This is quite simple to do; you just sift through the demolition rubble and soil and pick out the cobbles and then pile them up out of the way. The only trouble is that Alison found what she insists is a big cobble, I think it is a boulder and is about the size of a stone ball from a medieval siege engine catapult and probably weighs about a fifth of a ton. When Alison found this “super cobble” it was almost buried in the foundation trenches where the concrete foundations were due to go. It took me two evenings to dig it out and then roll it to the safety of the cobble pile before the cement arrived. It's not unusual to see me wandering around and suddenly stop to rescue up a lone cobble poking out of the soil and rubble.