Sentence:  They <WSD>are</WSD> huge areas which have been swept by winds for so many centuries that there is no soil left , but only deep bare ridges fifty or sixty yards apart with ravines between them thirty or forty feet deep and the only thing that moves is a scuttling layer of sand . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)