The Collaborative International Dictionary of English v.0.48:

Superincumbent \Su`per*in*cum"bent\, a. [L. superincumbens, p.
   pr. of superincumbere. See Super-, and Incumbent.]
   Lying or resting on something else.
   [1913 Webster]

WordNet (r) 3.0 (2006):

superincumbent
    adj 1: lying or resting on and exerting pressure on something
           else; "superincumbent layers of dead plants cut off the
           air and arrested decomposition"

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "superincumbent":
   beetle, beetle-browed, beetling, bridging, burdensome, cumbersome,
   cumbrous, demanding, exacting, exigent, greater, grievous, higher,
   imbricate, impendent, impending, incumbent, jutting, lapping,
   lowering, lumpish, massive, onerous, oppressive, over, overarched,
   overhanging, overhung, overlapping, overlying, pending, projecting,
   shingled, spanning, taxing, tough, unwieldy, weighty