Popular Science Monthly/Volume 20/February 1882/Vibration of Rocks in Patapsco Valley, Maryland
|VIBRATION OF ROCKS IN PATAPSCO VALLEY, MARYLAND.|
THE valley of the Patapsco River, through which the Baltimore and Ohio Railroad passes, presents many interesting subjects to the student of physics and of geology. After passing the junction of the Washington branch at the Relay House, eight miles west of Baltimore, the alluvial plain gives place to a narrow valley with steep and wooded hills on either side, and walled with conglomerate or metamorphic rocks. Above Ilchester, a few miles farther up the valley, begin the beds of solid granite, which are excavated by several quarries near Ellicott City, fifteen miles from Baltimore. It is worthy of observation that these granite beds are replaced, a few miles above, by separate rocks composed of the ingredients of the granite, or resulting from their decomposition. In the cuttings of the railroad five miles above Ellicott City, large masses of nearly pure silica are found, and in the same vicinity feldspar is abundant. The mica is sufficiently stratified to furnish a supply worth working for market; and asbestos, in more or less advanced stages of formation, is quarried to some extent for purposes not requiring a perfect fibrous structure. Kaolin of very good quality is found, but has as yet been little worked. These rocks will at some future time well repay exploration.
The object of the present paper is to record a phenomenon observed at Ellicott City, which lies in a narrow and deep valley, nearly at a right angle to the Patapsco, which is here a stream of some sixty yards in width, flowing very rapidly over a broken and rocky bed. About an eighth of a mile below the town, the turbulent stream enters a straight channel with a comparatively even bottom, and flows for nearly half a mile with steady current and a depth of three to six feet. The river is then checked by the dam of a large mill, and presents a fall of about ten feet, with a sheet of water seldom more than six inches in depth pouring over the edge of the dam.
The stream is here over sixty yards wide, and the body of falling water produces a sound which can not be heard more than four hundred yards below, or half that distance above the dam. The impact of the fall is so trifling that it causes no perceptible vibration in the immediate vicinity. Yet, in the valley where Ellicott City is built, more than half a mile above the fall, the houses on either hill-side, which have foundations deep enough to reach the granite bed-rock, are subject to occasional vibrations which shake their walls and cause a continuous rattling of the windows. This effect has, of course, been ascribed by the people, in an indefinite way, to the river, but it was some time before I collected the physical data for the explanation which is here offered.
In making the investigation, it was observed that the current of the river above and near the town was broken by rocks, and, although rapid, could produce no rhythmic vibrations because of the irregularity of its impacts. The apparently quiet flow of the stream below did not seem to offer any solution of the problem, and the fall of water over the dam had evidently a very trifling force of impact. But a more careful observation of this little water-fall convinced me that it was the motive power. First, a peculiar shimmer in the reflection of light from the edge of the dam, and on nearer approach a distinct rhythm in the movement of the falling water, were observed. This was found to occur only at certain times. If the river was either too high or too low, the water poured over the dam with regular and uniform motion and sound; when the vibration of the houses occurred, the pulsatory movement of the falling water was also noticed.
The vibrations of the water were estimated at eight or ten per second, or about half of the number required to give the lowest continuous sound. These facts being established, it still remained to account for the enormous mechanical effect necessary to produce the vibration of distant buildings.
The long stretch of steadily flowing water above the dam, whose sloping edge only partly arrested its course, suggested the divided current of air thrown into the pipe of an organ. And, as the small column of air in the pipe, when thrown into vibration, is known to produce in some cases a perceptible tremor in the massive walls of a church, it is probable that a column of water half a mile in length, when in rhythmic vibration, would have a mechanical effect in proportion to its mass. But, in order that such vibration of a considerable body of water should be produced, a peculiar combination of physical conditions must exist, and this I have never observed in any other place.
Of course, the fall of such masses of water as pour over Niagara and other great cataracts will cause a tremor of the earth to a certain distance. But this is due to the simple mechanical force of impact, and is in relation to that force only. The phenomenon which I have described is intermittent, and that fact points to the conclusion that it is due to a definite relation between the vibrations of the river and what may be called the keynote of the bedrock over which it flows.
The distance to which such motion may be transmitted will, of course, depend upon the mass of water in vibration. The direction will depend upon the continuity of the underlying rock. Where this is broken by seams, the vibrations will cease, and therefore the line of these remarkable tremors affords an indication of the structure of the granite bed which is the basis of the picturesque hills around Ellicott City.