USA > Colorado > History of the State of Colorado, Volume II > Part 3
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Of the Colorado rocks referred to this period only the lower, non- fossiliferous portion can be regarded, with any degree of probability, as the equivalent of Triassic beds elsewhere. The middle and upper por- tions, found to be fossiliferous in Southwestern Colorado, are probably the equivalent of similar strata in New Mexico; referred by Prof. New- berry, on palæontological grounds, to the horizon of the Rhetic beds of
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HISTORY OF COLORADO.
Europe, which are considered as passage-beds between the Triassic and Jurassic systems as there developed.
The Rocky Mountain Triassic series has been designated Jura-Trias by Hayden, Le Conte and others, in view of the possibility that the extreme upper portion may be of Jurassic age. There is very little doubt but the upper members of the system as developed in Colorado, are older than the Jurassic of Europe, while the lower members are probably referable to the Triassic proper, so far as they can be separated from similar fossiliferous rocks belonging to the Permo-Carboniferous. This separation is not easy anywhere in the State, and in a few places, notably on the eastern flank of the Sangre de Cristo, the passage-beds between the strata evidently of Carboniferous age on the one hand, and of Triassic age on the other, are probably over 2,000 feet thick west of the Spanish Peaks. To a less extent the same is true all over Colorado where these beds outcrop,-there is always a non-fossiliferous zone of heavy-bedded sandstone, merging into the recognizable Triassic above and into the Carboniferous below, without any defined line of demarka- tion between them.
On the eastern flank of the Front Range the entire series is non- fossiliferous, and rests directly on the Archæan. The strata are assigned to the Triassic principally on account of their position with reference to the overlying Jurassic beds, their lithological character, and prevailing brick-red color. This pronounced coloration, so commonly observed in the Triassic of the Rocky Mountains, has led to their being designated the "Red Beds," a term often applied to the system in the West. The red sandstone so much used for building in Denver is mostly of this age.
One of the most familiar occurrences of Triassic rocks is the red sandstone so conspicuously exposed at the gateway to the Garden of the Gods. The same bed of sandstone outcrops frequently along the base of the Front Range northward to the Wyoming line ; while southward it is found at Cañon City, in the Greenhorn Mountains, and along the eastern base of the Sangre de Cristo, where it is continuously exposed, underlying the Jurassic clays and shales as in Northern Colorado.
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HISTORY OF COLORADO.
The Red Beds are yet more fully developed west of the continental divide. From the northern to the southern boundary of the State, and throughout the western part, in localities high up toward the summits of the mountains, and in the deep gorges of all the principal streams, Triassic rocks are frequently exposed. Among the most noteworthy occurrences may be mentioned those along the main Grand River, and its tributaries, the Roaring Fork and Eagle River.
Conspicuous examples may be seen in the exposures at Red Cañon, Glenwood, and North Cañon Creek on the main stream ; and around Mount Sopris on the Crystal River branch of Roaring Fork.
The most complete series of Triassic rocks in Colorado is found in the southwestern part of the State. They are exceptionally well developed on the western slope of the San Juan Mountains,-on the Rio Las Animas,-on the Rio Dolores for a large part of its length,- on the Rio San Miguel,-on the Uncompahgre, in fact, on every prin- cipal stream tributary to the Grand and San Juan.
Probably the most typical section is that seen in the valley of the Rio Las Animas where, in addition to the Red Beds, all the geological terranes of the State, from the Carboniferous to the Wahsatch Tertiary inclusive, are clearly exposed in stratagraphical order, dipping westerly and successively disappearing as they reach the level of the river. In the Rio Animas section the Triassic includes three fairly well marked divisions, consisting of about 1,200 feet of brownish-red sandstone at the base, 200 feet of brick-red sandstone at the top, and at the middle about 200 feet of alternating calcareous conglomerate and drab-colored sandstones. This middle division is the fossiliferous zone of the series, and can be traced northward to the Rio San Miguel; but with the upper division thins out entirely just north of that stream. It does not appear in Northwestern Colorado unless represented by a thin bed of similar conglomerate, containing bone fragments, occurring on Red Dirt Creek near Grand River. The drab-colored sandstones have yielded imprints of land plants, and on the San Miguel, specimens of imper- fectly preserved fishes probably allied to the genus Catopterus common
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HISTORY OF COLORADO.
in the Triassic rocks of the Atlantic coast. The bands of conglomerate invariably contain reptilian remains consisting of teeth and scattered fragments of bone.
Throughout the Triassic Period the deposits were formed in shallow seas, and frequently subjected to the action of strong currents ; hence, the conditions were favorable to the production of sandstones and conglomerates, and unfavorable to the production of limestones and other rocks of deep-sea origin. The general absence of the latter, and of the remains of marine life, are marked features of the exposures of this age in the Rocky Mountains.
Reptiles, which first made their appearance near the close of the Palæozoic Era, are everywhere recognized as the dominant class in the animal life of the Triassic Period, and to have so continued through the remainder of the Mesozoic; for which reason the latter has been appro- priately styled the Age of Reptiles. The abundance of fragmentary saurian remains in the bone-conglomerate of the southwestern part of the State, and the paucity of all other animal remains in the same beds, indicates very strongly that the reptilian was also the dominant form during the Colorado Triassic; though as compared with other parts of the world the system is less well-defined, and the life but little known.
JURASSIC PERIOD.
The rocks of this period in Colorado are nearly co-extensive with the Red Beds which they succeed, and even in the few localities where they have not been recognized, certain beds are found which may be partly, or wholly, of Jurassic age.
Along the eastern base of the Front Range the system is represented principally by limestones, shales, and variegated clays, of which the uppermost strata are designated by Marsh the Atlantosaurus Beds, from the remains therein discovered of a genus of Dinosaurs,-the most gigantic of known reptiles, living or extinct. The enormous bones of this Dinosaur were first brought to light, by the explorations of Prof. A Lakes, of Golden, in the Jurassic beds along the foot-hills.
3 II.
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HISTORY OF COLORADO.
West of the continental divide the beds of this age are litho- logically similar to those of the Front Range, but have nowhere a thick- ness of more than a few hundred feet. They are usually present in the Mesozoic sections of Northwestern Colorado and have been identified by Hayden on the Rio Dolores, and elsewhere in Southwestern Colorado. Typical Jurassic beds have not been reported as occurring in the San Juan Mountains, though on the upper San Miguel a limited thickness of non-fossiliferous strata, sandwiched in between the Red Beds and the Dakota Cretaceous, and containing bituminous limestone, is thought to be of this age.
Along the eastern base of the Sangre de Cristo Range, from the Huerfano River southward to the line of New Mexico and probably beyond, there are exposures of typical Jurassic beds underlying the upturned, and usually quite prominent, Dakota sandstone.
Jurassic beds are likewise well exposed in Wyoming, from which the remains of marsupial mammals have been identified and described by Marsh.
The presence of some limestone in the Rocky Mountain Jurassic indicates the occasional existence of marine conditions. At other times lacustrine conditions prevailed, and the beds may be in part of brackish- water or fresh water origin.
While plant life is not represented, the remains of huge herbiverous reptiles point strongly to a luxuriant growth of land vegetation, probably confined largely to the low marshy shores of the shallow Jurassic seas.
The earliest known forms of mammalian life, the few small marsu- pials which first appeared in the latter part of the Triassic of Europe and Eastern North America, show an increase in the number of species in the Jurassic.
These diminutive forms appear in the Rocky Mountains, for the first time, in the Atlantosaurus beds of Colorado and Wyoming, associated with the remains of great numbers of gigantic Dinosaurian reptiles.
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HISTORY OF COLORADO.
CRETACEOUS PERIOD.
The Cretaceous is the most extensively developed of all the geolog- ical systems in Colorado, and is, economically considered, also the most important, since it contains our great coal-measures.
The rocks of this age form broad surface exposures, or are found immediately underlying the soil and drift throughout nearly the entire plains country east of the mountain border, the noteworthy exceptions being the eruptive overflows of Las Animas County,-the Monument Creek Miocene Tertiary, on the Arkansas-Platte divide,-the White River Tertiary in the northeastern corner of the State, and probably patches of Pliocene Tertiary along the eastern margin near the Kansas and Nebraska line. They are also prominently developed in the western half of the State, but are not to the same extent exposed owing to the presence of the more recently deposited Lower Tertiary beds occupying the Uinta and San Juan basins. 1186742
The Cretaceous system, as defined in the Rocky Mountains, has been separated into a number of well-marked terranes, distinguished from each other by remains of characteristic fossils, and more or less pronounced lithological features. The second epochs recognized are designated as Dakota, Fort Benton, Niobrara, Fort Pierre, Fox Hills and Laramie,-the relative age corresponding to the order given. Orig- inally they were known as Cretaceous No. I to No. 6 respectively.
The Dakota and Laramie terranes,-that is, the upper and lower,- were formed in shallow, brackish-water seas, and contain remains of land plants; for which reason they are always separated from the interme- diate members, the latter being altogether of marine origin. On this ground some geologists are inclined to combine all of the marine beds into one great group termed the "Colorado," referring all of the Creta- ceous above to the Laramie, and all that is below to the Dakota. The majority, however, restrict the name Colorado to the two lower members, the Fort Benton and Niobrara ; while the upper members, the Fort Pierre and Fox Hills, are by Hayden and others termed merely Upper and Lower Fox Hills. Recently the name " Montana " has been sug-
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HISTORY OF COLORADO.
gested to designate the group including the upper half of the marine beds, to avoid discarding the old formation name of Fort Pierre, while still retaining that of Fox Hills, both being comprehended under the term Montana group, where it is not possible or desirable to separate them.
A description of all these terranes in detail, would be out of the question, and to economize space they will be referred to as Dakota, Marine Cretaceous, and Laramie; the latter being the most important economically, will be considered at greater length than the others.
DAKOTA EPOCH.
The Dakota, or lowest of the Cretaceous beds in Colorado, is rep- resented by a varying thickness of sandstone up to 700 feet, the greatest development being in the southwestern part of the State, and the least along the eastern border of the Front Range. Wherever the sedimen- tary beds are upturned on the flanks of the Rocky Mountains the Dakota sandstone can usually be found projecting above the softer overlying and underlying shaly strata, and in Southwestern Colorado its exposures cover a comparatively large area of country. Along the eastern base of the Sangre de Cristo, and west of the Spanish Peaks, the upturned sandstone of this epoch stand up conspicuously above the adjacent country, forming in western Las Animas County what is called the "Stone Wall." Near Golden the Dakota contains the important bed of fire clay, and in Ouray, San Miguel, Dolores, La Plata and Mesa Coun- ties, it contains limited quantities of workable coal The coking-coal near Rico, the semi-anthracite near the mouth of Dallas Creek on the Uncompahgre, and the bituminous coal on the Gunnison near Grand Junction, belong to this epoch. Much of the sandstone used for building and paving is of Dakota age.
In a few places, notably at Golden, it affords remains of land plants, indicating nearness to the shores of a shallow, brackish-water sea. Al- though the oldest of the Cretaceous series in Colorado it is more recent than the Trinity and Comanche beds of Texas,-beds which are now
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HISTORY OF COLORADO.
regarded as the oldest Cretaceous of America. It is worthy of remark, however, that certain pinkish and light-colored massive sandstones, underlying the Dakota proper on the Rio Dolores, the Rio Las Ani- mas, and elsewhere, and reported as Lower Dakota by Hayden, may be the Rocky Mountain equivalent of the Trinity sandstones.
The Dakota Epoch marks the first appearance of the modern types of vegetable life in Colorado. The abrupt introduction of a radically new and dominant flora, differing so widely from that of the Jurassic, points to a great break in sedimentation, and an elevation of the land above ocean level for some distance away from the Jurassic shore-line during the early Cretaceous. While the latter conditions prevailed in the Rocky Mountains, a great thickness of sediments accumulated in Texas and along the Atlantic coast; represented by the Trinity and Comanche beds of the former, and the Potomac beds of the latter. It is in the last named that the earliest representatives of the modern types of plant life in America first appear,-types from which the existing ones have been, through long ages, gradually developed.
MARINE CRETACEOUS.
Succeeding the Dakota are marine beds consisting of shales, clays, limestones, and near the top, sandstones, aggregating usually from 3,000 to 3,500 feet, occasionally more. They form extensive exposures in Southeastern Colorado, and are prominent in the valleys of all the principal streams west of the continental divide. Along the eastern base of the Front Range, in Northeastern Colorado, the upper part is known to contain Fox Hills, Fort Pierre fossils. The same beds cover large areas in the western part of Kansas and Southern Nebraska, are exten- sively developed in Northern New Mexico, and to some extent in Eastern Utah. In a few localities the shales of the Marine Cretaceous outcrop high up on the mountains, and on the divide south of Mount Wilson there are typical exposures at an altitude of 11,000 feet above sea level. The lower members of the series, or what would be considered as be- longing to the Colorado group, are well shown in the vicinity of Pueblo,
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HISTORY OF COLORADO.
and along the valley of the Arkansas River. At Florence near Cañon City the Montana beds contain the petroleum for which that locality is. noted.
The Marine Cretaceous of Colorado abounds in the remains of the marine life of the times. Among the most interesting forms were the coiled and straight-shelled Cephalopods, which appear to have existed in vast numbers in the Cretaceous seas, and whose remains are common in many Colorado localities. The order of Cephalopods first appeared in the Lower Silurian, being then represented by the straight-chambered Orthoceras, which was followed later in the Paleozoic by the coiled Goniatites, Ceratites, Ammonites, Baculites, Scaphites, Heteroceras, Helicoceras, with other genera, appeared in the Mesozoic, and with the. exception of Ceratites are all abundantly represented in the Cretaceous beds of the Rocky Mountains and of Colorado. Of the Mesozoic Cephalopods only one genus, the Nautilus, has survived to the present time, although the order is still represented by a greatly diminished number of genera and species. The Cretaceous forms were prob- ably the progenitors of the Octopus, Cuttlefish and other genera of existing seas, and their gradual development from the ancient Ortho- ceras constitutes an interesting and instructive page in the history of marine life.
Of the vertebrate life of the Marine Cretaceous, so far as concerns. Colorado, little is known. The rich fauna obtained by Marsh, from the beds of this age in Kansas, no doubt indicates the life common also to the eastern half of Colorado. In what are designated by Marsh, the " Pteranodon beds," are found the remains of huge, toothless, flying lizards, allied to the Pterodactyles. Some species measured twenty-five feet between the tips of the wings. Other remarkable forms from Kan- sas are the Odontornithes, or birds with teeth, either arranged in grooves (Odontolcæ), or in sockets (Odontotormæ) which were first discovered, and their peculiar characters investigated by Prof. Marsh. Associated with these were countless numbers of Mosasauroid reptiles, highly characteristic of the age in America. They were slender, snake-
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HISTORY OF COLORADO.
like forms, provided with paddles, and some of the species were probably the longest reptiles that ever existed.
Other kinds of organisms characteristic of the Cretaceous generally were not wanting in Colorado. The chalk of Europe, which consists wholly of the remains of Foraminifera, is not represented, litholog- ically, in the Cretaceous of the Rocky Mountains; but there are, in Colorado, beds of calcareous shales, which appear to be mostly made up of the remains of Foraminifera similar to those of the chalk. These minute organisms still exist in countless millions, but only under pelagic conditions, or at great depths in the ocean, where the remains form the well-known deep-sea ooze. The absence of Foraminifera from shallow seas points strongly to the deep sea origin of all calcareous rocks con- taining them ; hence we may conclude that during the Marine Cretaceous, or rather during a large part of it, the Colorado archipelago was sur- rounded by deep seas, resulting from the final subsidence of the land which terminated the shallow water conditions of the Dakota Epoch.
With the close of the Marine Cretaceous ended the long period of true marine sedimentation in the Rocky Mountains. Previously there had been two grand revolutions in the geological history of the continent. First, the Appalachian at the close of the Paleozoic Era. Second, the Sierra Nevada revolution at the close of the Triassic. The third, or continental revolution, may be said to have begun at the close of the Marine Cretaceous ; though for some time there continued to be oscilla- tions of the land, which permitted occasional submergence, for brief periods, by the ocean, and the introduction of marine life. Brackish water sedimentation then began on an extensive scale, and probably con- tinued through the greater part of the succeeding or Laramie Epoch ; the last and most important of the Mesozoic terranes.
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HISTORY OF COLORADO.
CHAPTER II.
INTRODUCTORY REMARKS-LARAMIE EPOCH-EXTENT OF THE COAL MEASURES IN COLORADO-GRAND RIVER FIELD-VAMPA FIELD-LA PLATA FIELD-RATON FIELD -NORTHERN COLORADO FIELD-NORTH PARK FIELD-CANON CITY, SOUTH PARK, AND TONGUE MESA DISTRICTS-ESTIMATION OF THE AREA OF THE SEVERAL COAL FIELDS-ESTIMATION OF THE PROBABLE TONNAGE OF AVAILABLE COAL IN THE COMBINED COLORADO FIELDS-PHYSICAL CONDITIONS ATTENDING THE CLOSE OF THE LARAMIE EPOCH-LIFE OF THE LARAMIE.
The economic map of Colorado, included in the Geological Atlas published by the general government, is based on observations made by the geologists attached to the survey under Prof. Hayden ; and since their work was merely preliminary it is not to be expected that they could do justice to our coal fields, which no doubt ought to have been made the object of a special detailed survey. Indeed, it would have been more to the interest of the State if the matter of our coal resources had not been touched upon ; for nothing could be more unsatisfactory, not to say misleading, than the chapter on lignitic coals contributed by Marvine to Hayden's Report for 1873, based as it is on a mere inspec- tion of the Northern Colorado districts, which produce the most inferior coals mined in the State. So likewise, with the economic map, in which large sections of country, worthless for coal, are represented otherwise, and highly valuable workable areas are entirely overlooked.
The reports which appear from time to time in the United States Mineral Resources are creditable, and so far as they go, entirely just to Colorado mines ; but they are necessarily largely statistical, and lengthy descriptions, involving comprehensive details, would be out of place in such a work. Yet these reports and those of Hayden, contain the sum of our literature on this important subject. This being the case, where
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HISTORY OF COLORADO.
shall one who seeks for exact information regarding our coal resources. turn for aid ?
Clearly this question can only be answered when the results of an organized systematic survey, under State control, shall be given to the public. It would certainly seem that work of this kind, impressed with the stamp of official authority, would at this stage of our progress, be of great benefit to Colorado, and exercise a direct influence on its industrial development.
The lamentable want of trustworthy information, on the subject of our coal resources, is the writer's apology for bringing forward the brief and imperfect description of Colorado coal fields presented in this chapter.
LARAMIE EPOCH.
The strata of the Laramie were for a long time regarded by palæo- botanists as Lower Tertiary, for the reason that the flora first studied, and which was thought to contain many species common to the Eocene (Lower Tertiary) of Europe, was obtained near the very summit of the series; while the beds near Golden, that have afforded a large number of so-called Laramie species, are now known to be erosionally uncon- formable with the Laramie proper. The Golden beds extend over a large area in the Denver basin, and are hence termed the Denver Beds.
The question of their age is still unsettled. The flora is regarded by Newberry as Upper Laramie, a conclusion supported by the decidedly Mesozoic aspect of the vertebrate remains in which the Dinosaurs pre- dominate, though there are a few forms which in their affinities approach nearer to Tertiary types. At present, however, so far as regards the Laramie proper, few will question the propriety of its assignment to the uppermost Cretaceous, which makes it the closing epoch of the Mesozoic Era in Western North America.
The terms, "Post-Cretaceous," and, "Lignitic," often applied to the Laramie series, are now nearly obsolete, the former for the reasons just stated, and the latter for the reason that it originated in the erroneous impression that the coals were merely lignites; whereas, it is now well
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HISTORY OF COLORADO.
known that all the varieties of bituminous coal common to the Carbon- iferous are common to the Laramie also. What the Carboniferous is to the Appalachian region and to Europe, the Laramie is to the Rocky Mountains, it being, pre-eminently, the coal-bearing formation through- out the West.
The deep sea conditions of the Marine Cretaceous ceased with the beginning of the Laramie, when sedimentation took place in shallow, brackish-water seas, or alternated with periods during which extensive swamps, covered with an exuberant growth of semi-tropical vegetation, served for the accumulation of vast peat-like deposits, which were after- ward submerged and covered with sediments.
This alternation of conditions, due to the irregularity of the sub- siding movement, continued throughout the Laramie, or up to the time of the continental revolution, which closed the Mesozoic and permanently elevated the western half of the continent above the ocean level.
The rocks of the Colorado Laramie have everywhere nearly the same lithological characters. There is usually at the base,-and directly over- lying the Marine Cretaceous-a stratum of sandstone, from 100 to 200 feet thick, massive in the upper half, and often containing fucoidal remains (sea-weeds) in the lower half. Above this basal band of sand- stone, which is much used for building purposes, are others, separated from each other by shale-beds of varying thickness. These alternating shale-beds gradually decrease in thickness until, finally, at a distance of from 1,000 to 2,000 feet above the base, sandstones largely predominate. The workable coal-seams are confined to the lower portion of the form- ation, or to the lower 1,500 feet. In Northeastern Colorado there is also a series of shales and sandstones which has been referred to the Upper Laramie, and which contains coal,-seams of workable thickness, but too inferior in quality to furnish a marketable product. The Lower Laramie ranges in thickness from 3,000 to 5,000 feet; the Upper Lar- amie about half as much more, although the line of separation between the two formations is, to a great extent, arbitrary.
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