OCR Text |
Show BASALT FIELDS OF THE MAEKAGUNT. 201 the tributary ravines of Mammoth Creek (the main fork of the Sevier River), and reach a point about 6 miles from their origin. Besides this field of very recent basalt, remains of much more ancient basalt are found in the vicinity and in much larger amount. In truth, the basaltic eruptions go back to a period sufficiently remote to have permitted important changes in the configuration of the country to take place in the interval separating the present from the earliest eruptions of this class. During that interval a considerable number of outbreaks, separated by many centuries (probably hundreds of centuries), have occurred. Basalt fields of different ages are readily distinguished. Among the oldest, probably, are the first basalts spoken of in this chapter. Of an antiquity which may be quite as great are two large masses, lying respectively southeast and southwest of Panquitch Lake. The southwest field is much eroded, and consists of a tabular mountainous mass immediately overlooking the very recent basalt field just spoken of. The edges of the sheets composing this tabular mass project in bold cliffs around its fiat summit in the same manner as is frequently seen in lower regions, where buttes of sedimentary rocks owe their origin and preservation to a protecting mantle of lava. On all sides it is girt about by a talus of blocks, which have fallen by the sapping of the foundations of the mass through untold ages. Since this lava was disgorged broad valleys and deep ravines have been scored in the platform of the Markagunt, and the minor details of topography arising from the general process of surface sculpture have been carved out, and an older topography has been swept away or so completely remodeled that it cannot now be reconstructed. Southeast of the lake a wide expanse of country has been covered with ancient basalt, but only remnants are now left, covering mesas and buttes of sedimentary rocks and overlying fields of still older trachytes and volcanic conglomerates. Ravines of considerable magnitude and broad valleys have been cut into the country which they once covered, and these excavations have in several instances given passage to more recent floods of basalt, some of which extend as far east as the Sevier River. These later basalt fields are in an excellent state of preservation, but soil has accumulated upon them, and the face of the rocks shows deep weathering. |