Reason for Search:
One of my Christmas presents this year was a pair of Clarks Desert Boots. The rubber on the bottom seemed strange and the tag said “Crepe Sole.” So I searched.
Conclusions:
Crepe rubber is a crude form of rubber that can be cheaply manufactured into sheets (hence no tread). It is also a natural rubber, meaning that it is made primarily from a milky latex substance produced by rubber trees.
Opinions seem to be that they are not as durable as synthetic rubber soles but that they are more comfortable. Added bonus, they don’t track mud because they don’t have tread.
Useful Search Results:
http://www.tis-gdv.de/tis_e/ware/kautschuk/naturkautschuk/naturkautschuk.htm