The “Rock Wall” of Rockwall, Texas
For many years reports of a more or less definite nature have been circulated describing the wonders of the ancient wall surrounding the town of Rockwall, Texas. The writer was able during the past winter to spend a few days investigating this supposed historic structure. It proves to be not a wall, but a number of disconnected sandstone dikes, strictly speaking, not surrounding the town, but trending in many directions. As exposures are few, they have been discovered in such scattered localities in the town’s environs as to suggest the idea that they were fragments of a ruined wall.
Rockwall is located in a rich farming district about twenty-five miles east of Dallas. Black waxy soil covers the rolling hills, and only where erosion has been considerable can the underlying rocks be seen. These, when exposed, reveal blue limey strata of upper Cretaceous age in nearly horizontal attitude. A white clay, the decomposed product of the lime muds, generally occurs beneath the black soil. These lime muds are remarkable in their freedom from grit and in the peculiar property which causes them to decrepitate when exposed to the weather; notable also in that, on drying, cracks develop of various sizes. Within this series of semi-consolidated beds a few sandy layers occur. One is revealed by a drill record 1,800± feet below the surface; another may be seen near the town of Rockwall at the surface and consists of thinly bedded flaggy sandy limestone.
Though good exposures are infrequent, owing to the depth of soil, a peculiar condition affords ample opportunity to observe the dikes in place. These latter are natural courses for underground waters, and wells are often located on them. Though these walls are filled with water, the rock forming the dike, removed during the sinking of the well, may be examined at leisure.
The dikes are of various sizes, varying from an inch in thickness to eighteen inches or two feet. They stand vertically, or nearly so, and have in cases been followed downward fifty feet or more, always imbedded in the lime muds. They are composed of exceedingly fine grained quartz sands, cemented by calcium carbonate. So far as observed they do not vary appreciably in width through vertical range. Two joint systems, one nearly horizontal, the other vertical, have cut these dikes in such a manner as to suggest masonry walls, i. e., they are composed of oblong blocks in horizontal layers.
Certain facts may be noted, however, which preclude this view. In a photograph at hand exposing a portion of the dike near Rockwall, it may be seen that many of the vertical joints occur above each other, i. e., they are not broken, which condition would not exist in a wall constructed by hand. It may also be noted that the curve to the upper surface of one block exactly fits the curve on the under surface of the next block above, which leads to the same conclusion. The weathered sands between the joints, stained with iron oxide, have been mistaken for mortar.
To define accurately the steps which have taken place in the forming of these dikes is not as easy as to recognize the nature of the phenomenon. They may have originated in several ways. The sands may have come from above or from below. The cracks may be due to drying or to earth movements. The writer was not able to decide the direction from which the sands entered. Inasmuch as circulating waters have passed for long periods through the sands, dissolving and redissolving the cement between the grains, the original position of the latter can not be postulated. At present they show no signs of bedding. On breaking blocks, what might be called a stalagtitic fracture is obtained, i. e., cylindrical or tubular forms arranged in vertical position. As has been pointed out, this may well be secondary structure induced by circulating water.
The limey muds were probably deposited in very clear quiet waters. A slight elevation of the sea or an increased supply of material from the land may have altered deposition and spread fine sands upon the muds. Cracks formed by earthquakes may have permitted unconsolidated sand to enter as a filling. Again, the muds may have undergone a drying-out process since their elevation above the sea, cracks may have formed from this cause, and overlying sandy layers aided by percolating waters served to supply material wherewith to fill them.
The joints may be ascribed to forces arising from slight warping of the earth’s surface, acting on hard vertical masses imbedded in relatively plastic strata.
It is fair to say in conclusion that the believers in the theory which ascribes the origin of these dikes to prehistoric men are in the minority in the locality itself.