Mathematician and essayist James Warren looks at the Cornish megalithic stone circle Boscawen-Un, nineteen granite or quartz menhirs arranged in a rough oval, with a single sloping stone in their midst. The monument was possibly laid-out in Chalcolithic times for unknown reasons. James Warren measures the layout from an aerial photograph. The plan is not circular and the author demonstrates that the whole array and indeed any choice of three or more component stones cannot mathematically lie on an arc of a circle. The author shows that the stone circuit is plausibly comprised of four elliptical arcs and that these were likely generated by dragging a taut loop of rope or chain around four posts driven at the corners of a square.