The Cretaceous period
The Cretaceous period receives its name from the great deposits of chalk which are exposed on either side of the English Channel. It was here in southern England and northern France that these formations were first studied. However, it must be understood that chalk is the exception and not the rule in this period.
The Cretaceous was a long period and is divided into an Upper, or Later, and a Lower Cretaceous. During some of the period, a great part of North America was under water. The Gulf of Mexico overflowed and covered most of the southern United States, Cuba and Mexico. Texas, Oklahoma, Kansas and Wyoming gradually sank, and the waters crept across Canada and Alaska to join the Arctic Ocean. This great inland waterway known as the Colorado Sea, was more than a thousand miles across at its widest point, between Idaho and Wisconsin. Eastern Canada and the northeastern United States remained above sea level.
During the Cretaceous period, most of Europe was under water, and in the water the rocks were being deposited either as sandstone, clay or chalk, so that we find Cretaceous rocks in detached areas over most .f the continent, giving a record of the seas, lakes and rivers of the period. A map of Europe at that time would show the greater part of Spain, Italy, and the whole of Belgium, Holland, Prussia, Hungary, Switzerland, Rumania and north Russia as an almost continuous sheet of water. According to archeologists this flooding of the land during the Cretaceous was probably the most marked spreading of the oceans that has ever occurred.
Cretaceous rocks are found not only in Europe but all over the world - in the Arctic oceans, India, Japan, North America, South America, Australia and New Zealand. In California there are strata of Cretaceous rock as thick as thirty thousand feet. During parts of the period,, the seas were very shallow, and some of them became fresh. The rich coal beds of Alberta were laid down during this period.
The chalk rocks, like most other limy rocks, are composed of the limy shells of myriads of microscopic sea creatures. If a little chalk is ground up with water, it will be found to be full 0f these tiny shells (foraminifera), so small that about two thousand placed end to end stretch only about an inch. A cubic inch of this chalk will contain thousands of millions of shells. The seas of the period must have been swarming with these tiny sea creatures, and in the course of ages they accumulated on the floor of the sea and formed the white cliffs of Dover and all the other chalky rocks that we see now in the Cretaceous System. Because chalk beds formed during any period other than the Cretaceous are seldom found, this period is often known as the Age of Chalk.
The vegetation of the Cretaceous period is of special interest, for it was in the middle of this period that the first flowering plants appeared. These plants quickly spread throughout the world and took the place of the horsetails, ferns and cycads as rulers of the plant kingdom. The Cretaceous forests must have been very much like the forests we know to-day, although much richer and more luxuriant. Magnolias, myrtles, tulip trees, sassafras, oaks, beeches, elms, willows, palms and many other modern trees flourished in the warm, moist climate. When they died, they left impressions in the deposits of the Cretaceous period so that we have a record of them to-day. The Cretaceous rocks of northern Greenland have yielded up nearly two hundred species of plants, among them the breadfruit tree, which now grows only in tropical regions. More than a thousand Cretaceous plants have been discovered in North America. Because the flowering plants spread over the world so rapidly, the plants of Europe during this period were about the same as those growing in Greenland and North America.
The Cretaceous seas swarmed with life. In addition to the foraminifera, sponges, star-fish, sea-urchins, corals, polyzoa and crustaceans abounded. Sharks were quite common. Probably the largest fish was portheus, who reached a length of from eighteen to twenty feet and had a jaw a yard long, every inch of which was lined with long, sharp teeth.
SOME STRANGE REPTILES OF THE CRETACEOUS SEAS
Like the Jurassic, the Cretaceous period was an age of great lizards. One of the most interesting of these was the mosasaurus, which, when first discovered at Maestricht in Holland in 1780, was a great puzzle to the naturalists. At various times it was considered a fish, a whale and a crocodile. It was left for the genius of Cuvier to identify it as a reptile. The mosasaurus first found was only twenty-four feet long, but specimens have since been discovered about fifty feet long. These great creatures had fin-like paddles with which they propelled themselves through the sea. Like snakes, they had sharp teeth on the tops of their mouths, while their enormous jaws were jointed in such a way as to allow them to gape horribly and to swallow tremendous morsels. They must have been very formidable creatures. At the close of the Cretaceous they completely disappeared.
An amazing creature, too, was the elasmosaurus, which has been found in the chalk deposits of Kansas. It was sometimes fifty feet long, and twenty of this was a slender neck. It has been said that it probably often swam many feet below the surface “raising the head to the distant air for a breath, then withdrawing it and exploring the depths forty feet below without altering the position of its body.”
Another sea-dweller of the Cretaceous period, the turtle, has left a great number of descendants. The largest Cretaceous turtle is called archelon. It was twelve feet long and almost as wide. It swam by means of large flippers half as long as the body.
The iguanodon may perhaps be considered one of the most characteristic land reptiles of the period. It was first discovered in the Cretaceous rocks of Sussex; but no less than twenty-nine of them were found in a Belgian coal-mine. It was a herbivorous, or plant-eating animal, thirty or more feet long from nose to tip of tail, had a duck-like bill, and walked on its hind legs like a kangaroo. In the Royal Museum in Brussels nine skeletons of these strange creatures are mounted in one room. A North American relative of the iguarodon was the trachodon, a reptile that was as common during the Cretaceous period as deer were a few years ago. There is on exhibit at the American Museum of Natural History in New York the mumified remains of a trachodon. When he died millions of years ago, this animal must have lain exposed to the hot sun in a dry climate until his body was completely dried out and mummified. Then it was covered over by wind-blown sand which eventually became solidified into rock and preserved this unusual specimen. The trachodon skin was not so very different from the skin of some of our modern lizards.
THE MOST TERRIBLE CREATURE THAT EVER WALXED THE EARTH
Another Cretaceous dinosaur, tyrannosaurus rex, “King of the Tyrant Reptiles,” has been described as the most destructive life engine that ever lived. He roamed the ancient plains of Montana and Wyoming. His head towered from eighteen to twenty feet above the ground and his entire body was almost fifty feet long. His jaws were armed with curved, pointed, and double- edged teeth, the longest of which was six inches. Great claws enabled him to rend the flesh of his victims.
One of the most curious dinosaurs of the Cretaceous was the triceratops, a creature with a skull eight feet long and armed with three horns. Two of these horns were above his eyes and another grew from the middle of his snout. His skull projected backward over his neck to form a great bony collar that must have furnished an excellent protection against the attacks of such fierce animals as tyrannosaurus.
Besides the pterodactyls of the Cretaceous, there were a few true birds, some with teeth. Mammals were not yet plentiful.
