How You Can Easily Make A Very Simple, Inexpensive, Effective Trombe Passive Solar Wall From Stones And Wire...

The Trombe wall is a passive solar building technique where a wall is built on the winter sun side of a building with a glass external layer and a high heat capacity internal layer separated by a layer of air. Heat passes through the glass almost unhindered then is absorbed by the wall that then re-radiates and doesn't  pass back through the glass, hence heating the inside of the building. Trombe walls are commonly used to absorb heat during sunlit hours of winter then slowly release the heat overnight.
It was developed as an architectural element by the French engineer Felix Trombe with architect Jacques Michel in the 1960's.
All you need to do is site the wall, make up a timber frame, use a wire mesh on either side of the frame and start filling in layers so you're not dropping the beautiful river stones from a height.
The darker the river stones the better as they'll absorb more heat.
You're sure to figure it out...