He Ling, Ma Tao, Rolf Snethlage, and Eberhard Wendler
This article describes part of an interdisciplinary, joint
Chinese-German project on the preservation of the Dafosi grotto. It focuses on the properties of the sandstone, the influences of moisture and salts on the deterioration of the walls, and preliminary results on the conservation of the stone site. Aspects of Dafosi's art history, polychromy, stability, and rock mechanics are discussed elsewhere in this volume.
The following general description of the topography and geology of the area is based on the 80,000 Signs Report: The Study and Survey of the Environmental and Geological Situation and Deterioration of Dafosi Cave in Bin Xian (1991). Dafosi is situated in the loess plain in the northern part of Shaanxi Province, near the city of Binxian. The landscape is characterized by the broad valley of the Jing River, which cuts into the yellow earth and the underlying red Cretaceous sandstones. The grotto itself is excavated into the red sandstone of the valley flanks (Fig. 1). The stone floor of the grotto is covered by flood sediments of approximately 2 m thickness, which have been produced by periodic flooding of the Jing River throughout its history.
The sandstone is overlaid by loess layers of the Quaternary period (early, middle, and late Q3). The geological profile shows a horizontal layering of the strata with 180 m vertical distance between the base of the valley and the top of the hill. The height of the sandstone rock is 55 m. Typical for sandstone in general, clay-stone layers of various thicknesses are interspersed between the sandstone rock series. These layers form impermeable barriers against the water that flows from the top of the hill along the fissures as well as through the rock itself.
One of these clay-stone layers intersects the Dafosi grotto about 2 m above the present ground level, with an intersection length of 65 m. The water that seeps out of this horizon layer trickles into the sediment layers covering the floor. This seepage water is the main source of deterioration of the statues and the walls inside the grotto. The sandstone is particularly damaged along the clay-stone horizon. Behind the Great Buddha in the grotto, the sandstone is eroded back to 2.5 m from its original surface.
Figure 1
View of the Dafosi grotto.
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