蓟县北部山区第四纪冰川泥砾层的发现

蓟县北部山区居燕山中段南翼,以我国北方震旦纪地层发育著称。本区地质屡经地质工作者调查研究,唯第四纪地质至今尚少人注意。随着我国社会主义农业的发展,兴修水利工程,特别是山区寻找地下水源,都需要着重解决第四纪地质问题。 1974年春和1975年夏,我们先后两次在本区下营、小港等地进行调查,发现一些丘陵顶部黄土层之下埋藏着一种具有冰川作用痕迹的泥砾层,引起了我们的注意。

ON THE DISCOVERY OF QUATERNARY GLACIAL BOULDER CLAY FROM THE NORTHERN MOUNTAINOUS DISTRICT OF JIXIAN (CHIHSIEN) COUNTY

Around Hsiaying, Hsiokang and other villages in the nothern mountainous district of Jixian (Chihsien) County, to the south of the Great Wall of China extends from northwest to southeast a corridor-like belt of rolling hills, more than 30 km in length, and about 2-3 km in breadth. Along both sides of a river valley there exist terraces of three steps. The first-step terrace, made up of gravels, is five meters above the modern river bed. The second-step terrace is a cut and built terrace, covered with a glacial boulder clay and loess, 15-20 m above the modern river bed. The glacial boulder clay about 5 meter thick, with red, yellow and white spots, is called the mottled boulder clay. The third-step terrace is the bevelled hill tops, 50-60 meters above the modern river bed. On the bevelled hill tops in the piedmont belt remains of glacial boulder clay is buried under the loess with ferriginous concretions. The clay, a little more than 5 meters thick, reddish brown in colour, consolidated, is called reddish-brown boulder clay.Tracing along some water channels from the rolling hills up to the northern mountains, we may find the valleys of the channels toward the sourses take on the shape of a glacial trough and the relic of a glacial amphitheater. The bottoms of both the glacial troughs and the glacial amphitheaters are coverd with yellwish-brown boulder clay, locally attaining more than 10 meters thick.The difference in both the degree of consolidation and weathering colour among the three kinds of glacial boulder clay reflects the different periods of their accumulation, representing three ice ages in this district. The reddish-brown boulder clay represents the deposit of the earliest ice age: the mottled boulder clay deposit of the second ice age, while the yellowish-brown one the latest.The remains of the former two glacial boulder clays occur on the rolling hills, while the last one exists only in the valleys near the source up in the mountain. Evidently, the two glaciers appearing earlier are comparatively large, while the one appearing latest is small.