Important Features of the Canal Zone
Topography and Geography
Gaillard Cut, adjacent to which all slides of importance occur, traverses the higher part of the Canal Zone, from Gamboa on the Chagres River on the north (at level of Gatun Lake), through the Continental Divide, to Pedro Miguel Lock on the south, a distance of 8.75 miles. The general direction of the canal is nearly northwest and southeast. Individual stretches vary considerably from this direction, but for the sake of brevity in description, the canal will be regarded as running north and south; directions at right angles to the canal will be called east or west, and those parallel with it north or south.
The Canal Zone is irregularly hilly, with intervening areas of low relief. From the northern border of the Culebra slides to Gamboa, a distance of 6 miles, or two-thirds the entire length of Gaillard Cut, the canal passes through an undulating country with no elevations very near it attaining a height of more than about 200 feet above the bottom of the canal. The artificial banks nowhere rise more than 150 feet in this part of the cut.
In the vicinity of Gold and Zion Hills the canal enters a more hilly country. Several hills, the summits of which are more than 500 feet above sea, approach close to the canal, and Gold Hill, the highest, has an elevation of 660 feet, or 620 feet above the bottom of the canal. The Continental Divide, on the canal line, was between Zion and Gold Hills at approximately 360 feet above the sea. The country traversed by the deepest part of Gaillard Cut, at the Continental Divide, about 1 mile in length, in which all the really serious trouble has occurred, will be called the Culebra district; the hill upon which the village of Culebra stands will be called Culebra Hill.
The geological formations of the Canal Zone are mainly stratified, sedimentary beds of shale, sandstone, conglomerate, or breccia, made up for the most part of volcanic rock debris. There are some thin limestone and carbonaceous shale layers and some of the beds contain much clay. The formations now recognized vary from a few feet to over 600 feet in thickness. The distribution of these formations is of great importance in relation to the canal.
It may be well to explain at this point that the term "rock" is used in this report in its geological sense, meaning the material of any recognized unit or formation in the earth. The idea of hardness or firmness does not enter into this definition, and, in fact, some of the rocks of the Canal Zone are very soft.
In the northern two-thirds of Gaillard Cut, north of the Culebra district, and again for nearly 2 miles at the southern end of the cut, the rocks through which the canal passes vary in character from place to place, because of folding and faulting, and weaker rocks alternate with stronger ones in most of the vertical sections displayed in the banks. As will appear in discussing the development of the slides, the stronger rocks have to some extent supported the weaker ones in these parts of the cut, so that slides, while numerous, have been small, relatively speaking.
The Culebra district has peculiar and significant geological characteristics. Shortly north of the border of the district, the sedimentary formations assume a southerly dip (toward the point of view) and disappear beneath the canal, to reappear with a northerly dip on the southern border of the district. Thus a synclinal trough, about 1 mile wide at the canal level, crosses the cut directly at its deepest portion. This trough is filed by a single formation, called the Cucaracha, with a known preserved thickness of over 500 feet, and it original upper portion is no longer present.
The Cucaracha is a fine-grained, sandy, clayey formation, more nearly homogeneous throughout than most others of the region; and, as it is structurally weak, the failure of the banks in the Culebra district has been on a larger scale than elsewhere in Gaillard Cut.
The hills, adjacent to the canal in the Culebra district and to the south, are composed of intrusive basalt or a massive volcanic breccia. The breccias of Gold, Contractors, and some other hills have the appearance of being intrusive in their present position, as if forced up through softer formations by ascending basalt magma. These basalts and breccias are elements of strength in the canal banks, and their occurrence limits in various places the amount of the Cucaracha beds which can be involved in the slides. The discussion of the Culebra district and its slides will emphasize the importance of these geological relations.
The presence of a thick mantle of decayed rock and soil, such as is common the Tropics, has proved to be a source of much trouble in the Cucaracha slide area, where the underlying surface of soft rock is rather steeply inclined toward the cut.
The average rainfall in the Culebra district from 1884 to 1916 is reported as 87.68 inches per annum, and during the time of the excavation of the canal it was 84.75 inches per annum. This rainfall is almost wholly concentrated in eight months of the year, usually from April to December. The average precipitation for the eight rainy months during the period of excavation of the canal by the United States was 80.01 inches.
The general effect of rain water is sufficiently summarized by pointing out that it maintains a high level for ground water,which, occupying the large amount of pore space in certain rocks, adds material to the load to be supported by the canal banks; it may weaken the rocks adjacent to the cut, and after slides have started their motion is accelerated by the free access of a great amount of water into the broken masses.
Strong earthquakes are due to fractures of the rock and are usually connected with the formation of a new fault, or with movement on an old one. The damage done by earthquakes may be due to destruction along the fault line or to the rapid vibrations sent out from the region of the break.
Neither the records nor the topography indicate that severe earthquakes occur in the Canal Zone. Many fault fractures were brought to light by the digging of the canal, but none seems to belong to the present geological period. Volcanic activity appears to have been absent from the Canal Zone since mid-Tertiary times, and probably the same is true of earthquakes of local origin. But there are a number of earthquake centers not far from the zone, and shocks occurring there are often felt in the zone. The strongest recorded shock felt in the zone occurred September 7, 1882. Its origin was in northwestern South America, about 300 miles southeast of the Canal Zone.* It was an extraordinarily destructive shock in South America, and even on the Isthmus caused injury to the abutments and bridges of the Panama Railroad; the tracks also were said to have been bent in places. At Aspinwall (Colon) a building was damaged. No other shock has done any real damage in the Canal Zone, though a number of shocks have been felt and people have been alarmed. Los Santos Province, Chiriqui Province, and the neighborhood of Bocas del Toro seem to be the sources of these shocks. The nearest of these districts, Los Santos Province, is about 120 miles southwest of the southern end of the canal. The instruments at Balboa Heights have recorded a few shocks from nearer sources, but they have not been numerous or severe. The shock of May 27, 1914, originated about 80 miles from the southern end of the canal, probably in the direction of Los Santos Province. It made some small cracks in the Administration bui9lding at Balboa Heights. This building has a steel frame, which is filled in with hollow tiles and plastered over. The elasticity of the steel allowed some swaying, and the brittle character of the tiles caused them to crack slightly; but the strength of the building was not impaired. The shock of October 1, 1913, caused the dislocation of a part of the side of a sluice way at the top of the Cucaracha slide.
It is well known that water-saturated ground suffers more at the time of earthquakes than solid rock, and this has aroused fears lest great slides adjacent to the canal be started by earthquakes. Neither during the times of the old and the new French companies, nor during the American regime have earthquakes exercised any observed influence on the slides; and when the slides have come to rest under a low slope of equilibrium it is hardly possible that earthquakes could do more than start a slow temporary movement. They might cause some disturbance in the railroad fills crossing swamps or to the causeway leading to Naos Island. This seems the uttermost to be apprehended. The massive reinforced structures of the canal locks make them quite secure. The comparative security of such structures is illustrated by the fact that the concrete dam of the Crystal Springs Co., about 180 feet high, situated only about a quarter of a mile from the San Andreas fault, the seat of the great California earthquake of 1906, was not in the least affected by the earthquake.
The study of earthquakes is still in its early stages, and it is impossible to say that any region will not be visited by an earthquake; but there is no reason to fear that earthquakes will do any great damage to the canal.
*Since the above was written more information has come to hand regarding this earthquake. It seems probable that it was due to movement on a fault under the Gulf of Darien, which extended south into Colombia and northwest under the sea passing with a hundred miles, or less, of the isthmus. Nevertheless the conclusions arrived at in the text require no modifications.
Excerpt from: Report of the Committee of the National Academy of Sciences on Panama Canal Slides, Government Printing Office, Washington, DC, 1924
September 20, 1999
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