Sentence:  ...in porous beds, which are known as reservoir beds, which are usually sandstones or porous limestones, trapped there usually, not always, under <WSD>bifolds</WSD> in the rocks or arches in the rocks known as anteclines, and nearly all oil, not all oil, but nearly all oil wells are found where such reservoir beds are drilled into on the top of such anteclines. 
Meaning of the word marked with <WSD> tags:  A crease or turn that causes something to double back on itself.