The Grove of Ashtaroth
Superficially (actually, more than superficially) this story bears a resemblance to the work of Algernon Blackwood. It is about the effect of nature - or something beyond nature - on a particularly sensitive or susceptible person. Unsurprisingly, it shows its age in some of the attitudes it expresses, as well as what can - in charity to Buchan - be taken as out-of-date beliefs about the non-African origins of the iron age antiquities of Great Zimbabwe. Unfortunately the whole rationale of the story depends on the tower at the centre of the titular grove (explicitly linked in the story to Great Zimbabwe) being of Phoenician or otherwise ancient Semitic origin.