Mountains of material? Why not "Mountains of materials"?

Context:

WHAT IS THEISTIC EVOLUTION?
Mountains of material, in fact entire library shelves, are devoted
to the topics of Darwinian evolution, creationism, and Intelli-
gent Design. Yet few scientists or believers are familiar with the
term "theistic evolution," sometimes abbreviated "TE." By the
now standard criterion of Google search engine entries, there is
only one mention of theistic evolution for every ten about cre-
ationism and every 140 about Intelligent Design.