Creating composite factor and changing format from character to factor

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Dear Help-list, I have a dataframe containing 6 variables, 4 of  
which are factors, 2 numeric.  I want to create another factor  
variable (SitePos) by combining 2 existing factors (Site and  
Position).  I have tried a number of approaches based on trolling  
the R FAQs, various R webpages, etc., none of which work.  One  
approach e.g. Data1$SitePos <- paste(Data1$Site, Data1$Position)  
creates the appropriate "SitePos" values e.g. "CR core" but as  
character values not as a factor.
In one line you could have written:

  Data1$SitePos <- factor( paste(Data1$Site, Data1$Position) )

Or you could have created an interaction:

Data1$SitePos <- interaction(Data1$Site, Data1$Position)
 A linear model run on the updated dataframe works but notes that it  
coerces "SitePos" from character to factor e.g. Model.G =  
lmer(log10(SrCa) ~ SitePos + (1 | Eel), data = Data1) .  The next  
step of a multiple comparison test on the output of the linear  
model: Model.G.mct = glht(Model.G, linfct = mcp(SitePos = "Tukey"))  
fails because the mct does not recognize "SitePos" as a factor and  
gives error message: "Error in mcp2matrix (model, linfct = linfct):  
Variable(s) 'SitePos' of class 'character' is/are n!
ot contained as a factor in 'model'.  My final step is outputting  
the mct results:  summary (Model.G.mct, test = adjusted(type =  
"single-step")).  Any suggestions as to how to create a composite  
factor variable in the dataframe that would permit execution of the  
analysis code would be much appreciated.  I am using R version  
2.13.0 with packages lme4 and multcomp loaded.  Regards,B Jessop

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