Complex sentence: They propose that the incised symbols on the block represent the contents of three sacred bundles arranged broadly horizontally, to be read in boustrophon fashion from top to bottom and alternatively left to right and right to left.
Split into simpler sentences: They propose that the incised symbols on the block represent the contents of three'sacred bundles' arranged in three separate registers from top to bottom. The whole block is to be read in boustrophedon fashion from top to bottom and alternatively left to right and right to left.