This is the Phallus of Shiva, the male principle and is cylindrical, oval shaped and an upright piece of rock in wild nature.
What is Lingam?
This is the most ubiquitous symbol of Shiva ’the dark god of dark people and is pre Vedic and is regarded not simply as a symbol of the male sexual organ or of fertility and reproduction. But is also as sacred as the essential shape of the deity himself, it does not merely imply but directly express the presence of the divine. It proclaims Shiva as the supreme force of the universe.
Basically the lingam stood for the formlessness of nature, later for clear consciousness and much later for the masculine energy. It is venerated often alone with a dash of sindoor i.e. vermillion yellow white powder on the top, the symbol of blood, an atavism of animal sacrifice practiced in earlier times. As a ritual object, it is located in the sanctum sanctorum, accessible only by truly devout or the priest.