«The reflections I was led into on viewing this passage of the Potomac through the Blue Ridge were, that this country must have suffered some violent convulsion, and that the face of it must have been changed from what it probably was some centuries ago; that broken and ragged faces of the mountain on each side of the river; the tremendous rocks which are left with one end fixed in the precipice, and the other jutting out, and seemingly ready to fall for want of support; the bed of the river for several miles below obstructed, and filled with the loose stones carried from this mound; in short, every thing on which you cast your eye evidently demonstrates a disrupture and breach in the mountain, and that before this happened, what is now a fruitful vale, was formerly a great lake, or collection of water, which possibly might have here formed a mighty cascade, or had its vent to the ocean by the Susquehanna, where the Blue Ridge seems to terminate. Besides this, there are other parts of this country which bear evident traces of a like convulsion. From the best accounts I have been able to obtain, the place where the Delaware now flows through the Kittatinny mountain, which is a continuation of what is called the North Ridge, or mountain, was not its original course, but that it passed through what is now called the Wind-gap, a place several miles to the westward, and above an hundred feet higher than the present bed of the river. This Wind-gap is about a mile broad, and the stones in it such as seem to have been washed for ages by water running over them. Should this have been the case, there must have been a lake behind that mountain; and, by some uncommon swell in the waters, or by some convulsion of nature, the river must have opened its way through a different part of the mountain, and meeting there with less obstruction, carried away with it the opposing mounds of earth, and deluged the country below with the immense collection of waters to which this new passage gave vent. There are still remaining, and daily discovered, innumerable instances of such a deluge on both sides of the river, after it passed the hills above the falls of Trenton, and reached the champaign. On the New Jersey side, which is flatter than the Pennsylvania side, all the country below Croswick hills seems to have been overflowed to the distance of from ten to fifteen miles back from the river, and to have acquired a new soil, by the earth and clay brought down and mixed with the native sand. The spot on which Philadelphia stands evidently appears to be made ground. The different strata through which they pass in digging for water, the acorns, leaves, and sometimes branches which are found above twenty feet below the surface, all seem to demonstrate this.»
How little reason there is to ascribe to extraordinary convulsions the excavations which are made by water upon the surface of the earth, will appear most evidently from the examination of that natural bridge of which mention is made above, and which is situated in the same ridge of mountains, far to the south, upon a branch of James's River. Mr Jefferson gives the following account of it.
"The natural bridge, the most sublime of nature's works, is on the ascent of a hill, which seems to have been cloven through its length by some great convulsion. The fissure, just at the bridge, is by some admeasurements 270 feet deep, by others 205; it is about 45 feet wide at the bottom, and 90 feet at the top; this of course determines the length of the bridge, and its height from the water. Its breadth in the middle is about 60 feet, but more at the ends; and the thickness of the mass at the summit of the arch about 40 feet. A part of its thickness is constituted by a coat of earth, which gives growth to many large trees. The residue, with the hill on both sides, is one solid rock of lime-stone. The arch approaches the semi-elliptical form; but the larger axis of the ellipsis, which would be the cord of the arch, is many times longer than the transverse. Though the sides of the bridge are provided in some parts with a parapet of fixed rock, yet few men have resolution to walk to them, and look over into the abyss. You involuntarily fall on your hands and feet, and creep to the parapet, and look over it. Looking down from this height about a minute gave me a violent headache. If the view from the top be painful and intolerable, that from below is delightful in the extreme. It is impossible for the emotions arising from the sublime to be felt beyond what they are here. On the sight of so beautiful an arch, so elevated, so light, and springing as it were up to heaven, the rapture of the spectator is really indescribable! The fissure, continuing narrow, deep, and straight, for a considerable distance above and below the bridge, opens a short but very pleasing view of the north mountain on one side, and blue ridge on the other, at the distance each of them of about five miles. This bridge is in the county of Rockbridge, to which it has given name, and affords a public and commodious passage over a valley, which cannot be crossed elsewhere for a considerable distance. The stream passing under it is called Cedar Creek: it is a water of James's River, and sufficient in the driest seasons to turn a grist mill, though its fountain is not more than two miles above[24]."
Footnote 24:[ (return) ] Upon this occasion it may be observed, the most wonderful thing, with regard to cosmology, is that such remnants, forming bridges, are so rare; this therefore must be an extraordinary piece of solid rock, or some very peculiar circumstances must have concurred to preserve this monument of the former situation of things.
Thus both in what is called the Old World and the New, we shall be astonished in looking into the operations of time employing water to move the solid masses from their places, and to change the face of nature, on the earth, without defacing nature. At all times there is a terraqueous globe, for the use of plants and animals; at all times there is upon the surface of the earth dry land and moving water, although the particular shape and situation of those things fluctuate, and are not permanent as are the laws of nature.
It is therefore most reasonable, from what appears, to conclude, that the tops of the mountains have been in time past much degraded by the decay of rocks, or by the natural operations of the elements upon the surface of the earth; that the present mountains are parts which either from their situation had been less exposed to those injuries of what is called time, or from the solidity of their constitution have been able to resist them better; and that the present valleys, or hollows between the mountains, have been formed in wasting the rock and in washing away the soil.
If this is the case, that rivers have every where run upon higher levels than those in which we find them flowing at the present, there must be every where to an observing eye marks left upon the sides of rivers, by which it may be judged if this conclusion be true. I shall now transcribe a description of a part of the Vallais by which this will appear. (Discours sur l'Histoire Naturelle de la Suisse.)
«Après avoir passé le village de Saint-Leonard, on commence à monter la montagne de la Platière; cette route est on ne peut plus intéressante pour le naturaliste, etc.
«On se trouve fort élevé au-dessus du Rhône quand on est sur le haut de ce chemin, dont on découvre un de plus singuliers, des plus riches, et de plus variés passages qu'on puisse imaginer. On voit sous ses pieds le Rhône serpenter dans le lit qu'il se creuse actuellement, car il change et tout prouve qu'il en a souvent changé; une quantité prodigieuse de petites isles le séparent et le coupent en une multitude de canaux et de bras; ces isles sont couvertes les unes d'arbres, d'arbustes, de pâturages, de bosquets et de verdure, d'autres de pierres, de sable, et de débris de rochers; quelques-unes sont formées ou occasionnées par un amas de troncs d'arbres entassés avec de grands sapins renversés dont les long tiges hérissées de branches droites et nues représentent des chevaux de frise, et donnent l'idée de ces abatis destinés à preserver un pays contre l'approche de l'ennemi. Du côté du bas Vallais, on suit à perte de vue le fleuve dans ses sinuosités et ses détours, on l'apperçoit également dans le haut Vallais; des avances de montagne le cachent quelquefois: il reparoît et diminue insensiblement en approchant de ces monts élevés ou il prend sa source: le fond du vallon paroît être de niveau, s'abaisser seulement d'une pente douce du côté du bas Vallais: des mamelons, des hauteurs des monticules isolés, quelquefois groupés de différentes manieres, sont répandus dans cet espace, et rappellent la vue d'une pré dévasté par les taupes; plusieurs de ces hauteurs sont surmontées des ruines d'antiques châteaux, d'eglises, et de chapelles; des villages distribués ça et là enrichissent ce fond, qui d'ailleurs est couvert de pâturages, de champs d'arbres, de bois, et de bosquets; les enclos des possessions le coupent en mille figure bizarres et irrégulières. Ces monticules avec leurs fabriques s'élèvent au-dessus de tous ces objets variés; quelques-unes se distinguent par leur côtés écroulés qui sont à pic; la blancheur de ces éboulemens contraste singulièrement avec les verts qui sont les couleurs dominantes du vallon. Au-de-la des coteaux, des montagnes s'élèvent et vont s'appuyer et s'adosser à ces masses, à ces colosses énormes de rochers à pic élevés comme des murailles et d'une hauteur prodigieuse qui forment cette barrière qui sépare le Vallais de la Savoie. Les contours du pied de ces monts forment des entrées de vallons et de vallées d'ou descendent et se précipitent des torrens qui viennent grossir les eaux du Rhône; la vue cherche à pénétrer et à s'étendre dans ces espaces, l'imagination cherche vainement des passages dans effrayantes limites, parmi ces écueils et ces rochers amoncelés, elle est arrêtée partout; de noires forêts de sapin sont suspendues parmi ces rochers blancs-jaunâtres, qui se terminent enfin par une multitude d'aiguilles et de pyramides qu'on voit percer au travers des neiges et des glaces, s'élancer dans les nues, s'y cacher et s'y perdre.
«En examinant de plus près ces mamelons répandus dans le vallon, on voit qu'ils sont composés de pierres, de sables, et de débris rapportés et amoncelés sans ordre depuis des temps dont rien ne peut fixer l'époque: on voit que les eaux du Rhône ont coulé à leurs pied, qu'il en a miné plusieurs et a occasionné leurs chutes et leurs ruines. On voit actuellement quelques mamelons qui subissent ces mêmes dégradations, et fournissent au Rhône les matériaux dont il va former plus loin ces atterissemens dont nous avons parlé. La confusion et le désordre qui se remarque dans la composition intérieure de ces mamelons prouvent qu'ils ne sont pas le produit de la mer ou des eaux qui ont travaillé successivement et lentement à la formation de la plupart des terrains; mais que le fond de ce vallon a été rempli des décombres et des débris des montagnes supérieures, qu'ils y ont été entraînés par des inondations et des débordemens subits; que les eaux du Rhône ensuite ont parcouru ce vallon qu'il a souvent changé de lit; que c'est en tournant et en circulant dans ce terrain nouvellement formé, qu'il a creusé les espaces qui sont entre ces mamelons, et que c'est en creusant le terrain qu'ils se sont élevés; leurs formes et leurs pentes allongées vers le bas Vallais, sont de nouvelles preuves que ce sont les eaux actuelles qui ont changé la surface de ce terrain, nous verrons de nouvelles preuves de ce que nous disons en avançant d'avantage vers le haut Vallais; il n'y a peut-être point d'endroit plus propre à étudier le travail des eaux que ce vallon qu'on a la facilité de voir et d'examiner sous des aspects différentes.»