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conditions of instability that have resulted in so-called "slides” or “breaks.” Fortunately, man controls the means to restore equilibrium. This phase of the canal work will be more fully described later on.
From the reports of the commission's geologists it is learned that the oldest rocks are exposed along the canal in the Atlantic end of the Culebra Cut in the region about Bas Obispo. There is also a smaller outcrop of these rocks near Miraflores at the Pacific end of the Culebra Cut and the evidence is that the two are joined by deep-lying rocks of the same character. These are of a type known as volcanic breccia, with local deposits of meta-conglomerates, and originally composed of fragments from volcanic craters but which, in the course of ages, have been compacted and cemented into a fairly hard rock. The whole in past ages has been under heavy stresses, which have caused faults and shears; that is, the rocks have cracked and the parts have slid by one another vertically, and this has resulted in very much broken masses.
From Empire to Paraiso, at the head of Miraflores Lake, and thus including the greater part of the Culebra Cut, there existed in a former age a great basin or sag. This basin no longer forms a valley, for during succeeding geologic periods it was filled with new formations. These are not of a regular character and the variations in the rocks and in their disposition are greater than in many other parts of the world.
A complete and detailed description would interest only the geologist or the engineer in advance of his constructive operations. The basin rocks are overlaid by the next younger formation, composed of light gray limy sandstones and sandy limestones in beds or lenses, and separated from each other by thin beds of friable shale with occasional masses of carbonaceous matter. These formations contain fossils of marine fauna which also occur at other points entirely across the Isthmus, and as these are formed only in the shallow estuaries of the sea, it is proved that the materials in which the fossils are now lodged were deposited under water; and their disposition across the Isthmus is considered by geologists as proof that the Atlantic and Pacific Oceans were joined at that time. The latest marinedeposited rock is composed of coral and shell limestones and is of non-continuous occurrence. During succeeding ages, these formations slowly rose, as they are now above the level of the sea. There was then aerially deposited a thick bed of greenish fine-grained volcanic clay rock, which fills the depressions in the marinedeposited rocks — the remains, probably, of former estuaries and channels. This deposit is also not uniform, but contains beds of gravel, sandstone and lava flow. The whole is weak and crumbly.
The most recent geologic formations consist of masses of volcanic rock; molten lava from below forced its way through the softer rocks along the lines of least resistance, sometimes spreading between the layers of softer rock and sometimes breaking through in great rifts, forming dikes. Great volcanic masses have also risen, due to the pressure exerted from below, and, forcing their way upward in a cold condition through several hundred feet of softer overlying rocks, are said to have formed such elevations as Gold Hill and Contractors Hill.
The geologically recent volcanic eruptive rocks are generally hard, and they fortunately serve to greatly strengthen the banks of the canal. The various rocks are in irregular formation, and have in some cases been dislodged from their former relative position by faults or vertical sliding of unstable masses, and by the breaking and cracking into blocks of masses of contiguous rocks.
With this brief description of the local geology, the problem ahead of the engineer in planning and making the Culebra Cut becomes a little clearer. It is to be remembered that most of the information now available was unknown before the excavation exposed materials hundreds of feet below the surface. Where the canal passes through a deep cut, the exact slopes to be given to the sides are of the greatest importance. If they are too steep the material will slide or fall into the canal, and if the slope is flattened, even a little, unnecessary excavation and greater cost will be incurred, increasing rapidly with the depth of the cut. The side slopes must be determined in advance of excavation, as far as possible, because the shovels in the beginning cover the full width and gradually work to the lower and narrower levels. The cutting of any additional material from the side to flatten the slope will be hazardous and costly. As the cut progressed, the bulk of the side slopes proved stable. But as already mentioned, certain areas of instability developed into breaks and slides.
Fig. 12. — Miraflores upper locks, general view, looking north from lower locks. Steam-shovel excavation for lower locks in foreground. November 8, 1911. The two cranes on extreme left and on extreme right are the “berme" cranes containing concrete mixers. The two center cranes are chamber cranes and handle materials to center part of the locks.
Fig. 13. — Concrete handling cranes at Miraflores. The booms on the right carry the buckets which pick up the sand and stone from the storage piles and dump in bins over the mixers in center of crane. The booms on the left carry the concrete for dumping in the lock walls. The whole crane travels on a track. The booms on the left move through a small angle horizontally.