From my stat/sed prof.... 
"Slot canyons don't seem to be a speciality of anyone around WWU,  
and I 
don't know of any web sites specifically covering them or how they 
form.  (There are some pretty pictures out there, though.)  I do know  
that 
they aren't restricted to sandstones, since I've seen them in limestone 
and Doug  remembers one in marble in Death Valley.  In order to  
form 
a slot canyon, there must be rock that can be eroded but that is strong 
enough to hold near-vertical slopes.  It must also be relatively 
homogenous, with no weak strata along which mass-wasting can cause  
slope 
failures.  To the best of my knowledge, they are characteristic of arid 
regions, as a guess, that could be because running water is restricted 
rather than there being a lot of overland flow." 
  
took her almost a month to say this....geologists are such slackers!