George Orwell's grave: coefficient of friction

When we were last at Crinan, we were able to look out over the water to the remote farmhouse on Jura where George Orwell wrote 1984. Within a year he was dead. It turned out we were staying in Oxfordshire near his grave. He wanted to be buried in an English country churchyard. I wonder what he'd make of the coins left by other visitors. Notice that as the edge of the grave stone gets steeper, the coins won't stay. Friction isn't strong enough to resist the component of weight pulling down the slope. Friction F is directly proportional to normal contact force N. The constant of proportionality is called the coefficient of friction mu.

 Above is the force diagram. Three forces act on a coin. I take components of the weight mg.

At the point at which the coin is just about to move, the perpendicular components must be equal and opposite. The same is true of the parallel components. So I can make the substitution shown at the bottom. Lastly, coefficient of friction = tan(theta). The angle at which the coins come off is about 30 degrees so the coefficient of friction for stone and metal is thus about 0.6.