Liz-- This is probably too late to help, but when I was working in
Williamsburg in similar conditions I would more otr less keep the site
covered with plastic at all times except parts actively being excavated. Used
tires, available for free at your local garage, are the ideal weighting
device!  As far as preserving the profile, the only really succesful example
I have seen is at Historic Halifax, NC. There they built a "shadow structure"
over an excavated 18th century foundation. You might contact Linda
Carnes-McNaughton for further info. E-mail her at:
[log in to unmask]
 
Good luck, Carl Steen