boxes, to be carried home with equal care. Often much of the face of the cliff is covered with slip or rainwash, and overgrown with vegetation. Sometimes a large slip exposes a good hunting ground.

Now let us walk along the shore, and try to read the story these Tertiary beds tell us. We will begin in Whitecliff Bay. Though easily accessible, it remains still in its natural beauty. The sea washes in on a fine stretch of smooth sand sheltered by the white chalk wall which forms the south arm of the bay. North of the Culver downs the cliffs are much lower, and consist of sands and clays of varying colour, following each other in vertical bands. Looking along the line of shore we notice a band of limestone, at first nearly vertical like the preceding strata, then curving at a sharp angle as it slopes to the shore, and running out to sea in a reef known as Bembridge Ledge. This is the Bembridge limestone; and the beginning of the reef marks the northern boundary of Whitecliff Bay, the shore, however, continuing in nearly the same line to Bembridge Foreland, and showing a continuous succession of Eocene and Oligocene strata. The strata north of the limestone are nearly horizontal, dipping slightly to the north. In the Bembridge limestone we see the end of the Sandown anticline, and the beginning of the succeeding syncline. The strata now dip under the Solent, and rise into another anticline in the Portsdown Hills. North and south of the great anticline of the Weald of Kent and Sussex are two synclinal troughs known as the London and Hampshire basins. Nearly the whole of our English Eocene strata lies in these two basins, having been denuded away from the anticlinal arches. The Oligocene only occur in the Hampshire basin, the higher strata only in the Isle of Wight.

Fig. 3

COAST SECTION, WHITECLIFF BAY.
BMBembridge Marls. BBarton Clay. ChChalk.
BLBembridge Limestone. BrBracklesham Beds. PPebble Beds.
OOsborne Beds. BgBagshot Beds. SSandstone Band.
HHeadon Beds. LLondon Clay.
BSBarton Sand. RReading Beds.