Sentence:  <!--One of [the] defining characteristics of [the northern Horn of Africa] is its separateness, and this is partly due to its physical diversity. A detailed description of physical geography falls outside the scope of this book. It may simply be noted that the region comprises highlands that are bounded on the east by the precipitous escarpment bordering the Danakil lowlands and the Red Sea.--> To the west, the country descends more gradually to the extensive plains of the Nile Valley but is <WSD>riven</WSD> by the rugged valleys of the Takezze and other Nile tributaries. 
Meaning of the word marked with <WSD> tags:  To break apart; to split.