Page:Popular Science Monthly Volume 53.djvu/608

588 Although the geological water way was open during this period (more recent than has generally been suggested), it was afterward closed for a time, and the subsequent connection between the Gulf of Mexico and the Pacific is the most interesting to us, as reaching down almost to the historic period. The date of the older strait was prior to the Lafayette epoch (about the close of the Pliocene or commencement of the Pleistocene period), while that of the later one obtained well on in the Pleistocene period. The separation of the seas was in part effected by gentle warpings of the district, but the low Cordilleras seem to have been further squeezed upward by a movement referred to on a preceding page. Out of the floor of the present divide, the rocky islands of the recent strait became prominent knobs (such as one shown in Fig. 9). The newly formed isthmus was only a mile or two across. Its once nearly level floor, composed of soft, earthy sandstone, has since been rounded into a succession of hummock-like hills by the subsequent action of rains and rills. This feature is shown in Pigs. 8 and 9. For a time the narrow isthmus was penetrated by a geological canal, one hundred and fifty feet deep and less than a quarter of a mile wide, the features of which



are now perfectly preserved in the channel, about a mile long, which dissects the hummocky divide of the Tehuantepec Isthmus. The northern end of this canal is shown in Fig. 9. The floor of this passageway is still covered with water-worn gravel, which was deposited by the former currents. Connected with this late channel and at the same level, gravel terraces mark the shores of the Gulf