SUPERIMPOSITION

"An easily erodable formation, alluvial, or colluvial deposits (hillslope material)
 that could have acted as a covermass"

  Covermasses are critical evidence in support of superimposition and consist of alluvium, colluvium, or an easily erodable bedrock layer.  However, even though they must cover the bedrock high, often times they quickly erode away because they're easily erodable.  For this reason, I don't know of a natural example for this criterion where the covermass is not gravels related to the transverse drainage but some weaker formation or colluvial deposit.  The image below illustrates the striking Black Creek Canyon transverse gorge, which possibly developed through superimposition.  The covermass would have been the weak clay rich Chinle Formation, but this hypothesis is highly speculative.  Willow Creek Canyon in the lower image provides an example of a superimposed transverse drainage that utilized gravels or alluvium as a covermass to cross the bedrock high.