On exposed cliff faces, you will often see that the rocks are arranged in layers, rather like a sliced loaf placed on end. The layers, or strata, may be level, tiled or bent into rounded wrinkles, or folds. Folds are caused by sideways pressure from colliding plates in the Earth’s crust. The tremendous pressure created by sideways moving plates can squeeze otherwise level rock strata into gigantic folds which form lofty mountain ranges. The world’s highest ranges, including the Himalayas where the rock layers have been compressed by as much as 650 km (about 400miles) are fold mountains. Upfolds are called anticlines and downfolds are synclines.