I see the black one, that appears to be a monument or head stone of some kind in the back ground.
As for the gray/tan mass just behind the stacked cannon balls, I enlarged the picture as far as I could without matrixing it out.
IMO, it appears to be a part of the tree in the background. It appears "disconnected" due to the cannonballs in the forefront, but if you follow the angle of the tree and the shading, you'll see that the trunk is partly showing just to the right of the top of the cannonballs... with a possible old limb that was cut off long ago as it appears weathered and rounded...
I don't see anything else though. Looks like a great picture for the album for that visit.
Have a great day!