习题6-4
褶皱构造是由地壳运动的挤压作用，将原始水平平展的岩层挤成一系列的弯曲，分为背斜和向斜。向斜是向下凹的弯曲，而背斜是向上拱的弯曲，因此背斜的位置一定是山，向斜的位置一定是谷（）
The fold structure is the squeezing effect of crustal movement, squeezing the original horizontally flat rock into a series of bends, which are divided into anticlines and synclines. The syncline is a concave downward curve, and the anticline is an upward curved curve, so the position of the anticline must be a mountain, and the position of the syncline must be a valley ()