The National Geographic Magazine, Vol. III. (Complete), 1891

by Various

YAKUTAT SYSTEM.

The rocks of this system are of gray and brown sandstones and nearly black shales. They are uniform in lithological character over a large area, and are usually greatly crushed and seamed. So great has been the crushing to which they have been subjected that it is difficult to work out a hand specimen with fresh surfaces. Fragments broken out with a hammer are almost invariably bounded by plains of previous crushing, and are usually somewhat weathered.

These rocks form the bold shores of Yakutat and Disenchantment bays, and were the only rocks seen along our route from Yakutat bay to Pinnacle pass. The whole of the Hitchcock range is composed of rocks of this series, as are also the Chaix hills and the hills west of Icy bay and the southern portion of the Samovar hills. North of Pinnacle pass there are rocks undistinguishable lithogically from those about Yakutat bay. These are exposed in Mount Owen and on each side of Dome pass; they also form the bold spurs about the immediate bases of Mount Augusta, Mount Malaspina, and Mount St. Elias. In the three instances last named these rocks dip beneath the schist forming the crest of the St. Elias range, and it is probable that a great overthrust there took place before the formation of the faults to which the present relief of the mountains is due.

All the mountain spurs of Mount Cook, so far as is known, are composed of sandstones and shales of the Yakutat series, with the exception of the Pinnacle pass cliffs. Nearly all the débris on the glaciers from Disenchantment bay to the Seward glacier, and probably beyond, is derived from the rocks of this system. The distribution of the rocks from which the débris was derived may be ascertained in a general way by tracing out the sources of the glaciers. Medial moraines on the Hayden and Marvine glaciers, however, have their sources on the northern slope of Mount Cook, and are composed of gabbro and serpentine. These rocks were not seen in place, and their relation to the Yakutat series can only be conjectured.

Although the rocks of this system are stratified, it is impossible to determine their thickness, for the reason that they have been greatly crushed and overthrust. This is well illustrated in the Hitchcock range, which, as already explained, trends about northeast and southwest, and is composed of strata of shale and sandstone, having a nearly east-and-west strike and a uniform dip toward the northeast. Were the rocks in normal position their thickness would be incredible. In addition to this negative evidence, there is the crushed condition of the strata to show that movement has taken place all through their mass; and in a few instances thrust faults were distinguished, dipping northeastward at about the same angle as the lines of bedding. In the crushing to which the rocks have been subjected the shales have suffered more than the sandstones, and have been drawn out into wedge-shaped masses, the sharp edges of which usually point toward the northeast, which is presumably the direction from which the crushing force acted.

The hypothesis that the rocks in the St. Elias region have been crushed and overthrust explains many otherwise inharmonious facts, and accounts for the superposition of the St. Elias schist upon rocks of the Yakutat system.

Coal has been discovered in the rocks of the Yakutat system about two miles west of the southern end of Disenchantment bay, and is reported to be of workable thickness. I saw thin lignite seams at the surface at this locality, but as the shafts were filled with water I was unable to examine the coal in the openings, and cannot vouch for its thickness. Samples obtained from the mine show it to be a black lignite which would apparently be of value for fuel. Fossil leaves are reported to occur in connection with the lignite, but these have never been seen by any one who could identify them.

The rocks of the Yakutat system, wherever seen, dip northeastward, except when greatly disturbed near fault-lines. East of Disenchantment bay the inclination of the beds is from 15° to 20°; farther westward the dip increases gradually all the way to the Hitchcock range, where the prevailing inclination is from 30° to 40°, and frequently still greater. Beneath Mount Malaspina and Mount St. Elias the Yakutat sandstones dip northeastward at an angle of about 15°, and in the hills west of Icy bay the dip is about the same. Exceptions to the prevailing dips occur along the immediate shore of Yakutat bay, northwest of Knight island, and at the southern extremity of each of the mountain spurs between Yakutat bay and Blossom island. At these localities the rocks are frequently vertical or nearly so, owing their high dip to the proximity of lines of displacement. The faults indicated by these unusual dips also mark the boundary between the mountains and the seaward-stretching plateau of alluvium and ice.

The crushing, overthrusting and faulting that has affected the rocks of this system render it doubtful whether the coal seams which occur in it, even if of requisite thickness, can be worked to advantage. Some of the samples of coal obtained at the openings made near Yakutat bay were slickensided, showing that movements in the coal seam had there taken place.

As already stated, the rocks of the Yakutat series are remarkably uniform in character throughout the extent now known, and offer but little variety. The sandstones are intersected in every direction by thin quartz seams, which stand in relief on the weathered surfaces, giving the rocks a peculiar and characteristic appearance. The first important change in the geology along the route traversed by us was met on reaching Pinnacle pass.