The plain circa 2540. (TV: Planet of the Daleks)
The Plain of Stones was a rock formation on Spiridon.
The stones absorbed heat in the day and released it at night. Many creatures often congregated near the formation. The Third Doctor, Jo Grant and a Thal party spent a night at the formation. The Spiridon slaves reported this to the Daleks. (TV: Planet of the Daleks)