Some Engineering Problems Of The Panama Canal In Their Relation To Geology And Topography

GENERAL GEOLOGY OF CANAL ZONE. 31 STABILITY OF THE ISTHMUS. A brief reconnaissance of the geology of western Panama was made possible by a cooperative arrangement between the Isthmian Canal Commission, the Smithsonian Institution, and the United States Geological Survey. The information thus gathered, coupled with that obtained from a study of the Canal Zone geology, leads to the conclusion that the isthmian land first appeared as an archipelago of islands in a shallow sea. During the Oligocène period, when the Cu- curacha formation was laid down, there seems to have been established, possibly for the first time, a land connection between the two conti- nents. At least there is, as yet, no positive geologic evidence of an earlier connection. Before the close of the Oligocène, the sea again swept over the submerged land, leaving only a few islands above the surface of the water. It appears that the Miocene began with an emergence, and that until its close the isthmian barrier once more effectively separated the oceans. With, the beginning of Pliocene time, the land again sank until only the highest points were left above the surface of the sea. Another uprise ushered in the Pleis- tocene, and before this period was well advanced the land stood several hundred feet higher than now. Again sinking began, and continued until the land was 6 to 30 feet below present sea level. The last uprise, according to the fresh appearance of the old raised sea beaches and other evidence, must have taken place within very- recent time, probably within the last thousand years. In all, there is a clear record of four oscillations and the beginning of another elevation. Hence the question: Is the canal in danger from this uprise? Should the emergence bo rapid, of course it would be in some danger; but the fact is, that the average rate oi uplift for say, the last 1,000 years has been something less than 0.03 foot a year, or less than 3 feet in 100 years. Dredging could, of course, take care of this rise with little additional expense above the ordinary dredging necessary for tho annual upkeep of the canal. Then, too, there is always tho chance that this motion will stop entirely, and a sinking begin. Arguing then from what has happened in recent geologic time, it may bo concluded that the canal is not in any appreciable danger from the geologic instability of the isthmian land. It is believed that the sinking of tho ocean bottom outside the rela- tively shallow depths of the isthmian shore waters has been the chief cause of tho earthquake periods that have, so far as the record go, visited the isthmus every 30 to 35 years, and that each of these seismic disturbances has resulted in some increased uprise of the land mass. This matter is more fully discussod under the subject of “Earthquakes.”