Popular Science Monthly/Volume 23/August 1883/A Natural Sea-Wall
|A NATURAL SEA-WALL.|
ALONG the New Hampshire sea-coast, in the towns of Rye and North Hampton, stretches a curious and massive formation, which at first sight appears as if built at enormous expenditure of time and labor. On closer examination, however, it proves to be only one of Ocean's eccentric freaks, executed in this case with almost human intelligence and care.
A sea-wall, compactly formed of water-worn pebbles of all sizes, shapes, and materials, runs along the beach for about six miles, here and there broken by rocky points and little inlets, somewhat modified by its situation, but preserving with astonishing regularity several remarkable features. In places it is so high and wide that one can hardly believe it anything but a carefully constructed dike, designed to shelter the adjoining fields. Along part of its extent, where it separates the ocean from an extensive salt-marsh, it is utilized by the farmers of the neighborhood for a cart-road. Along another stretch, a plank-walk surmounts it for half a mile.
It first appears in the form of a low wall composed of three terraces, near Little Boar's Head, in the town of North Hampton, thirty rods south of the slight projection known on the charts as Fox Hill Point. This portion of the wall is only about twenty rods in length, and seems much like a stone facing to the steep beach ward slope. Some forty rods north of the point it reappears, this time in the form of a large and compact dike, and extends along the water-line in a crescent form for at least fifty rods, terminating at a small cove directly east of the well-known Farragut House. This section of the wall is by far the most symmetrical and characteristic, and is the one selected for a more detailed examination and description. Beyond this point the wall runs with occasional breaks to its northern terminus without presenting any novel features.
The annexed diagram, enlarged from measurements by the United States Coast Survey, shows that part of the wall between Fox Hill Fig. 1. Point and the Farragut House. It has the form of a shallow crescent, and follows high-water mark quite closely, just east of the highway. It has the general peculiarities very strongly marked, but near its northern extremity is so modified by an adjacent shoal as to give an easily-followed clew to the method of its formation. It has the three neat terraces common to the whole of the wall, and, at the time of examination, had not been marred by the walk previously mentioned. This walk was built directly along the summit, which was smoothed for this purpose, thus almost obliterating the upper terrace, and lowering the whole crest. Previously, the wall has remained substantially as originally formed, for its steepness and height were such that, as a path, it was far from alluring. But now along its landward base runs the road from Little Boar's Head to Rye Beach; the situation of this road has been materially changed by the encroachments of the sea during the last fifty years, and its old location is approximately indicated on the diagram by the dotted line.
At the highest point of the crescent near its middle the road is about twelve feet below the summit of the wall, and only four or five feet above high-water mark. Diagram number two is a cross section at this place, drawn to scale from personal measurements. The slope inward, it will be noticed, is comparatively gradual and quite regular, while the sea-face is formed in terraces, very regular and individually steep. The general seaward angle of inclination is fully twice as great as the landward slope, and possesses some decidedly interesting characteristics. In the first place, it is singularly regular, varying but a very few degrees throughout its entire length; secondly, it is almost exactly the angle that such a mass of pebbles Fig. 2. would take if uninfluenced by the action of the waves. Constant washing has adjusted it precisely as if the pile had been left to its own conditions of equilibrium.
The general angle of the upper terrace is 29º. It has undoubtedly been lessened by travel, and by the fact that it is out of reach of any but the higher waves, and the constant tendency, therefore, is toward undermining it. This terrace is composed of rather large stones, quite unmixed with sand or gravel.
The second terrace is steeper, 32º nearly. It is exposed to the high tides, and its surface is made up of rather small stones, distributed with great regularity, still unmixed with gravel, but compacted by the waves to a remarkable degree of solidity.
The lower terrace has nearly the same angle as the middle one, but its composition is very different. The pebbles are of small size, mixed with much coarse gravel and sand, and pounded by the constant washing of every tide into a hard, smooth, regular slope that effectually resists any attempt at undermining.
The general angle of the wall as a whole is within a few minutes of 30º, as nearly as could be determined. Owing to the rough instrument employed—a simple arrangement of plumb-line and semicircular protractor the angles could only be measured to the nearest half degree, but the number of measurements taken leaves little doubt as to the accuracy of the result.
The size of the stones in some parts of the formation may give an idea of the tremendous force of the rushing waves that produced it. Stones of from twenty to thirty pounds in weight are common along the crest of the wall, fifteen feet at least above high-water mark. And not only this, but stones of the same size have been thrown completely over the wall into the road, more than a hundred feet from the water. After a severe storm the road is sometimes strewed thickly with these great pebbles, and some trouble is necessary to clear it. So, at least, says popular tradition, but it is probable that very many of the larger stones, found some rods inland, were deposited there before the wall existed in anything like its present condition. For it is almost certain, from the character of the formation and its known history, that it has been piled up in comparatively recent times.
Some of the old inhabitants assert that the terrific storm that in 1851 devastated the whole New England coast and snapped like pipe stems the iron pillars of the Minot's Ledge light-house, is responsible, too, for the wall. While that noted storm certainly did perform some tremendous feats, and in some places permanently changed the contour of the shore, no one storm could do its work in a very systematic or regular fashion. No one storm could have formed the three terraces that compose the sea-face of our wall. The Minot's Ledge storm may have greatly augmented an existing pile and changed its shape somewhat (as there is reason to believe it did), but, powerful as it was, it did not do anything more than this.
It is said that the above-mentioned storm so blockaded the existing highway that it was moved a short distance inland, and there is evidence to show that this road once ran near the present high-water mark, on or about the line now occupied by the wall.
It is, then, probable that a double process has been going on. The sea has been encroaching on the beach, and at the same time obstructing its own course with the débris of former invasions. It is worthy of notice that the wall has only been thrown up along the sandy beach, where the waves had an unobstructed passage. On the ledges that are of frequent occurrence no signs of any extensive deposit are visible. The wall is absent or much modified where a shoal intervenes. These facts show that the deposit has been the result of successive storms heaping up the material, and the ordinary course of the waves and tides molding and arranging it. When the angle of the pile exceeded the natural slope of such materials, growth in that particular plane ceased, and a terrace was formed. Thus the wall, as far as its seaward side is concerned, seems to be a sort of concretion, the terraces being formed in succession, partly out of new material furnished by annual storms and partly from what was left after the first terrace had reached its present angle. Constant pounding of the waves has solidified the wall, though various storms have partially undermined it and necessitated the re-formation of its face. To these storms is due the motion landward that has from time to time taken place. The materials of the wall have been collected from a large area, as is shown by their diverse character, and why they should have been deposited at this particular point is a matter of some doubt. It may be that submarine ledges off this part of the coast have furnished a quarry for the waves. The method of formation, however, has been substantially as above recorded, and the results have been of such a curious character as to be well worth a visit from those interested in the influence of the sea in modifying and eroding the coast.