Sentence:  <WSD>Tarns</WSD> are found in some of the vales, and are numerous upon the mountains. A <WSD>Tarn</WSD>, in a "Vale", implies, for the most part, that the bed of the vale is not happily formed; that the water of the brooks can neither wholly escape, nor diffuse itself over a large area. Accordingly, in such situtions, <WSD>Tarns</WSD> are often surrounded by an unsightly tract of boggy ground; but this is not always the case, ... 
Meaning of the word marked with <WSD> tags:  A small mountain lake, "especially" in Northern England. (from late 14th c.)