Sentence:  The 15 th Street deposit is not to be confused with the nearby famous Mayflower Hotel cypress swamp on 17 th Street reported in The Washington Post , August 2 , 1955 , which was probably formed during the second interglacial period and <WSD>is</WSD> therefore much younger . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)