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题目材料：
Contradicting earlier notions that mud glyphs--etchings made in wet clay on cave surfaces--represented the dominant form of cave art in the southeastern United States, discoveries of additional sites have revealed at least three forms of cave decoration. Pictographs, produced by adding pigments to cave walls, while the rarest form of cave art in the Southeast, are most common in Missouri. Because they involve mineral pigments on rock surfaces, pictographs are as fragile as mud glyphs, or more fragile, where cave walls are unstable. Petroglyphs, which are carved directly into cave walls and ceilings, are most common: nearly two-thirds of known sites contain petroglyphs. Because petroglyphs were engraved directly into rock, they are less fragile than mud glyphs.
以上解析由 考满分老师提供。
