Jefferson's response to this question includes not only observations on Virginia's waterfalls and caverns (with temperature measurements), but his unsolicited inclusion of his wonderment at the Natural Bridge, the geological phenomenon in Rockbridge County that was among his landholdings.
QUERY V.
Its Cascades and Caverns?
The only remarkable Cascade in this country, is that of the Falling Spring in Augusta. It is a water of James' river, where it is called Jackson's river, rising in the warm spring mountains, about twenty miles south-west of the warm spring, and flowing into that valley. About three quarters of a mile from its source, it falls over a rock 200 feet into the valley below. The sheet of water is broken in its breadth by the rock, in two or three places, but not at all in its height. Between the sheet and the rock, at the bottom you may walk across dry. This cataract will bear no comparison with that of Niagara, as to the quantity of water composing it; the sheet being only 12 or 15 feet wide above, and somewhat more spread below; but it is half as high again, the latter being only 156 feet, according to the mensuration made by order of M. Vaudreuil, Governor of Canada, and 130 according to a more recent account. In the lime-stone country, there are many caverns of very considerable extent. The most noted is called Madison's Cave, and is on the north side of the Blue ridge, near the intersection of the Rockingham and Augusta line with the south fork of the southern river of Shenandoah. It is in a bill of about 200 feet perpendicular height, the ascent of which, on one side, is so steep, that you may pitch a biscuit from its summit into the river which washes its base. The entrance of the cave is, in this side, about two thirds of the way up. It extends into the earth about 300 feet, branching into subordinate caverns, sometimes ascending a little, but more generally descending, and at length terminates, 20 in two different places, at basins of water of unknown extent, and which I should judge to be nearly on a level with the water of the river; however, I do not think they are formed by refluent water from that, because they are never turbid; because they do not rise and fall in correspondence with that in times of flood, or of drought; and because the water is always cool. It is probably one of the many reservoirs with which the interior parts of the earth are supposed to abound, and which yields supplies to the fountains of water, distinguished from others only by its being accessible. The vault of this cave is of solid lime-stone, from 20 to 40 or 50 feet high, through which water is continually percolating. This, trickling down the sides of the cave, has incrusted them over in the form of elegant drapery; and dripping from the top of the vault generates on that, and on the base below, stalactites of a conical form, some of which have met, and formed massive columns.
Another of these caves is near the North mountain, in the county of Frederick, on the lands of Mr Zane. The entrance into this is on the top of an extensive ridge. You descend 30 or 40 feet, as into a well, from whence the cave then extends, nearly horizontally, 400 feet into the earth, preserving a breadth of from 20 to 50 feet, and a height of from 5 to 12 feet. After entering this cave a few feet, the mercury, which in the open air was at 50°. rose to 57°. of Fahrenheit's thermometer, answering to 11°. of Reaumur's, and it continued at that to the remotest parts of the cave. The uniform temperature of the cellars of the observatory of Paris, which are ninety feet deep, and of all subterranean cavities of any depth, where no chymical agents may be supposed to produce a factitious heat, has been found to be 10°. of Reaumur, equal to 54 1–2°. of Fahrenheit. The temperature of the cave above mentioned so nearly corresponds with this, that the difference may be ascribed to a difference of instruments. At the Panther gap, in the ridge which divides the waters of the Cow and the Calf pasture, is what is called 21 the Blowing cave. It is in the side of a hill, is of about 100 feet diameter, and emits constantly a current of air, of such force, as to keep the weeds prostrate to the distance of twenty yards before it. This current is strongest in dry, frosty weather, and in long spells of rain weakest. Regular inspirations and expirations of air, by caverns and fissures, have been probably enough accounted for, by supposing them combined with intermitting fountains; as they must of course inhale air while their reservoirs are emptying themselves, and again emit it while they are filling. But a constant issue of air, only varying in its force as the weather is drier or damper, will require a new hypothesis. There is another blowing cave in the Cumberland mountain, about a mile from where it crosses the Carolina line. All we know of this is, that it is not constant, and that a fountain of water issues from it.
The Natural Bridge, the most sublime of nature's works, though not comprehended under the present head, must not be pretermitted. It 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 only 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 this thickness is constituted by a coat of earth, which gives growth to many large trees. The residue, with the bill on both sides, is one solid rock of lime-stone.—The arch approaches the semi-eliptical form; but the larger axis of the elipses, which would be the cord of the arch, is many times longer than the transverse. Though the sides of this bridge are provided in some parts with a parapet of fixed rocks, yet few men have resolution to walk to them, and look over into the abyss. You involuntarily fall on your hands and feet, creep to the parapet and peep over it. Looking down from this 22 height about a minute, gave me a violent head-ache. If the view from the top be painful and intolerable, that from below is delightful in an equal extreme. It is impossible for the emotions arising from the sublime, to be felt beyond what they are here: 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' river, and sufficient in the driest seasons to turn a grist mill, though its fountain is not more than two miles above.*
See Jefferson's original, hand-written manuscript, including edited draft content that reveals insights into Jefferson's writing process. Available courtesy of the Coolidge Collection of Thomas Jefferson Manuscripts at the Massachusetts Historical Society.
