Nice texture!! I can see why you'd want to try and do that!
First off - I am not a wet felter so I do not "know" how to create that texture, but I could take a logical guess or two:
In the first image you posted the cobbles all appear to me to be from the same original layer of fiber which makes me think that the piece was partially felted and then the cobbles were cut (not cutting through all layers) and the cobbles were given some individual attention to smooth their edges before continuing to felt the whole piece.
The first piece might also be achieved by laying out the foundation fibers. Laying on top of that a plastic web or plastic that has had holes cut into it that are large enough to form a good bond between the foundation and the cobble layers and then laying out the cobble layer fiber. Wet felting, cutting the cobbles and continuing to wet felt the whole. The plastic would act as a resist preventing the edges of the cobbles from felting to the underlying foundation.
In the second piece I suspect that the cobbles were made separately and then added to the overall piece. They did not appear to be as dimensional as the cobbles in the first piece.
As a needle felter - I would create the bag on a larger than desired foam pillow. I would lay out the foundation fibers, tacking them in place and then creating and adding the cobbles. The whole could then be wet felted to really bond the layers together. The fibers won't significantly felt to the foam pillow so you can make a cut, use your hands to free the fiber from the foam pillow - remove the pilllow and then continue wet felting if desired.
I created this rock by a combination of needle and wet felting which allowed me great control over what colors went where.
Hope that will give you some ideas on how you might create your cobble bag!! =^..^=