Chapter XVI: Part 4: The Geology of the Country Traversed by the Peking-Kalgan Railway

From Peking to Nan-k’ou the railway traverses part of the Chihli plain. As one approaches Nan-k’ou, low limestone hills, outliers from the main range, can be seen. In these the very marked strata dip at an angle of about 45° to the south-east. The same thing may be seen in the lower slopes of the main range. The strata of the higher levels, however, are horizontal, and, though the actual contact cannot be clearly made out from the train, suggest the existence of two unconformable formations of limestone. This is the case, the sharply dipping strata of the lower slopes and outliers having been identified as belonging to an older formation called the Nankou series, which finds its equivalent in Shansi in the Hu-t’o series already mentioned.

Ascending the valley, at the mouth of which Nan-k’ou is situated, one soon reaches a level above the Nankou limestone, and finds the Ta-yang limestone (Sinian), the equivalent of the Ki-ch’ou limestone in Shansi, lying in horizontal strata. Further on up the valley these commence to dip to the north-west, and are interbedded with blue clays and shales. Finally the strata become contorted, and are replaced by plutonic rocks, consisting chiefly of light buff-coloured granite. This formation continues to well beyond the crest of the ridge, till the mountains give place to a wide plain—the valley of the Ta-sha Ho, which joins the Ta-yang Ho a little to the south.

Crossing this plain in a north-westerly direction, a second range is encountered, which runs from east-north-east to west-south-west, and consists, as far as can be judged from the train, of limestone formation (probably Ta-yang limestone).

Near Hsia-hua-yüan, where the Ta-yang Ho cuts through this range in a narrow and deep valley, the sedimentary rocks become rather complicated. Coal-bearing sandstone and shale strata lie underneath the limestone, an unconformity between the two formations being distinctly discernible. This can only be explained by the existence of an overfold or overthrust. The coal-bearing strata are more or less horizontal and somewhat folded, while the limestone shows perpendicular or very sharply dipping strata.

North-west of Hsia-hua-yüan, for a considerable distance, horizontal strata of purple shale and sandstone occur. These next give place to limestone, which extends to within six miles of Kalgan, when it is replaced by the volcanic rocks of the Kalgan district. In our rambles through the hills round this town we found porphyry, chiefly of a brown or brick-red colour, and other volcanic rock in great confusion. Specimens of serpentine and metamorphosed felspathic rocks were also obtained. To the north-west, basalt is exposed in the deep cutting of the stream bed, which ends in the Han-nor-pa pass. Further up this valley extensive beds of volcanic ash occur, which at first sight might be mistaken for the sedimentary rocks of the Shansi formations. The deposits are of a bright red colour, and in places assume the form of conglomerates, with large well-rounded boulders. This suggests that these beds were laid under water. Nearer the pass, these deposits are much lighter in colour, being pink or buff and light blue-grey. In places the original substance has degenerated into soft clay, while, wherever the strata assume the form of conglomerate, the fragments of harder rock are rough-edged and not rounded as they are nearer Kalgan. Beyond these beds basalt occurs right up to and beyond the Han-nor-pa pass.