I think the burial accomplishes two things. One, it keeps the dough
ball away from the air and other opportunities for contaminantion
(Dick's famous nose puppies come to mind <g>). Two, it dries the
surface of the doughball rather quickly. This makes surface
contamination less likely since a dry surface doesn't readily support
mold or other nasties.

I have tried it both ways, burying and not burying, and burying works
better. One never sees slime on buried dough pieces.

One of the anecdotal mysteries, and after all what is desem without
mystery? is that one needs to bury the dough ball in 5 pounds of fresh
flour and it must be several inches deep and it must be below 65
degrees, and so forth. My starters have done fine buried in about a cup
of flour. They haven't shown any preference for deep burial, a modest
cover will do. Cool works better than warm. The one key thing I've
found is that the initial hydration of the doughball should be slightly
higher than a regular dough. So something in the 75% range is good.
Dryer makes for a very slow start and the dough ball loses moisture to
the surrounding flour so that must be factored in.

I did come across this interesting factoid in the yahoo group
wholegrain baking:

Jeffrey Sheinberg wrote:

"There are some good tips regarding starter maintenance in Wing &
Scott, be aware of this advice from Michael Ganzle contained
therein...

with greater than 30% inoculum, yeast growth will dominate
over Lactobacteria SF, eventually, the Lactobacteria SF will
die out, to be replaced by Lactobacteria Pontis."

So the relationship of cultured flour to new flour has more bearing
that one might have otherwise assumed.

Will