Jane,
 
Machine trenching once you found the features? I recommend against that  
technique because it is so destructive it would render the collection valueless  
due to breakage and mixing. Why not construct a single-wall building shell with 
 a corrugated steel roof to protect the site and excavate, then move the 
shell  over the next location. This would give you security and allow for 
controlled  archaeology methods. Another idea would be to seal the features you have 
exposed  to vandalism with temporary protective fill and perhaps post signs 
warning of  deadly chemicals or "flesh-eating germs" to keep the bottle hunters 
away.  Although I am sure others in the room would disagree, I think mechanical 
 trenching is only to be used to find features and not to do the actual 
digging.  Recordation of the context is everything in archaeology.
 
Ron May
Legacy 106, Inc.