Chapter XV: Part 3: The Geology of the Upper Fên Ho Basin, Western Shansi

The geology of the country lying north-west of the T’ai-yüan Fu plain, owing to the extensive fault running down its north-western margin, is in many ways a repetition of that of the country traversed by the Chên-T’ai Railway. A new feature, however, is presented in the form of extensive outcrops of plutonic and metamorphic rocks.

Hitherto this country has scarcely been touched by any geologist; so that the following notes, though far from complete, may be considered as breaking new ground. They were gathered on the trip described in the second chapter of this book.

Let us take a line running north-west from T’ai-yüan Fu, through Tsing-lo Hsien, Wu-chia-kou, Tien-p’ing to Wu-tsai Hsien, and consider in section the different formations.

First after crossing ten or twelve miles of alluvial plain, loess and stony river bed, we encounter the extensive escarpment of Ki-ch’ou limestone, caused by the fault running north-east to south-west. Entering a deep gorge at Kuan-k’ou, we note that the strata at first are horizontal, though slightly folded; but that further on they dip slightly in a general north-westerly direction. Continuing along the bottom of the ravine, which rises steadily through the limestone, Pei-hsiao-tien, ten miles from Kuan-k’ou, is reached. This is a small town situated at an altitude of 4,570 feet in an open valley in the Shansi coal measures, where bituminous coal of the usual quality is mined. We have thus ascended roughly 2,000 feet, through the Sinian limestone to the Shansi formation.

On leaving Pei-hsiao-tien the road continues north-westward, ascending a narrow valley. Here metamorphic rocks, consisting chiefly of schist, with veins of quartz running through it, abruptly replace the sedimentary rocks. Ascending the steep pass, first a broad vein of dark basalt containing a high percentage of iron is encountered: subsequently two high ridges formed by a massive dyke of granite are crossed.

The metamorphic rocks, which are not very extensive, may owe their existence to the granite, as, judging from the stratigraphy of the district, it seems probable that the granite is an intrusion of later date than the limestone and coal measures.

On the western descent the outcrops show the same rocks as on the east (i.e. granite and schist). These, however, are soon replaced by Ki-ch’ou limestone of an equal height with that east of the passes, with a slight north-westerly dip. As may be expected, this gradually gives place to the carboniferous series (Shansi formation) once more. Soon the Fên Ho is reached (at Tsing-lo Hsien), crossing the valley of which, we still find the sandstone and shales of the Shansi formation. The strata here, however, are now horizontal for a short distance, subsequently dipping more and more sharply to the south-east, till they are almost perpendicular. They are suddenly replaced by almost perpendicular strata of limestone (Ki-ch’ou). These form a high ridge, with a dip slope on the south-south-east of at least 80°, and a very steep escarpment on the north. This clearly indicates the existence of a great synclinal fold in the sedimentary rocks, extending from the north-west of the granite dyke, near Pei-hsiao-tien to this point.

Wu-chia-kou lies in the valley to the north of this ridge of limestone. It must be noted that our section crosses this ridge somewhat obliquely. The northern slope of this valley is of red sandstone belonging to the Man-t’o Shales, the strata of which lie unconformably against schists of the Wu-t’ai series, and also dip sharply to the south-east. The rocks of the Wu-T’ai series are soon replaced by a great mass of basal complex, probably equivalent to the T’ai-shan complex. The outcrops of granite here are very extensive, and form the mass of the mountain chain, which rises to a great height (10,000 feet), and is about five miles wide.

On the north-western slope of this massive range of archaic rock, limestone, similar to that on the east, lies unconformably against the older systems; but, dipping sharply to the north-west, it is soon replaced by sandstone of the Shansi formation, also dipping sharply to the north-west and merging into the Wu-tsai plain.

The occurrence and dip of the sedimentary rocks on this side of the range, suggest that the strata lie in the form of an immense anticline, the apex of which has been denuded, exposing the archaic formations beneath. On the north-western side of the range the strata flatten out and probably continue horizontally up to and beyond the Yellow River. I did not notice any sign of the Hu-t’o series, but doubtless they exist.

North-east from Wu-chia-kou the geology of the country immediately adjoining the Ning-wu Fu district, as already stated in my geological notes in _Through Shên-Kan_, shows a series of shale and sandstone ridges (Shansi formation), the strata of which dip to the east at angles varying from 30° to 80°. The ridges are formed by escarpments on the west and dip slopes on the east. It is in some of the higher and more gentle hollows, caused by this configuration, that lie the fresh-water lakes, mentioned in Chapter II of this book as having been newly discovered in January of 1910.

Here again the Shansi formation is replaced westward by the Ki-ch’ou formation (limestone), the strata of which dip to the east at very sharp angles. The outcrop of archaic rock, which has extended unbroken north-east from the Wu-chia-kou district, has tapered down, disappearing altogether before Ning-wu Fu is reached.