Narmadeshwar Shivling is found in River Narmada in Maharashtra and Gujrat region. It is believed where there is Narmadeshwar , Lord Shiva himself resides there It is never in the same shape. Historically it’s dates back to centuries and the formation of this Shiva Linga is very unique in its own right.
When the waves of Narmada River hits the stones in the river then it changes into oval shape creating the Shivlinga. This Process takes thousands of year. It is also known as Banaling.