We have built 4x4 retaining walls around existing rocks and concrete that we were unable to get rid of. The idea was - and is - use something fill the gaps between the lumber and the rocks / cements. Given that some of the gaps are over 2 inches, what is the best - and easiest - way to fill these gaps?
I would use mortar , if you wanted to get fancy you could add pigment and match the color of the rocks. The one in the corner looks to have some color. I have not done this exactly but have seen it done at first I thought he had chiseled the rocks but then I realized he matched the rock color and filled with mortar.