Sentence:  The <WSD>Fucoid</WSD> beds are quite variable in appearance in composition ... The rock unit got its name from the early belief that these markings were the compressed remains of seaweeds (fucoids). Now it is thought that they are the trails left by feeding creatures on the sea floor. 
Meaning of the word marked with <WSD> tags:  Of sandstone: bearing seaweed-like markings.