Public domain
[p. 742]
The Cretaceous period was ushered in, so far as North America is concerned, by a notable encroachment of the sea. In the United States, the system is found in (1) the Atlantic Coastal Plain; (2) the Coastal Plain of the Gulf, both east and west of the Mississippi ;
(3) the Great Plains, from the Gulf of Mexico to the Arctic Ocean;
(4) at many points in the western mountains, and (5) over considerable areas along the Pacific coast. While the distribution of this system has much in common with that of the Comanchean, it is much more wide-spread (Fig. 504). Unlike the Comanchean, the system is chiefly marine.
The Atlantic Border Region[1]
The^ Cretaceous beds come to the surface in a belt near the western margin of the Atlantic Coastal Plain, just east of the outcrop of the Potomac series (Fig. 504). The beds have been but little disturbed, and still dip, as when deposited, slightly to seaward, and in that direction pass beneath younger formations. The formations are chiefly of unindurated clay and sand, with not a little greensand marl, which is rather characteristic of the system. There is also some limestone.
The distinguishing constituent of the greensand marl is glauconite, primarily a hydrous silicate of potash and iron,[2] which occurs in grains. Glauconite is now making in some parts of the sea, and from the situations in which it is formed, it is inferred that [p. 743] the conditions necessary for its development are the following: [3] (1) Water of moderate depth, 100 to 200 fathoms being the most favorable; (2) a meager supply of land-derived sediment; and (3) the presence of foraminifera. The production of the glauconite seems to be effected by chemical changes induced in sediments perhaps as the result of decomposition of the organic matter contained in the foraminiferal shells. The abundance of greensand marl, which is not a common formation outside the Cretaceous, in corresponding systems of different continents, is one of the many striking inter-continental resemblances.
[p. 744]
The aggregate thickness of the Cretaceous beds along the Atlantic coast nowhere exceeds a few hundred feet.
The subdivisions now generally recognized are the following, commencing with the lowest: 1. Matawan formation, 2. Monmouth formation; 3. Rancocas formation; 4. Manasquan formation.
These formations are not severally continuous throughout the coastal region. Thus the Matawan formation does not appear at the surface south of Maryland, being overlapped in that direction by later beds. All the formations show notable variations when traced along their strikes, and borings to the east of the outcrops show that they also vary seaward from their landward margins.
Though the beds have been but little changed since their deposition, certain alterations are worthy of note. The porous beds of greensand marl have been changed, locally, to brown, by the decomposition of the silicate and the formation of ferric oxide. Cementation, chiefly by ferric oxide, has indurated certain beds at some localities, and many of the conspicuous hills within the area of Cretaceous outcrops are due to a capping of this ironstone. The cemented layers are most likely to occur at the junction of formations of different texture, a generalization which holds in other partially indurated series.
The Eastern Gulf Border
The Cretaceous formations of the Eastern Gulf states appear at the surface some distance from the coast (Fig. 505), and dip [p. 745] seaward at a low angle. Near the Mississippi, the belt of their exposure extends northward to Kentucky. If any of the formations once had greater extensions to the northward, as is probable, they have been worn away except for possible meager remnants, not certainly identified.
In Alabama,[4] where the system is best known, there are three principal divisions : the Eutaw below (mainly clays and sands, some greensand, 300 feet), the Selma Chalk (Rotten limestone, 1,000 feet) in the middle, and Ripley (mainly sand, 200-500 feet) above. The Eutaw is believed to be the equivalent of the Matawan formation of the Atlantic coast, and the Ripley is thought to be older than the Rancocas.
[p. 746]
The Cretaceous beds of the Gulf coast (Alabama) have been disturbed rather more than the corresponding beds along those parts of the Atlantic coast where the system has been carefully studied. They have been bent into low anticlines and synclines in some places, and even faulted to a slight extent.
The Western Gulf Region[5]
The general stratigraphic relations of the system here are the same as farther east, but deposition seems to have been well under way in Texas before the oldest beds of the system farther east were laid down. Alternating beds of sand, shale, limestone, and marl, most of which are of marine origin, make up the system. They attain a maximum thickness of about 4,000 feet. Three principal subdivisions are recognized:[6] (1) The Dakota; (2) the Colorado; and (3) the Montana.
The Dakota formation, 600 feet and less thick, is largely of sandstone, with some lignite, and is, for the most part, of nonmarine origin. The Colorado series contains much limestone (or chalk) of marine origin. Its thickness is about 1,000 feet. The Montana series is more largely clastic, and from it the oil of the Corsicana oil field of Texas is derived. Locally, the system is much faulted. From Texas it is continued northward into Arkansas, and westward into New Mexico.
The Cretaceous of the western Gulf region differs from the corresponding system farther east, in its greater thickness, and in greater proportion of calcareous matter, largely in the condition of chalk. Of limestone or chalk, the Cretaceous of the Atlantic coast contains little, that of the eastern Gulf region (Alabama and Mississippi) more, and that of Texas much; nor is the chalk confined to Texas, as will be seen.
The Western Interior
The Cretaceous system of the western interior consists of the following subdivisions, commencing at the bottom: 1. Dakota; 2. Colorado (including the Benton and the Niobrara formations) ; 3. Montana (including Ft. Pierre and Fox Hills); and 4. Laramic.
[p. 747]
The Dakota formation. The Dakota formation, mainly of nonmarine origin, is wide-spread in the Great Plains, though buried over the greater part of the area. It extends westward beyond the Rocky Mountains, though in the mountain region the area of deposition was interrupted by elevations which rose above the lakes, marshes, or river flats where the sedimentation took place. The formation is largely sandstone, though it contains much conglomerate and clay, and some lignite.
The formation was formerly regarded as lacustrine, but it is perhaps to be regarded rather as the joint product of subaerial and fluviatile deposition. The presence of bird tracks in Kansas[7] and the wide-spread abundance of fossil leaves of angiosperms, in a condition which precludes much transportation, imply subaerial sedimentation to a notable extent at least. The upper part of the formation carries some marine fossils. North of Texas the formation is in apparent conformity with the Comanchean in some places, though in others, as in the Wasatch and Uinta mountains, it rests on older formations.
The formation is an important source of water in the semi-arid plains. It takes in the water where it outcrops near the mountains and the water follows the beds down their dip to the eastward. Along the east base of the Rocky Mountains, where the beds have been tilted, the less resistant formations associated with this sandstone have been removed or worn down, leaving the outcropping edges of this formation as ridges or “hogbacks” (Fig. 120), characteristic of the east base of the Rocky Mountains much of the way from New Mexico to Canada.
The Colorado series.[8] The Colorado series records an extensive invasion of the western interior by the sea, the invasion going so far, probably, as to establish a connection between the Gulf of Mexico on the south and the Arctic Ocean on the north, over the site of the Great Plains. The western border of this sea appears to have been in Arizona, Utah, eastern Idaho, and western Montana, and the eastern as far east as Minnesota, Iowa, and Kansas. Clastic [p. 748] formations predominate in the Colorado series as a whole, hut there are beds of chalk comparable to those of Europe, in Texas, Kan Iowa, Nebraska, and South Dakota. Occasional beds of coal are present, probably formed about the borders of the sea, or about the islands which stood above it. Charred wood and even charcoal in the series point to the existence of fires during the epoch. The aggregate thickness of the series is locally as much as 3,000 feet, as strata are measured, though its average thickness is much less. The earlier formations of the Gulf region are probably not older than the Colorado series of the western interior.
The origin of the chalk.[9] There has been much different o| opinion concerning the origin of chalk. Its resemblance to the foraminiferal ooze of the deep seas long since led to the belief thai it was a deep-sea deposit; but closer examination has thrown doubt on this conclusion, for the differences between the chalk and foraminiferal ooze are as striking as their likenesses. Both consist [p. 749] largely of the shells of foraminifera; but with them are associated shells of other types, some of which are similar in the two formations, and some dissimilar. The echinoderms, the sponge spicules, and the secretions of certain microscopic plants correspond in a general way with those of the oozes now forming, and are consistent with the deep-water origin of the chalk. The molluscan shells of the chalk, on the other hand, seem to point with clearness to water no more than 30 to 50 fathoms deep. The distribution of the chalk and its relations to other sedimentary beds indicate its deposition in shallow water, not in water comparable in depth to that in which oozes are now formed. On the whole, the balance of evidence is in favor of the view that the Cretaceous chalk was deposited in relatively shallow water. The conditions for its origin seem to have been clear seas, with a genial climate. Its materials may accumulate as well on the bottom of a shallow sea as on the bottom of a deep one, if clastic sediments are absent.
The Montana series. Following the Colorado epoch, there were changes in the sedimentation and in the life of the western interior sea. The Montana series is chiefly clastic, but the area of sedimentation was somewhat contracted. The beds are, for the most part, marine, and the water shallowed as the epoch progressed. Local beds of coal give evidence of marshy conditions. Like other parts of the Cretaceous system of the west, the Montana series abounds in concretions, some of which attain great size. The thickness of the series is variable, and its maximum is great. From 8700 feet in Colorado, it thins to 200 feet in some parts of the Black Hills. The Ripley formation of the Gulf region is of about the same age, probably, as the Montana series.
The Laramie.[10] Deposition continued in the Great Plains and to some extent west of them through the last epoch of the Cretaceous period, but most of the sedimentation was non-marine. Fresh-and brackish-water beds are widely distributed. The Laramie series may be said to record the transition from the marine conditions of the Montana epoch, to the fresh-water and land conditions of the [p. 750] Tertiary in the same region. This change probably did not take place everywhere at the same time, so that beds correlated with the Montana series at one point, are perhaps the time equivalents of beds classed as Laramie at other points. The series consists primarily of sandstone, and shale, with some conglomerate; but with these clastic formations there is much coal. Both shale and coal are more abundant below than above, while in the upper part of the series conglomerate is not rare. The materials of the Laramie formation seem to have been derived principally from the pre-Paleozoic rocks of the mountain region. The thickness of the Laramie is estimated at 1,000 to 5,000 feet, exclusive of the transition (Mesozoic-Cenozoic) beds to be mentioned below. In not a few places there is an unconformity in the great group of strata heretofore classed as Laramie, and there is difference of opinion as to whether the part above this unconformity should be called Laramie. The present tendency is to regard it as Eocene.[11] It may be added that there is difference of opinion as to whether there is one widespread, major unconformity at the horizon indicated above, or whether there are diverse local unconformities. If the former, it should have great weight in determining the classification. The latter would be less significant.
In a considerable area of northeastern Wyoming, and in a large area farther north,[12] some of the Laramie lignite has been burned in the ground. The burning was relatively recent, and locally is still is progress. The firing appears to have taken place at the outcrops on hill and valley slopes. The burning was accompanied by fusion, semi-fusion, and baking, resulting in lava-like slag and brick-red banks of indurated clay. The slag, etc., has had a notal >le effect on the details of the topography (Fig. 507) developed by wind and water, and the color effects of the burning are striking.
Coal. The Cretaceous is pre-eminently the coal period of the west. Coal-beds occur in every one of its principal divisions in [p. 751] this part of the continent. The total amount of coal, chiefly in the Laramie, is perhaps comparable to that in the Pennsylvanian system, though the Laramie coal is not now so accessible, and its quality is inferior. It is estimated that along the east and west bases of the Rocky Mountains there are more than 100,000 square miles of coal-bearing lands, and Colorado alone is estimated to have 34,000,000,000 tons of available coal,[13] most of which is Cretaceous. The coal is largely lignite, though in Colorado not a little of it has been advanced to coking bituminous coal, and even to anthracite.[14] Anthracite referred to the Laramie also occurs farther south in localities where it has been affected by intrusions of igneous rock. The areas of Laramie coal are indicated in Fig. 438.
Transition beds between Mesozoic and Cenozoic. There are diverse, more or less local, terrestrial formations in the west which have been referred now to the Cretaceous (Laramie, — or more [p. 752] exactly, to the upper Laramie or post-Laramie) , and now to the Tertiary (Eocene). They have heretofore been regarded, most commonly, as the transition beds between the Cretaceous and the Tertiary. It has been proposed recently to group these formations together under the name Shoshone, and to class them as Eocene.[15] These formations are, generally speaking, unconformable on the Laramie and, in some places seem hardly separable from the recognized Tertiary[16] (Fort Union). Their reference to the Eocene seems to be justified both on stratigraphic and paleontologic grounds, so far as present data are concerned.
The Pacific Coast[17]
The Cretaceous system is represented on the Pacific coast by the marine Chico series. At the time of its origin, this series probably extended along the coast from Lower California to the Yukon. The Chico series rests on the Shastan unconformably in some places,[18] and overlaps it at others.[19] The fauna of the Chico series is littoral. Its oldest portion is older than the fauna of the Colorado series, and its youngest is older than the fauna of the youngest Cretaceous beds in some other places.[20]
Close of the Period
About the close of the Cretaceous period a series of disturbances was inaugurated on a scale which had not been equalled since the close of the Paleozoic era. These changes furnish the basis for the classification which makes the close of this period the close of an era. These disturbances continued into later times, but the close of the Cretaceous may be said to have been the time when the changes had advanced so far as to make themselves felt profoundly. They consisted of deformative movements, a part of which were orogenic, and of igneous eruptions on an almost unprecedented scale.
[p. 753]
General movements. In the closing stages of the period, the sea which had lapped over the Coastal Plains of the Atlantic and the Gulf of Mexico was withdrawn toward the abysmal basin. At about the same time, the Appalachian Mountains, which had been reduced to a peneplain by this time, were bowed up again. This later movement was chiefly vertical, while the Permian deformation was primarily horizontal, — a folding movement.
In the western interior, the sea had abandoned the area of the Great Plains, and other tracts in the mountains farther west, before the close of the Laramie. It is probable that most of the Cordilleran region was elevated bodily at this time, though not to a great height. Without further details, it may be said that enough is known to make it probable that a large part of the continent was affected by deformative movements of a gentle sort.
Orogenic movements. The growth of mountains locally by folding was probably in progress in the closing stages of the Cretaceous period from Alaska on the north to Cape Horn on the south, — more than a quarter of the circumference of the earth. Folding [p. 754] movements probably affected the Antillean mountain system,[21] between the southern end of the Cordilleran and the northern end of the Andean systems, at the same time, for in several of the Antillean islands later formations rest unconformably on the deformed Cretaceous beds. Where the Eocene rests conformably on the Laramie, the disturbances of this time are not clearly distinguishable from those of later date, which increased the folding initiated in this epoch. Some of the folded ranges of the western mountains began their history at this time, others had a new period of growttt and still others date from a later time; yet the close of the Laramie was a time of general orogenic movement in the western part of [p. 755] North America. The Rocky Mountain system may be said to have had its birth at this time. That these mountains are not older is shown by the deformation of the Laramie beds along with those of greater’ age. That some of the folding was not younger, is shown by the lesser deformation of the Tertiary beds in the same region. It should be added that most of the western mountains which began their history at this time are unlike the Appalachians, as developed at the close of the Paleozoic. In the first development of the latter, horizontal movement was the great factor involved; in most of the former, vertical movement.
Faulting. The growth of mountains at the close of the Cretaceous was accompanied by faulting on a somewhat extensive scale throughout the region of movement, though the faulting of this time cannot always be distinguished from that of later date. In the Rocky Mountains of British Columbia, one overthrust fault has been located which crowded the Cambrian rocks obliquely up over the Cretaceous. The horizontal displacement is estimated to be as much as seven miles[22] and the throw 15,000 feet. Near the national boundary, the displacement of wThat appears to be the same fault crowded the Proterozoic up over the Cretaceous[23] by a movement of equal magnitude (Fig. 510). The exact date of these faults has not been determined, but was, perhaps, mid -Tertiary.
Igneous eruptions. The close of the Cretaceous was marked by the inauguration of a period of exceptional igneous activity, continuing far into the Tertiary. During this period, great bodies of igneous rock, both extrusive and intrusive, were forced up. Eruptions occurred in other lands at about the same time.
Europe. The distribution of the Upper Cretaceous strata of Europe shows that extensive transgressions of the sea occurred at the beginning of the period, for in some parts of the continent, marine Cretaceous formations overlap all older Mesozoic systems.
[p. 756]
During the closing stages of the Upper Cretaceous, fresh-water beds appear in localities (alpine region) where marine sedimentation bad been in progress earlier in the period, showing that the movements [p.757] which were to mark the close of the era were making themselves felt.
Limestone is the dominant sort of rock in the Upper Crustaceous of southern Europe, showing that clear seas still prevailed, as in the Early Cretaceous period. From a characteristic genus of fossils, much of the limestone is known as Hippurite limestone. In the system farther north, there is more clastic material.
The most notable petrographic feature of the Upper Cretaceous of Europe is the chalk. Both in England and France it attains an aggregate thickness of several hundred feet, though much of it is far from pure. It grades into marls and clays on the one hand, and into sandstone on the other. Chalk is, however, by no means coextensive with the system, for it has little development outside of the Anglo-French area. The name “Cretaceous,” therefore, as generally used, is as inappropriate as a name could well be, having no applicability to the Lower Cretaceous, and fitting only a relatively small area of the Upper. Even within the areas where chalk occurs, it is not everywhere the dominant sort of rock. Greensand occurs in the Upper Cretaceous as well as in the Lower. The principal subdivisions of the system recognized in western Europe are 1. Albian; 2. Cenomanian; 3. Turonian; 4. Senonian, and 5. Danian, numbered from the base up.
Asia. The submergence of Europe and North America at the beginning of the Upper Cretaceous finds its parallel in other continents. There are extensive areas of Hippurite limestone in southwestern Asia, closely connected with that of Europe on the one hand, and with that of North Africa on the other. The Himalayan region seems to have been still beneath the sea, for Upper Cretaceous formations are found in the mountains at great elevations. Greensand occurs in the Salt Range of India.[25] South of these marine beds there appears to have been a large tract of land, including much of the peninsula of India, which has been thought to have stretched southwest to Africa, though the configuration of the sea-bottom does not lend this view much support.
Upper Cretaceous beds occur on the eastern coast of China, and in Japan. In many places they rest on formations older than the Lower Cretaceous, and therefore record an increased submergence [p. 758] dating from the beginning or early part of the Upper Cretaceous. On the other hand, northern Asia, which was largely submerged during the earlier Cretaceous period, was largely land during the later.
Late in the Upper Cretaceous occurred the extensive lava-flows of the Deccan. These flows, 4,000 to 6,000 feet in thickness, cover an area of something like 200,000 square miles, and are perhaps the most stupenduous outflows of lava recorded in the earth’s history. The fossils in sediments interbedded with the lava, show that the flows were subaerial.
Africa. In northern Africa, the Lower Cretaceous beds are confined to the northwestern mountains, but the Upper Cretaceous beds, which overlie the Lower unconformably,[26] spread southward and cover most of the desert, indicating great submergence in the north African region. South of the Sahara, no Upper Cretaceous beds are known except in a few small areas about the coast, where they rest on crystalline schists with no Lower Cretaceous beds beneath, so far as now known.
South America. In South America, the sea invaded eastern Brazil, where marine Upper Cretaceous beds cover and overlap the non-marine Lower Cretaceous. In some parts of Brazil, however, the Upper Cretaceous is represented by fresh-water beds only. Farther west, marine Upper Cretaceous beds rest unconformably on the Lower Cretaceous, and form the summits of much of the eastern Andes, occurring up to altitudes of 14,000 feet at many points, and locally even higher. Upper Cretaceous beds also occur in southern Patagonia. There appears to have been great volcanic activity in the Andean system (Chile and Peru) during the late Cretaceous.
Australia. The phenomena of Australia are in harmony wit those of the other continents. The Upper Cretaceous beds wide-spread, locally resting on formations older than the Lower Cretaceous. Furthermore, the Upper Cretaceous (Desert Sand-stone) is in many places unconformable on the upturned and denuded Lower Cretaceous, showing that there were deformative movements, as well as movements which changed the relations of [p. 759] sea and, land, after the deposition of the Lower Cretaceous beds and before the deposition of the Upper.
General. In general it may be said that there was little marine sedimentation in the Late Cretaceous period north of the parallel 60° north, while the Jurassic and Lower Cretaceous systems are there more wide-spread. Between the parallels of 20° and 60°, on the other hand, the zone where marine Lower Cretaceous is but slightly developed, the Upper Cretaceous system is wide-spread. Outside of China, the Upper Cretaceous system is wanting over no considerable land-area within these limits. In the equatorial and south temperate zones, the Upper Cretaceous seas were also expanded much beyond the limits of the waters of the preceding period.
Climate
The climate of North America throughout most of the Cretaceous period seems to have been rather uniform and warm throughout a great range of latitude. In Greenland, Alaska, and Spitzbergen, the climatic conditions seem to have been similar to those in Virginia. Toward the close of the period the temperature was perhaps lower, for the Laramie flora is a temperate, rather than a tropical one. The fresh-water fossils of central Europe indicate a climate comparable to that of Malaysia.[27] As this seems to have been a period of low land, widely extended epicontinental seas, extensive calcareous deposits, and slow consumption of carbon dioxide in rock solution and carbonation, there was a combination
conditions regarded as favorable for a mild and uniform climate.
The Land Plants
Angiosperms predominated in North America at the beginning of the Cretaceous, and during the period genera now living came to be numerous, giving the flora a modern aspect. Among the living genera of angiosperms (Fig. 513) that made their appearance were those which include the birch, beech, oak, walnut, sycamore, tulip-tree, laurel, cinnamon, maple, holly, sweet-gum, ivy, and oleander. Among the gymnosperms, there was a notable development of the sequoias, which now include the giant trees of California. The modern Cycas was present, and the ginkgo had some prominence, though never a leading type. Worthy of special note was the presence of genera in Europe and the United States which are now confined to the southern hemisphere, as Podocarpus, the leading pine of the southern hemisphere. Some of these remained in the northern regions till the early Cenozoic.
[p. 760]
[p. 761]
Previous to the later stages of this period, monocotyledons played but an insignificant part in the floral record, but they now began to assume a position of importance. Palms were plentiful, even in northerly latitudes, before the close of the period, and some of them were closely allied to existing palms. Of even more interest, because of their relations to the evolution of grazing animals, was the appearance of grasses, which do not, however, appear to have attained prominence until later.
It is worthy of remark that the introduction of dicotyledons, the great bearers of fruits and nuts, and of monocotyledons, the greatest grain and fodder producers, was the groundwork for a profound evolution of herbivorous and frugiverous land animals, and these in turn, for the development of the animals that prey upon them. A zoological revolution, as extraordinary as the botanical one, might naturally be anticipated; but it did not follow immediately, so far as the record shows. The reptiles seem to have roamed through the new forests as they had through the old, without radical modification. The zoological transformation may have been delayed because the appropriate animals had not then come into contact with the new phases of plant life. With the opening Tertiary, the anticipated revolution in the animal life of the land made its appearance, and advanced with great rapidity.
The new flora spread widely. Not only was the European flora essentially the same as the American, but there was a close resemblance between the flora of mid-Greenland (70°-72° Lat.) and that of Maryland and Virginia, indicating climatic conditions of remarkable uniformity. Not only this, but the flora was of a sub-tropical type.
[p. 762]
The Land Animals
The terrestrial animals had the same general aspect as in the preceding period. In Europe, where the sea made great inroads upon the land, there was some decline in their abundance, variety and proportions; but in America, the area of land was not small enough to restrain greatly the evolution of the reptiles. On both continents the aquatic reptiles seem to have made greater pro<: than the terrestrial forms.
The dinosaurs still retained the leading place among the land reptiles, though the carnivorous forms (Theropoda) were less abundant and varied than before. Among them was a leaping, kangaroo-like form (Laelaps- or Dryptosaurus) with a length of 15 feet. The most singular dinosaurian development was among the herbivorous branch, some of which were very large, of quadrupedal habit, with enormous skulls which extended backwards over the neck and shoulders in a cape-like flange (Fig. 514) . Added to this was a sharp, parrot-like beak, a stout horn on the nose, a pair of large pointed horns on the top of the head, and a row of projections around the edge of the cape. One of the larger skulls measures eight feet from the snout to edge of the cape. This excessive provision for defense was not unnaturally accompanied by evidences of lowmentality, in the form of a very small brain cavity. Marsh remarks that they had the largest heads and the smallest brains of the reptile race. They were doubtless stupid and sluggish. The ornithopod division was well represented (Fig. 516). Their posterior parts were strongly developed, their limbs were hollow, and their footprints indicate that they walked in kangaroo-like at i it ude". Save for the thalattosaurs, some of the dinosaurs (of the Niobrara) were among the first distinctively American air-breathing forma since the Permian.
Distinctively terrestrial turtle remains are found in the Dakota sandstone, and the fossils of species inhabiting fresh waters have been found in the late Cretaceous (Belly River) deposits of Canada. Of true lizards, which appeared in the Triassic, only one later Me zoic form is known, and that of small size and uncertain affinities from the Laramie. Snakes made their first appearance, so far known, in the later part of the period, but they were small. Among the crocodiles, the long-snouted teleosaurs persisted, in North America at least, until well into the Cretaceous; but for the most part the order underwent a marked change early in the period, developing into the modern type of crocodiles and gavials. A few small salamanders, of modern type, are known from the late Cretaceous.
[p. 763]
[p. 764]
The flying reptiles made so distinct an advance in specialization, that Williston regards them as having come to excel all other flyini vertebrate animals. Some attained a wing-spread of perhaps -<» feet. In some of the genera (Fig. 517) the development of the anterior parts was disproportionately great, while the posterior parts were so very small and weak that it is doubtful whether thry could stand on their feet alone. The Cretaceous forms were all short-tailed, and for the most part toothless, though the toothed [p. 765] forms persisted for a while. Their bills resembled those of modern birds.
Terrestrial birds existed, but the fossil record of them is meager. There were some curious aquatic forms, which will be mentioned in connection with the sea.life. The mammals thus far recovered from the Cretaceous indicate little advance upon those of the Jurassic. Mammals appear to have played a very inconspicuous part in the fauna of the period.
The Sea Life
Vertebrates. The ichthyosaurs and plesiosaurs which had dominated the Jurassic sea lived on into the Cretaceous. The former dropped into an insignificant place soon after the beginning of the period, but the plesiosaurs attained their highest development and perhaps their greatest size, during the period. The American types of plesiosaurs indicate lack of intermigration between this continent and Europe.
The aquatic branch of the scaled saurians (Squamata) attained great importance as veritable sea serpents. The long-necked, lizard-like reptiles of the Comanchean period were the forerunners [p. 766] and perhaps the direct ancestors, of the mosasaurians (Fig. 518), a family which nourished in the Cretaceous, and ranged from North and South America to Europe and New Zealand. Their short career seems to have ended with the period, and no direct descendants are known.
Marine turtles seem to have appeared first in this period, and to have deployed into many and diverse forms. The largest were broad, and flat, degenerate in having the carapace reduced to the ribs alone, and probably covered with a soft skin, like some living marine turtles. Some of them had skulls larger than those of horses and must have measured fully twelve feet across the shell.
In the long interval between the first known appearance of birds in the Jurassic, and the later Cretaceous when they reappeared, important changes took place, among which was the loss of the elongate, bilaterally feathered tail. The Jurassic birds were terrestrial, while the Cretaceous were aquatic. The Cretaceous birds include about 30 species belonging to two widely divergent ordi Hesperornis and Ichthyornis. The former (Fig. 520) were large, [p. 767] flightless, highly specialized divers, with aborted wings and remarkable legs. This implies that, following the evolution which had produced the wings, there had been a degenerative history long enough for them to dwindle almost to the point of extinction. Concurrent with this, and doubtless its cause, was an extraordinary development of the legs. They were not only very powerful, but the bones of the feet were so joined to the legs as to allow the feet to turn edgewise in the water when brought forward, thus increasing their efficiency as paddles. Furthermore, the legs were so joined to the body frame as to stand out nearly at right angles to it, like a pair of oars, instead of standing under the body like walking legs.[28] Apparently, walking as well as flying had been abandoned, and the organism was specialized for swimming and diving only. For this purpose, the head, neck, and body were admirably adapted. The jaws were armed with teeth set in a groove in primitive saurian fashion, and, like the jaws of snakes, were separable so as to admit large prey. As these strange birds attained a length of six feet in some cases, they were doubtless formidable enemies to the soil life on which they choose to feed, and their victims may have embraced fish and reptiles of considerable size. As they have been found in Kansas, Montana, North Dakota, New Jersey, and England, they probably frequented the epicontinental seas somewhat widely, and belong more to the sea life than to the land life from which they sprang.
[p. 768]
The second type, Ichthyornis (Fig. 521), was scarcely larger than a pigeon, endowed with great power of flight indicated by the strong development of the wings and keel. At the same time, their legs and feet were small and slender. They had teeth set in socket s. Their biconcave vertebrae and other skeletal features, as well as their small brains, suggest reptilian relationships. Their habitat was the same as that of Hesperornis, and yet the two were farther apart, structurally, than any two types of birds now living (Marsh).
An important change took place in the fish of the sea. in the transfer of dominance from the older types to the teleosts. This change set in during the Comanchean, and was complete by the middle of the Cretaceous. Though modern in type, the specie! were in the main ancestral, and some of them are not yet extinct. The sharks and rays were chiefly of the modern types, though not of living species.
Invertebrates. The most notable departure from the p dents of the preceding ages is the prominent place which the rhizopods or foraminifers take in the record. They made large contributions to the chalk of the period, and they were concerned in the formation of the greensand, scarcely less characteristic of the period than the chalk. While some of these minute organisms live on shallow bottoms, on fixed algae, and in abysmal water, they are chiefly inhabitants of the surface waters of the open sea.
[p. 769]
[p. 770]
Sea-urchins were quite abundant, and lent one of its characteristic aspects to the fauna, while corals and crinoids, so long associated with clear seas, were not abundant.
[p. 771]
In the clastic formations, pelecypods and gastropods are abundant and characteristic fossils (Fig. 522). It will be seen by a glance at the figures that they have a modern appearance. Cephalopods were still abundant, though ammonites were in their decline and were showing erratic divergencies of form, attended by excessive ornamentation, comparable to that which marked corresponding stages of the trilobites and crinoids. Odd forms of partial uncoiling, or of spiral and other unusual forms of coiling, were common (Fig. 523). Interesting forms, perhaps to be classed here, were the Baculites (i), which resumed the straight form of the primitive Orthoceras, while retaining the very complicated sutures of the Ammonites ©.
Map work. Folios of the U. S. Geological Survey containing good maps for the study of the Comanchean and Cretaceous systems are the following: Arizona, Bisbee, Clifton, Globe; California, Colfax, Lassens Peak, Mother Lode, Redding, Sacramento, San Luis; Colorado, Anthracite and Crested Butte, Elmoro, La Plata, Nepesta, Pueblo, Spanish Peaks, Telluride, Walsenburg; District of Columbia, Washington; Delaware-Maryland-New Jersey, Dover; Oklahoma, Atoka, Tishomingo; Montana, Fort Benton, Little Belt, Livingston, Three Forks; New York, New York City (Staten Island and Harlem sheets); Oregon, Roseburg, Coos Bay, Port Orford; South Dakota, Edgemont, Oelrichs; Texas, Austin, Nueces, Uvalde; Virginia, Fredericksburg; Wyoming, Alladin, Cloud Peak-Fort McKinney, Devils Tower, Hartville, Newcastle, Sundance, Yellowstone.
Both Comanchean and Cretaceous are classed as Cretaceous in the folios, though often distinguished in the text and on the maps.
Besides the State Reports referred to under the Comanchean, see (lark, Bull. Geol. Soc. of Am., Vol. I, 1897, pp. 315-358, and Weller, Jour. Geol… Vol. XIII, p. 71. ↩︎
Glauconite is usually impure, and, as it occurs in nature, contains several other ingredients. ↩︎
For brief summary concerning the origin of greensand marl, sec Chirk, Jour. Geol., Vol. II, p. 161. For a fuller account, see Challenger Report on Deep Sea Deposits. ↩︎
For an account of the Cretaceous of Alabama, see Smith, Report of the Alabama Survey for 1894. See also Safford, Geology of Tennessee, 1869, and Hilgard, Geology of Mississippi, 1860. ↩︎
Hill and Vaughan, 18th Ann. Rept., U. S. Geol. Surv., Pt. II, pp. 238-242 ↩︎
Hill, 21st Ann. Rept., U. S. Geol. Surv., Pt. VII, p. 114. ↩︎
Williston, Univ. of Kans. Geol. Surv., Vol. IV, p. 50. ↩︎
For subdivisions of this series, see Logan, Jour. Geol., Vol. VII, pp. 83-91, and folios of the U. S. Geol. Surv. ↩︎
Wallace, Island Life. Also Chamberlin and Salisbury, Vol. Ill, p. 149. ↩︎
For a full discussion of the Laramie (up to 1892) see White (C. A.), Bull. 82, U. S. Geol. Surv. A brief statement by the same author is found in the Proc. A. A. A. S., 1889, Vol. XXXVIII. See also folios of the Great Plains region, U. S. Geol. Surv. ↩︎
The Laramie question is well reviewed by Cross, Washington Acad, of Sci., Vol. XI, pp. 27-45, 1909. Other recent discussions by Veatch arc found in Am. Jour. Sci., Vol. XXIV, 1907, and Jour. Geol., Vol. XV, 1907. ↩︎
Allen, Proc. Boston Soc. Nat. Hist., Vol. XVI, p. 246, L874; also Bastin, Jour. Geol., Vol. XIII, p. 408. These phenomena wore noted and correctly interpreted by Lewis and Clark. See report of their expedition. ↩︎
Storrs, 22d Ann. Rept., U. S. Geol. Surv., Pt. III. ↩︎
Anthracite-Crested Butte folio, U. S. Geol. Surv. ↩︎
Cross, see footnote, p. 750. ↩︎
Here belong the Arapahoe, Denver, Middle Park, and Animas beds of Colorado, the Carbon, Evanston, and Ceratops beds of Wyoming, and the Hell Creek, and perhaps the Livingston beds (at least in part) of Montana. ↩︎
Papers of Diller, Stanton, and Turner, cited under the Lower Cretaceous (Shasta), p. 7:1.:;. ↩︎
Fairbanks, Jour. Geol., Vol. Ill, p. 426. ↩︎
Fairbanks, Am. Jour, of Sci., Vol. XLV, 1893, p. 478. ↩︎
Stanton, Jour. Geol., Vol. XVII. ↩︎
Hill, Nat. Geog. Mag., Vol. VII, p. L75. ↩︎
McConnell, Geol. Surv. of Canada, Vol. II, Rept. D, p. 33, 1886. ↩︎
Willis, Bull. Geol. Soc. of Am., Vol. XIII, pp. 307, 331-335. ↩︎
The term Comanchean has not been applied outside of North America, and the Cretaceous system will therefore be referred to as Upper Cretaceous. ↩︎
Seeley, Geol. Mag., 1902, p. 471. ↩︎
Kayser, Geologische Formationskunde, p. 443. ↩︎
Neumayr, Erdegeschichte, Bd. II, p. 383. ↩︎
Lucas, Animals of the Past, 1901, pp. 81-85. ↩︎