THE third period of the Mesozoic era, or middle age, is called Cretaceous, and the rocks formed during this closing period of the era are known as the Cretaceous system. The chalk which gives its name to this system (the Latin word Creta means chalk, hence the name Cretaceous) was first studied in the Anglo-Parisian basin. The chalk cliffs of England have long been well known, and the chalky limestone of Kansas belongs in this system.
But although the name Cretaceous was originally given to rocks containing chalk, the rocks of the Cretaceous system as we now know it contain chalk in relatively few places. The name illustrates a common custom in geology and also one of the serious drawbacks resulting from this custom. Many rock formations are named for some local lithologic characteristic. The Exwyzee sandstone is a sandstone at Exwyzee, where it was first examined and named, but when this formation was traced laterally it was found that the Exwyzee sandstone consists of shale at some places and of limestone at other places. It therefore becomes necessary to explain that the Exwyzee formation is not everywhere a sandstone; it is a sandstone only at the place from which it was named.
During the Cretaceous period the sea overwhelmed the continents, submerging their low lands. The Cretaceous was a period of change in many waysa period of advance toward present conditions. The lower orders of plants, which had held first place, now took a subordinate position, and the higher orders of plants, such as we are familiar with today, assumed the leading role. The Cretaceous period marks the end of the age of reptiles. Some of these creatures, however, were then still numerous, but, like the plants, they were destined to give place to more modern forms. They did not long survive the geologic revolution that brought the Cretaceous period to an end. This revolution involved the emergence of continents from the sea, the formation of new ranges of mountains, and the introduction of animals of modern types.
Mesa Verde National Park
Rocks of Cretaceous age are conspicuously exposed in Mesa Verde National Park. The sandstone that caps Mesa Verde is harder than the shale that underlies it and has resisted erosion more strongly, so that it now forms the top of the most notable table-lands of southwestern Colorado. The Mesaverde formation, which was named from this mesa, is of commercial interest, for it contains vast deposits of coal, and it is of historic interest, for in the shelter of its protecting cliffs many of the ancient cliff-dwellers found safety.
The ruined houses of the cliff-dwellers (Plate XXIX), perched in almost inaccessible places, appeal powerfully to the imagination. Unfortunately those who dwelt in these natural fortresses left no written records. Their language is unknown. Many of the dwellings are well preserved and some of them contained utensils. But when we try to find who the cliff-dwellers were and why they sought such well-protected but difficult dwelling places we are baffled.
In 1888 Richard and Alfred Wetherill found the ruins known as Cliff Palace and Spruce Tree House. Since that time great numbers of other ruins have been found.
The interest in the ruins induced Congress to set aside Mesa Verde as a national park in 1906, eighteen years after they were discovered and after many of them had been looted.
Mesa Verde National Park is an area of irregular outline containing 48,966 acres, situated on Mesa Verde, a Spanish term meaning Green Table. This mesa is one of the conspicuous geographic features of southwestern Colorado. It rises from the surrounding lowlands by steep, shaly slopes surmounted by a sandstone cliff. It reaches its maximum altitude, 8,575 feet, in Park Point, but its most conspicuous summit as seen from below is Point Lookout, which has a height of 8,429 feet. The walls of the mesa rise steeply about 2,000 feet above the floor of Mancos Valley on the east and about 2,300 feet above the floor of the broad Montezuma Valley, which lies north and west of the Park.
W. H. Holmes, who visited this mesa in 1875, says of it:
The general level is well sustained by the massive layer of sandstone of the Upper Escarpment [now called Cliff House sandstone], not as an unbroken mass or block of strata, of course, for the erosive forces have invaded it on all sides and the edges are scalloped by a thousand gorges. Not only have the destroying forces encroached from the edges on all sides, but the mesa has been entirely severed by the canyon of the Rio Mancos, which cut through it from north to west. This cañon sends out a multitude of side cañons, which seem literally to have honeycombed the interior of the tableland. [The Rio Mancos swings to the west just south of the park boundary and into it empty all of the southward-flowing streams of the mesa.]
The outer rim of the mesa is intact. Viewed from the side the cap rock seems to be continuous, but in reality it is a mere skeleton. The outer rim has been likened to the tire of a wheel and the long strips of rock that separate the canyons to spokes of the wheel. The sandstone that caps the mesa was once continuous instead of trenched by long, narrow gorges, as it is now. Before it was trenched the surface sloped gently to the south, as the remnants of the cap rock do now.
The Little People
The modern Pueblo Indians of the Southwest, who dwell in community houses, are supposed to be the descendants of the cliff-dwellers. The objection of these Indians to the exploitation of the ruins by white men and their belief that the ancient dwellings are inhabited by spiritsthe Little Peoplegive color to this supposition. Their attitude was shown by their opposition to the installation of the telephone. As published by the National Park Service, the story runs as follows:
Indians of today shun the ruins of the Mesa Verde. They believe them inhabited by spirits whom they call the Little People. It is vain to tell them that the Little People were their own ancestors; they refuse to believe it.
When the National Park telephone line was building, in 1915, the Indians were greatly excited. Coming to the supervisor's office to trade, they shook their heads ominously.
The poles wouldn't stand up, they declared. Why? Because the Little People wouldn't like such an uncanny thing as a telephone.
But the poles were standing, the supervisor pointed out. All right, the Indians replied, but wait. The wires wouldn't talk. The Little People wouldn't like it.
The poles were finally all in and the wires strung. What was more, the wires actually did talk and are still talking.
Never mind, say the Indians, with unshaken faith. Never mind. Wait. That's all. It will come. The Little People may stand it for a while; but wait. The supervisor is still waiting.
Story of the Rocks
The mode of formation of the rocks, the differences in their hardness, why some contain fossil shells and others contain beds of coal and impressions of plants, are made clear by studies grouped under the head of geology. Why the mesa stands out so prominently above the surrounding country, why it is carved so ornately, how the sheltered nooks and alcoves were formed in which the cliff-dwellings were built, are made clear by a study of natural processes grouped under the head of physiography. The conclusion reached may be made sufficiently clear by presenting in story form the ancient events in order as they occurred, leading up to the production of the features as we see them.
The story may begin at the time when the oldest beds of the park were formed. These beds constitute the Mancos shale, so called from the town of Mancos, which stands on them. They are made up of mud that was deposited in Cretaceous time, when this part of North America was occupied by a sea. Mollusks lived and died in this sea and their shells were buried in the mud.
The sea was gradually filled with sediment, which finally consisted mainly of sand, until the water became shallow. At this time was formed the lower sandstone of the mesa rim, now exposed on Point Lookout,the sandstone forming the lowest layer of the rocks called the Mesaverde formation.
A little later the sea was expelled entirely from this part of the basin, which became a low-lying coastal plain on which a great variety of plants grew, and swamps were formed in which accumulated the vegetable matter that later turned to coal, for the Mesaverde formation contains valuable beds of coal.
As time went on the swamps, with their beds of peat, were buried under sand and mud washed by the streams from the highlands farther west, and the alternating beds of coal, shale, and sand formed a mass about 400 feet thick. This mass, which lies upon the Point Lookout sandstone, has been named the Menefee formation, from Menefee Mountain, which stands east of the park, just south of the town of Mancos.
Conditions favorable to the accumulation of coal recurred many times in late Cretaceous time as new beds of sand accumulated on some new coastal plain. At some times the greater part of the basin was occupied by the sea; at other times sediments accumulated to such thickness that the water was expelled from large parts of it. The resulting migration of the shore line has never been worked out in detail, but enough is known to indicate that the history of the time is complex.
Some of the coastal plains that were developed in later Cretaceous time were 200 or 300 miles wide and reached from central Utah eastward at least as far as the Rocky Mountains, in central Colorado.
The plants which furnished the material that formed coal left many records in the rocks. Innumerable leaf impressions and petrified trunks of trees have been found. Some of these impressions represent modern types of plants. In some places fossil leaves are particularly abundant. These plants belong to species that indicate a warm, moist climate for western America during Upper Cretaceous time.
The animals of the Cretaceous epoch are no less interesting than those of the Jurassic. Reptiles were still rulers of earth, air, and sea. Their remains in America have been found chiefly in the West. The earlier horned dinosaurs differed in many ways from their descendants, especially in having a horn on the nose instead of horns over the eyes. The jaws of Triceratops, one of the later forms, terminated in a sharp beak, like that of a turtle, and the neck was protected by a scalloped bony shield called the frill.
The primitive horned dinosaurs ranged at least from New Mexico northward to Canada. They were peaceable plant-eaters and obviously needed protection from their enemies, the carnivorous dinosaurs. Hence the head was armed with horns, the neck was protected by the bony frill or shield, and the body was covered with a tough, scaly hide. These were the ancestors of Triceratops, which roamed the western plains at a later time (Plate XXXII, B).
The rulers of the air during this epoch were the flying reptiles called pterodactyls. Some of them were great dragons measuring 18 or 20 feet from tip to tip of wings. Their long jaws were armed with teeth, and their bat-like wings were formed of a thin membrane connecting greatly elongated fingers. These flying engines of destruction have been called the prototypes of the modern airplane.
Other inhabitants of the air in western America were certain toothed birds. These were connecting links between the reptile-like Jurassic birds, which had toothed jaws without beaks, and the modern birds, which have beaks but no teeth.
The fossil remains of the birds and of the reptiles of this time show many puzzling resemblances, which make it difficult to distinguish them. The question was debated whether Archeopteryx was a bird or a reptile, and it is now recognized that birds and reptiles are closely related. On this subject Dr. W. H. Ballou writes as follows in the Scientific American (1919):
In the sense that clothes make the man, feathers make the bird. Hence the bird is merely a flying reptile, feathered more or less, according to species. Man has little of the reptile structure, but a bird has little else. Feathers, then, merely conceal the reptile. When Robin Redbreast lifts up his head and pours out his morning song, the brain that guides it is almost identical with that of the young alligator, which, while it can not sing, bays and roars pretty loudly. Mrs. Robin lays an egg and so does Mrs. Alligator.
The rulers of the Mesozoic sea were the swimming reptiles, the mosasaurs. These were powerful swimmers and found their way to all parts of the interior Cretaceous sea. Their long, slim bodies have suggested the name ancient sea serpents. Their powerful jaws were armed with long, conical, sharp-pointed teeth. These they used for capturing prey and in fighting among themselves.
Fossil bones of mosasaurs marked by the teeth of other mosasaurs have been found. One of these bitten bones has a tooth embedded in it. The victor in the combat did not escape without injury. Apparently the victim struggled so fiercely as to break away from its enemy. The wound healed, incasing the enemy's tooth, and later, after the death of the escaped mosasaur, the bone, with the tooth in it, was buried and preserved as a fossil.
Last Updated: 31-Dec-2009