Complex sentence: The scene on the face of the medal is derived from Randolph Caldecott's illustrations for "The Diverting Story of John Gilpin" where Gilpin is astride a runaway horse, which in turn was based on a poem from 1782 by William Cowper.
Split into simpler sentences: The scene on the face of the medal is derived from Randolph Caldecott's illustrations for "The Diverting Story of John Gilpin", in which Gilpin is astride a runaway horse. The story, in turn, was based on a poem from 1782 by William Cowper.