I don't guess. Bagginsbill and I reserached this issue thouroughly about two years ago. I documented the findings, http://www.book.artbeeweb.nl/?p=632 . It;s somewhere in this forum too, in some very lengthy Cloth Room for Dummies threat.

Note that this page is in the middle of the "how to get your PhD in Cloth Simulation" so you might need some asperine as well :-).
Also note that this part on Cloth Dynamics starts at http://www.book.artbeeweb.nl/?p=594, you can download the entire PDF from there, the section mentioned is on page 70.

In short: this kind of cloth behavior, ridging and crumbling, happens especially in quad meshes. Not using quads resolves the issue, smaller quads make smaller ridges. And... 

"Increasing shear and/or fold resistance will push the crumbles out. But high shear and fold resistance will affect other behavior of the cloth as well, life is a compromise. Reducing mass (density), friction and other effects will make this pushing more successful. So instead of raising fold/shear resistance, reducing density is an option. And as shear and fold resistance work together in this case, you can do with less fold and more shear."

Notes on other posts:

 - perhaps not very intuitive, but higher resolutions make finer, thinner, more flexible cloth. Low res makes coarse, thick cloth behavior. Lowering the resolution will not make less ridges, but perhaps might make them less apparent.

 - Min Shadow Bias addresses the shadowing on displacement mapped surfaces, and e.g. prevent skin (brick, etc) pores to make shadows. Has nothing to do with cloth behavior. Sorry. Raytrace Bias (in the reflect node) does a similar thing for reflections on a displacement mapped surface.

All the best.

- - - - - 

Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though