Source : Webster's Revised Unabridged Dictionary (1913)

Superincumbent \Su`per*in*cum"bent\, a. [L. superincumbens, p.
   pr. of superincumbere. See {Super-}, and {Incumbent}.]
   Lying or resting on something else.

Source : WordNet®

superincumbent
     adj : lying or resting on and exerting pressure on something else;
           "superincumbent layers of dead plants cut off the air
           and arrested decomposition"