"My Hope Is Built on Nothing Less"

A number of years ago, I noticed a small feature in "My Hope Is Built on Nothing Less."  The hymn is paired with two melodies in The Lutheran Service Book, "Magdalen" (#575) and "The Solid Rock" (#576), but what I noticed is specific to "Magdalen."

The refrain is "On Christ, the solid rock, I stand; / All other ground is sinking sand," sung to these musical phrases:

The line "All other ground is sinking sand" is sung to a descending melody (spanning an octave: C to C), musically giving a sense of that "sinking."