[logback-user] logback-classic and Tomcat

I'm using Logback Classic pretty much everywhere in my code to log lots of things (mainly exceptions and debug messages). I do *not* use Logback Access at all. I develop this webapp locally using an embedded Jetty instance, and the logging works fine: all logging messages are written out to the Eclipse console as expected. But on production we're using Tomcat 6.0.18 (installed on Ubuntu 8.10 using apt) and I can't find any log files with logback messages anywhere. I'm sure I just have some settings wrong somewhere - can anyone tell me how to configure logback *classic* (not access) to actually write out logging messages to a log file under Tomcat 6.0.18? Thanks, Zach