Prior to its insertion into the granite frame, there was likely more of an outer border. Now there is a hint of a thin width, two line border around parts of this stone.
There is a solid two line, thin width border between the abbreviated tympanum and the illustrated top panel. That illustrated panel further intercedes between the and the tablet.
Remark
The design in the tympanum seems like a sun setting on the horizon. With that assumption, the border line becomes the horizon line and the illustrated panel are the waves underneath.