The “round mound” or cairn is one of the simplest megalith type –consisting of a rounded heap of earth or rubble, usually marking a burial spot. As the size of the cairn grows it needs to be contained using various structural techniques such as containment using large boulders as bounding circle, stone slabs embedded in the earth around in a circle etc. Using examples from various megalithic sites in the subcontinent, an attempt is made to understand a possible evolutionary sequence of the Buddhist stupa using the structural necessities of containment. A unique megalith at Mallasandram, near Krishnagiri, which represents a “proto-stupa” or an intermediate type betweena dolmen and a stupa, is discussed. It is stressed that this is not presented as a linear evolutionary sequence, but rather as how the evolved form of the stupa derives from the structural requirement to build higher and more extensive mounds