Popular Science Monthly/Volume 53/September 1898/Geological Water Ways Across Central America
|GEOLOGICAL WATER WAYS ACROSS CENTRAL AMERICA.|
By Dr. J. W. SPENCER.
Introduction.—From the days of early discovery, adventurers and explorers have repeatedly sought for accessible routes across Central America. For more than half a century engineers have ransacked the dense mountain forests for easy passes through which to connect the two great oceans by water or by rail. The number of low passes which dissect the Cordilleras and plateaus between North and South America is much larger than is popularly known. Indeed, there are so many deep breaks in the plateau regions as to suggest that many of them at no distant date were water ways between the Atlantic and Pacific. On account of the narrowness of the Isthmus of Panama and the low altitude of the divide, this region has been most noticed as a possible geological passageway between the Antillean waters and the Pacific Ocean. But it has been discovered that other water ways across the American continent can be even more surely established, and that neither the narrowness of the Isthmus of Panama nor the lowness of the divide form any additional evidence of the late interoceanic connection.
The early Tertiary period—the time when water connection across Panama had been suggested—was so long ago that the land animals have completely changed their characteristics, and even the sea shells have been mostly transformed into more modern species; accordingly, the time has been sufficiently long for several subsequent changes in the physical barriers between the Atlantic and Pacific. Indeed, geological straits across the continent have existed at different epochs since the early Tertiary period, and the Last was so recent as to place it almost in modern times.
In searching for the geological water ways, the phenomena of the recent elevation of Mexico and Central America became so apparent as to establish the theory that the uplifting of the American barrier to its great altitude occurred principally since the period when the West Indian islands formed a high plateau bridge between the eastern portions of the two American continents (as described by the writer in The West Indian Bridge between North and South America). Indeed, the discovery of the recent elevation of Central America and Mexico forms a most important sequel to the story of the now submerged West Indian lands, for it shows that Central America was low at the time when the Antilles stood at a great height—though it has subsequently been elevated, while the eastern region has been largely submerged. The discoveries of these great changes of level in recent periods are the necessary complementary phenomena of those found in the Antillean region.
Physical Features of Mexico and Central America.—The Table-Lands.—Much the larger area of Mexico and Central America is occupied by plateaus from six to eight thousand feet above the sea. Indeed, some of them have an altitude of over ten thousand feet. These elevated table-lands rise abruptly for thousands of feet above the inner margins of the coastal plains, which gradually slope upward from the seashores. The elevated plains, traversed by many mountain ridges, are, to the eye, apparently level. Out of their floors there also rise numerous island-like hills called cerros, as well as great volcanic cones. Thus the volcanoes of Popocatepetl and Iztaccihuatl, capped with perpetual snow, tower to a height of eight thousand feet or more above the Mexican plateau, as shown in Fig. 1.
The floors of the plateaus are substantially base levels of erosion—that is to say, the more ancient land surfaces were in olden days ground down by the action of the rains and rills to such low elevations above the sea that the streams could not further deepen their channels, whereupon the mountains were worn down into rolling plains. The hills and the mountains traversing the table-lands are often only the remnants of higher plains which have, so far, escaped destruction. Thus these elevated plateaus themselves demonstrate their subsequent great elevation above the sea.
In proceeding from Mexico into Central America, the high plateaus are not only necessarily of smaller extent, but they are more broken, and on the Isthmus of Panama they are almost replaced by mountain ridges rising from one to three thousand feet, except along a few lower passes. Interruptions of the Plateaus corresponding to the Antillean Basins.—In the paper upon The West Indian Bridge between North and South America it was pointed out that the Antillean seas formed three distinct basins—the Gulf of Mexico, the Sea of Honduras (between Cuba, Haiti, and Jamaica), and the Caribbean Sea, although the two latter are united at their surface under the name of the Caribbean Sea. In the investigations of the river valleys, now drowned beneath the West Indian waters, it was found that they were traceable to the floors of the three basins just mentioned. Accordingly, the hypothesis that these three basins were formerly drained into the Pacific Ocean called for corresponding depressions
Fig. 1.—View of the Mexican Plateau from near Puebla, showing the plains out of which rise isolated cerros, with the volcanic cones of Popocatepetl and Iztaccihuatl in the background.
across Central America and Mexico; and only such are found to occur (as shown in map. Fig. 2), although the continental barriers are now raised to considerable heights above the level of the sea. Thus the Mexican plateau rapidly narrows to a breadth of only twenty-five miles in the Tehuantepec Isthmus, where for a distance of sixty or eighty miles it is reduced so that the higher points do not exceed four thousand feet, and for a distance of perhaps over twenty-five miles the ridges do not reach an altitude of more than two thousand feet, with a repetition of base levels of erosion only nine hundred or a thousand feet in altitude among them. Indeed, these lower divides are further reduced at seven or eight passes which are only about eight hundred feet above the sea. Beyond these low depressions there are higher passes in the isthmus having an altitude of about twenty-seven hundred feet with accompanying plains. The Tehuantepec depression through the Mexican plateau corresponds in position to an extension of the basin of the Gulf of Mexico, toward which, from the Pacific side, there is a similar indentation of the continental mass as shown on map, Fig. 2. The Honduras basin, indenting the Central American plateau, has a corresponding depression across it at an altitude of about twenty-seven hundred feet. The Caribbean Sea proper has a more uniform
Fig. 2.—Map of Central America, showing the indentations corresponding to the Mexican, Honduras, and Caribbean basins. (Soundings in feet.)
breadth than the other basins, and in like manner the American barrier, which is here narrower than farther north, is dissected in several places, as might be supposed. These depressions make it appear that the Caribbean basin was connected by several channels across the continent with the Pacific Ocean. The Nicaragua depression is the most northern. The elevation of the valley east of Lake Nicaragua is about three hundred and fifty feet above the sea, although the San Juan has further dissected it so as to reduce its channel to one hundred and three feet above tide. Between Lake Nicaragua and the Pacific the divide is reduced to two hundred and thirty feet, but a subsidence of the land to three hundred feet or so would connect the lake basin with the Pacific Ocean by eight or ten straits between as many islands.
While most of the Isthmus of Panama is traversed by mountain ridges from one thousand to three thousand feet high, these are dissected so that five of the deeper depressions are reduced to an altitude of from five hundred and fifty-three to eleven hundred and forty-two feet, while the Chagres River Pass, along which the railway is built, has a natural altitude of two hundred and ninety-nine feet, although the artificial cut reduces it to two hundred and fifty-four feet.
Besides the Nicaraguan and Panaman depressions across Central America, the valley of the Atrato and San Juan (to Buenaventura) forms a third and equally interesting depression between the Caribbean and Pacific basins. This valley is about three hundred miles long (direct course) and crosses Colombia between the Coast Range and the great Cordillera. The Atrato Valley is from forty to sixty miles wide, and for long distances above its mouth (Gulf of Darien) it is forty feet deep. At two hundred miles from its mouth the river is only forty-seven feet above the sea. Above this point, the tributary Rio Quito is still navigable for steamers to a distance of two hundred and seventy-three miles from the Gulf of Darien, and for eight miles farther canoes freely ascend the stream Santa Monica. Between this point and the navigable waters of the San Juan, only three miles distant, upon the other side of the divide, the elevation is so low that during high water canoes can even pass from the branches of one river to those of the other. Here the divide is reduced to about a hundred and fifty feet above the sea. The whole valley of the Atrato suggests a comparatively recent connection between the Caribbean Sea and Pacific Ocean. Across the Coast Range, separating the Atrato Valley from the Pacific, there are several passes at an elevation of only one thousand feet or less.
The Atlantic and Pacific Coastal Plains.—Prom the seashore, the coastal plains, on the Atlantic side of the continental plateau, slowly rise until they abut against the edges of the table-lands. These plains may have a width of only a few miles, or they may extend for a distance of considerably more than a hundred miles from the Gulf of Mexico. Back of Vera Cruz the coastal plains have a breadth of more than fifty miles and reach an elevation of seventeen hundred feet, while in the Tehuantepec Isthmus they extend for a distance of eighty miles from the sea and deeply indent the plateau region (see map, Fig. 7, page 588). Similar coastal plains form the eastern border of Central America, extending sixty-five miles into Honduras and a hundred miles into Nicaragua; but at Panama they are reduced to a width of five or eight miles. The Pacific coastal plains of Panama are wider than those on the Caribbean side; but, generally speaking, the plains upon the western side of the table-lands are much narrower than those upon the Atlantic side. In Nicaragua they are reduced to eighteen miles in width, and even at Tehuantepec, where they set in from the Pacific Ocean and indent the plateau region, their breadth is only twenty-five miles (see map. Pig. 7, page 588). The coastal plains represent the lands which have been most recently elevated above the sea. When they were covered by the oceanic waters the Isthmus of Tehuantepec was only twenty-five miles across, and still less where penetrated by dissecting arms of the sea. The Panama Isthmus was reduced to five miles across.
Characteristics and Terraces of the Valleys descending from the High Plateaus.—The plains, having no barriers to obstruct their drainage, and terminating at the margins of the high plateaus, indicate former base levels of erosion, and their great altitude shows the extent to which they have been elevated above
Fig. 3.—The Base Level or Elevated Floor of the Salazar Summit of the International Railway.
the sea. Their final surfaces may have been due in part to shallow accumulations of lake deposits leveling up the slight undulations of the old base-level surfaces, or these irregularities have at times been smoothed over by wind deposits. These superficial accumulations do not modify the general inference that the floors of the great table-lands were the effects of atmospheric agents, acting throughout a long period, in wearing them down to near sea level. The remains of such elevated plains were observed at an altitude of even ten thousand six hundred and thirty-six feet above the sea, as shown in Fig. 3. This represents a level plain bounded by mountain ridges about a mile apart. It is the summit of a divide, also about
Fig. 4.—The Valley at the City of Orizaba, showing broad terrace plains bounded by sculptured mountain scenery.
a mile long. Both ends of the floor of this mountain-bounded depression are open, and terminate in abrupt steps of three hundred feet or more to the next lower and very much greater plains. The margins of this flat summit are indented by short ravines of recent date (the character of which is illustrated in Fig. 6, page 585), that have not yet had time to be extended backward into the summit plain and to dissect it into sharp ridges and deep ravines.
Equally abrupt are the margins of the plateau back of Vera Cruz (at about eight thousand feet above the sea), which may be taken as a type of the high base levels of Mexico. The borders of the high plateaus are dissected by deep valleys, which, when compared with the extent of the elevated plains, are remarkably short—often only a few miles long. The distance between Esperanza, upon the edge of the plateau (at an altitude of eight thousand and forty-two feet), and Atoyac (at fifteen hundred and twelve feet), near where the valley opens on the coastal plain, is only about forty miles in a direct course. The valley is from one to two or more miles in width, and is of course very deep. Upon its broad terrace plains the city of Orizaba and other large towns are situated, as shown in Fig. 4. Compared with the age of this large, deep valley, which is sufficiently old to have what were once the precipitous bounding walls of a great cañon now rounded off into sloping mountains, the Mexican plateau, of hundreds of miles in extent (at an altitude of eight thousand feet more or less), which is dissected for a distance of only forty miles, is of great age. Here again the elevation of the tableland has been too recent to have been converted into rugged mountain ridges and deep valleys.
This Mexican valley back of Vera Cruz, whose grandeur and beauty are perhaps unsurpassed, and depend more or less upon its youthful features, is not exactly modern, for since its first development it became partly refilled with muds and gravels, which the streams are again removing. The slope of its floor is not uniform, but it is represented by great series of gradation plains bounded by abrupt steps, as shown in Fig. 5. These steps may be only five or ten feet in height, or several may coalesce into one great terrace. Three of the greater steps are perhaps five hundred feet above the floor below. The valley heads abruptly in an amphitheater which forms a step a thousand feet in height. These gradation plains, thus abruptly separated from each other, represent pauses in the elevations of the table-land, when the streams were depositing muds and gravels and forming low flats near sea level. With the subsequent rise of the land these became terraces which the streams soon began to dissect and form in them deep channels and cañons. The newness of these cañons and smaller channels is shown by the fact that they are seldom more than a quarter of a mile long, although varying in depth from several hundred to a few feet; and the loose gravel floors, except at the margins of the terraces, have not even been
Fig. 5.—Section of the Valley between Esperanza and Atoyac, showing its gradient to he composed of numerous abrupt steps.
removed. These short cañons are characterized by beautiful waterfalls, as illustrated in Fig. &.
The characteristics of the slopes and terraces of the valley descending from the high plateaus have been described because they illustrate evidence of great elevation, which has occurred in Mexico and Central America in recent geological times, with the later movements belonging to almost modern days.
In place of an elevation of from six thousand to ten thousand feet obtaining, as shown in many parts of the Mexican table-lands, the uplift in the Tehuantepec Isthmus closing the outlet of the Mexican basin into the Pacific Ocean apparently amounts to less than a thousand feet; and indeed an elevation of seven hundred and seventy-six feet has been enough to withdraw the waters from the last canal across the continental barrier. But of this again.
The same story of the rise of the land and the separation of the seas is repeated in Central America—in Honduras, in Nicaragua, and in Colombia, whether at Panama or not, as appears from the observations of Mr. J. Crawford, Dr. W. M. Gabb, Dr. G. A. Maack, and especially of Mr. F. C. Nicholas, in the Atrato Valley. But the phenomena accompanying the great changes of level have not been so fully studied farther south as in Mexico.
The Manner in which the Elevation of the Plateau occurred.—There are various great terrestrial movements known to geologists. Extensive continental areas are gently rising and others are sinking. These oscillations, so far as time is concerned, appear slow—as, for instance, the sinking of the New Jersey coast at the supposed rate of a foot in half a century, or of the New Orleans district, of a foot in twenty years; while the rising of the northern part of the continent, as between the Adirondack Mountains of New York and the St. Lawrence River, is at the rate of a foot in twenty or twenty-five years. Within a generation the changes of level in the physical geography of a special district are no more apparent to the ordinary inhabitant than the shallowing or deepening of small channels which interfere with or favor the passage of canoes or small boats, or the gentle backing of waters which make swampy grounds or the reverse. Even with their effects, accumulated for centuries, the changes do not often affect the course of the drainage or obstruct it. By this gentle continental movement the mountains are not formed, nor are ridges squeezed up into existence, although they often give rise to high land. Since the beginning of the Glacial period, the accumulations of such movements have raised the same superficial formations of the coastal plains of America to an altitude of one hundred feet near Cape Hatteras, to eight hundred feet farther south, to fifteen hundred feet in the neighboring mountain region, to only three hundred or four hundred feet in the Mississippi Valley; while the same coastal plain in Mexico has been elevated to seventeen hundred feet. Farther south, in the Tehuantepec Isthmus, the corresponding change of level has not been more than eight hundred feet. These movements do not bend, crumple, or break the strata, or appreciably affect their horizontality. The amount of elevation just mentioned was sufficient to cause the final separation of the Mexican Gulf and the Pacific Ocean, but inadequate to account for the high plateaus.
Another class of terrestrial movements gives rise to mountain folds, thrusts, and faults, with a general dislocation and disturbance of all the formations. Just how much of a plateau was elevated by these forces is not apparent, for the surface base levels of erosion still retain their older courses of drainage. Consequently, one is led to suspect that the great escarpments are not entirely due to denudation, but that a third class of earth movements has obtained, lifting the plateaus abruptly to a considerable proportion of their height (six thousand feet, more or less) above the inner margin of the coastal plain, without greater deformation of their surface than that of the coastal plains themselves. This third movement seems to be a sort of squeezing up of great segments of the earth's crust, without tilting its surface by more than a few feet per mile; consequently, it implies a great dislocation and slip or fault along the margin of the table-land, since modified upon the surface by the atmospheric denudation. The analysis of these complex movements is far from complete, and, although we do not fully understand them, this ignorance of causes does not affect the evidence of the very recent elevation of the plateaus.
The Periods of Great Elevation.—The old geological formations of the plateaus are mostly buried beneath secondary accumulations; so that the present physical surface features are largely due to the atmospheric agents which have been at work since about the end of the Cretaceous period, as out of the formations of that date a large portion of the base levels of erosion have been molded. These great base levels required a long time for their development, which was provided for during the greater portion of the subsequent Tertiary period, when the present table-lands were mostly low continental plains interrupted by mountain ridges. In the meanwhile, the present coastal plains (now rising to seventeen hundred feet) were submerged and were receiving the older Tertiary accumulations under the waters of the Gulf of Mexico. The Tehuantepec Isthmus (Fig. 7, page 588) was throughout the late Miocene and Pliocene periods covered by deep water, as seen by the occurrence of such fossils in the horizontal formations accumulated there, thus showing that there was a strait or water way across the continental barrier, subsequent to the great physical dislocations and changes of level occurring in the earlier Miocene period.
Without considering the minor oscillations of land and sea which raised the coastal plains during the later Tertiary period, it has been found that these plains were covered with water about its close (end of the Pliocene period). Until about this time the present tablelands do not appear to have been elevated. The lavas and other similar rocks derived from Orizaba and sister volcanoes upon the present edge of the great plateau date back only to about the close of the Pliocene period, and appear to have originated with the elevation of the region. The excavation of the great valleys, such as that described back of Vera Cruz, has been subsequent to the commencement of this volcanic epoch, and consequently the elevation of the plateau can not date back prior to about the commencement of the Pleistocene period (or the beginning of the ice age). After the time of the first great elevation with the formation of the original valley, the region was more or less depressed, when gravels and muds were accumulated upon its floor during midglacial epochs; since when the table-lands have attained their great elevation, with the formation of the terrace steps already described, so recently that the streams have not yet removed these loose deposits, and are only in the early stages of making new cañons.
Prom all that has now been ascertained, it appears that the great elevation of Mexico and Central America was chiefly effected by the Pleistocene changes of level; and from the magnitude of continental movements in both directions and the excavation of large valleys out of very hard rocks, it would seem that this period must have been one of long duration.
The Geological Water Way across the Tehuantepec Isthmus.—As has already been mentioned, the great continental tablelands are here broken down for a distance of perhaps eighty miles, with the resulting lower ranges for a length of twenty-five miles penetrated by numerous lower channel ways. The reduction of the width of the plateau is shown in map. Fig. 7, where the shaded portion represents the coastal plains setting into the highland mass, which is only about twenty-five miles across. While this district was a strait during the Pliocene period, in which deep water organisms were living, the existing gap in the Cordilleras was being widened so as in part to complete the great interruption in the American plateau. Although the geological water way was open during this period (more recent than has generally been suggested), it was afterward closed for a time, and the subsequent connection between the Gulf of Mexico and the Pacific is the most interesting to us, as reaching down almost to the historic period. The date of the older strait was prior to the Lafayette epoch (about the close of the Pliocene or commencement of the Pleistocene period), while that of the later one obtained well on in the Pleistocene period. The separation of the seas was in part effected by gentle warpings of the district, but the low Cordilleras seem to have been further squeezed upward by a movement referred to on a preceding page. Out of the floor of the present divide, the rocky islands of the recent strait became prominent knobs (such as one shown in Fig. 9). The newly formed isthmus was only a mile or two across. Its once nearly level floor, composed of soft, earthy sandstone, has since been rounded into a succession of hummock-like hills by the subsequent action of rains and rills. This feature is shown in Pigs. 8 and 9. For a time the narrow isthmus was penetrated by a geological canal, one hundred and fifty feet deep and less than a quarter of a mile wide, the features of which
Fig. 7.—Map of the Isthmus of Tehuantepec. The unshaded portion represents the high plateau region as indented by the coastal plains (shaded).
are now perfectly preserved in the channel, about a mile long, which dissects the hummocky divide of the Tehuantepec Isthmus. The northern end of this canal is shown in Fig. 9. The floor of this passageway is still covered with water-worn gravel, which was deposited by the former currents. Connected with this late channel and at the same level, gravel terraces mark the shores of the Gulf of Mexico when its waters were last connected with the Pacific Ocean, although the barrier has now been raised to seven hundred and seventy-six feet above the sea. Since the elevation of the land, which finally separated the Gulf of Mexico from the Pacific in post-
Fig. 8.—The Divide of the Isthmus of Tehuantepec at Chivela, eroded into hummocks, with a gravel plain in front (northern side).
Glacial times, the excavation by the streams has produced only narrow cañons, at most a mile or so in length and three hundred or four hundred feet in depth, while the gravel floors and terraces remain almost intact. Indeed, the final elevation seems to have been even later than the birth of Niagara Falls. Thus the separation of the Antillean and Pacific waters is found to have been much later than has hitherto been supposed. So recent is the geological canal of Tehuantepec, that it would be reasonable to suspect its existence even since the advent of man upon the earth, although the proof of his occupancy of the district and his utilization of the passage has not been found. Had the appearance of the barriers been retarded for only a little while longer (geologically speaking), the engineering difficulties of constructing interoceanic canals would have been avoided.
A sister canal to that described occurs a few miles toward the east, and probably similar narrow channels are found in the other half-dozen low passes of the region which have an altitude of about eight hundred feet.
The Atrato and other Water Ways.—The low country of the Atrato-San Juan Valley of Colombia has been mentioned as an important depression in the barrier between the Caribbean Sea and Central America, and Dr. Maack and others have suggested a connection at some previous time. By Mr. F. C. Nicholas enough data have been gathered to make it appear that a geological strait and later a canal or canals obtained similar to that of Tehuantepec. The barrier between the low navigable waters on each side of the divide is only eleven miles across, and of this distance, the valley is so low that except for three miles of its course it is navigable for canoes at all times. During floods even the remaining three miles can be crossed in small boats. This passage is only about a hundred and fifty feet above the sea, but it is a narrow canal a hundred
Fig. 9.—The Divide of the Isthmus of Tehuantepec (view on larger scale than in Fig. 8), showing the northern end of the geological canal dissecting the divide to a depth of about one hundred and fifty feet. Knob to the left represents the end of a ridge rising out of the old geological strait.
feet or more below the summit of the dividing ridge. The barrier in part appears to have been produced by the delta deposits brought down from the lateral mountains by the streams at a time when the valley was occupied by the waters of a great strait from twenty to forty miles wide, connecting the Caribbean Sea with the Pacific Ocean. Some of these gravels contain large quantities of gold. The gold-bearing gravels and other alluvial deposits which rise upon the hillsides and occur on the divide seem to mark the different changes of level corresponding to those in the Tehuantepec Isthmus.
Besides this Atrato Strait, connecting the Caribbean Sea with the Pacific Ocean, there was another connection by way of Nicaragua, as suggested by Mr. Crawford's studies, but the latest geological canals across this region have been obstructed by barriers rising to two hundred and thirty feet. The more open country of the Panama district is about four hundred and sixty feet above the sea, but it is dissected by valleys such as that followed by the railway, whose natural divide is two hundred and ninety-nine feet. It is only reasonable to suspect that here may have been a former strait or canal, but so far the terraces and gravel floors similar to those of the Tehuantepec divide have not been described, as this is a feature of only recent scientific inquiry. The interoceanic connections referred to by Dr. Maack belong to the older Tertiary period, preceding the sculpturing of the physical features of the region, which were long anterior to the water ways here described.
Biological Evidence of Interoceanic Connections.—The fishes, shells, sea urchins, and other organisms of the West Indian basins belong to modern types, which to a large extent seem to have migrated from the Atlantic Ocean. Their recent appearance suggests great changes in the physical history of the West Indian seas, which are only explicable on the theory of the Antillean bridge, as set forth in the May number of this journal, and extended in the present paper. For example, the West Indian region was a high continental mass with extensive plains where the sea basins now occur, draining into the Pacific Ocean across the central part of America. With the subsequent subsidence in the mid-Pleistocene epoch the Antillean basins became seas, into which the modern Atlantic forms of life gained access. The deep-sea fishes have absolutely no relationship with those living in the Pacific Ocean, thus showing that the American barrier obtained sufficient height for their exclusion; but the littoral or shallow-water fishes and shells of the Pacific Ocean, to a notable degree, are found in the West Indian waters. The term deep-sea fauna here used applies to those forms living at greater depths than from three hundred to five hundred feet, according to circumstances. Thus it would appear that there were shallow water connections with the Pacific Ocean in or since the mid-Pleistocene epoch, such as have been shown from the study of the physical features. But these water ways were either too small or of too short duration for the general admission and commingling of Pacific and Atlantic types. The modern characteristics of the deep-sea fishes of the West Indian basins suggest that the older forms of Antillean life had been expelled from the region, so as not to permit of the development of their successors, as should have occurred if the continental area, now drowned, had not been generally drained. The modern facies of the marine West Indian life thus supports the physical evidence of a great Antillean continent or connection between North and South America just prior to the introduction of modern species. There is no inconsiderable degree of satisfaction to the student in finding that the biological phenomena support the physical testimony of the recent great changes which have occurred in the Central American and West Indian regions.
On the Confirmation of the West Indian Continent by Central American Phenomena.—The Reconstruction of the Antillean Continent, and The West Indian Bridge between North and South America, have been largely based upon the testimony of the great river channels being traceable as drowned valleys of natural proportions across the submerged plateaus or margins of the land to the floors of the Gulf and Caribbean basins, which are characterized by large plains. This fundamental feature is supported by a great array of facts collected by the writer and described in the papers named. It was during the early part of the Pleistocene period that the West Indies were united into a high plateau and the floors of the sea basins were transformed into plains, to the margins of which the river valleys have been traced. The continuance of the river valleys to the floors of the sea basins is the result of their formation upon the surface of the land before the region was submerged beneath the oceanic waters. Satisfactory as was the evidence of the high continental elevation to the east, the Central American region now appears as a barrier against its former drainage to the Pacific, thus producing inclosed sea basins. On a smaller scale, there is a perfect analogy in the basin of Lake Ontario, which is only the valley of the St. Lawrence, mostly closed by warping of the earth's crust so as to raise up a barrier of several hundred feet in place of thousands of feet, as is the case in Central America. It was very significant that the depressions in the Mexican and Central American table-lands correspond only to the extensions of the West Indian basins, thus suggesting the location of their outlet. But the discovery of the recent general elevation of the American barrier to thousands of feet, and the preservation of the last water ways across the divide, are found to show that the barrier did not obtain at the time when the Antillean basins, according to the hypothesis, should have been drained into the Pacific. Thus the low altitude of Central America at the time of the high continental elevation to the east and the presence of the recent water ways to the Pacific Ocean, lately discovered, complete, in a manner, the story of the great oscillations of land and sea in the regions between North and South America.
Of the two miles or two miles and a half in height of the Central American barrier above the floor of the sea basins, probably half that amount has been produced by gentle warping of the earth's crust, amounting to only a few feet per mile, like that across the outlet of Lake Ontario—so gently developed as not to produce bold features. On the other hand, portions of the barrier have been squeezed up by local movements, causing the plateaus, whether above or below the surface of the sea, to rise rather suddenly from the plains, in front of them.
Thus the story of the West Indian bridge and the geological water ways across Central America are only different chapters of the great changes of level of land and sea which have occurred in the most recent geological times, illustrating terrestrial movements now in progress, which have the power of completely altering the physical features of the earth, transposing tropical and arctic climates, and scattering or exterminating animal and plant life of continental regions.
- ↑ Appletons' Popular Science Monthly, 1898, vol. liii, pp. 10-30.
- ↑ Bulletin of the Geological Society of America, vol. vi, pp. 103-140, 1894.
- ↑ Appletons' Popular Science Monthly, vol. liii, pp. 10-30, 1898.
- ↑ For further details, see Great Changes of Level in Mexico and the Interoceanic Connections. Bulletin of the Geological Society of America, vol. ix, pp. 13-34, 1897.