An ice age would only be one factor to consider ( climatic change), usually societal collapse is based on multiple causes working together. The fact that there are plenty of deserted ancient civilizations located far away from the coastal areas is testament to that. However, if you have a global infrastructure that can no longer be supported for various reasons how long would an inland city be able to maintain the same standard of living and support it's population if dependent on outside resources? I wouldn't think it would be for very long. If stone was not used for buildings then I would think very little would be left to excavate unless it was something suddenly cataclysmic, like Pompeii, that would preserve the signs that buildings were once there. I have this on my "to read" list, maybe you've already read it:
http://www.amazon.com/Collapse-Compl.../dp/052138673X