That anyone can imagine that the presence of civilization will not be readily
detectable 65 Myr from now boggles one mind.
It depends where one looks, with what technology, and with what
preconceptions.
I doubt seriously that our civilization could have detected and
accepted the evidence of a prior civilization circa 1960 unless we
had been lucky enough to stumble upon the remains of a major urban
center that had been buried reasonably intact. Even today, the actual
urbanized area is a small fraction of the continental surface. I keep
hearing that there are some periods of geologic time poorly
represented in terrestrial sediments -- to keep on topic, some parts
of the dinosaurian fossil record that are very poorly sampled. If
some putative past civilization fell into one of those epochs, it
would be hard to find today.
THe odds would be much better if the civilization left behind
something to attract the attention of geologists and paleontologists
-- e.g., caused a mass extinction. Neolithic humans appear to have
wiped out most of the Pleistocene megafauna (or at least their
appearance coincides with that extinction), so that's one marker
future paleontologists could look for. A widespread technological
civilization would have left trace markers behind, but detecting them
would require technology comparable to what we used to detect the
iridium enrichment at the KT boundary.
Perhaps ultimately we will come up with some technology that can
survey the entire buried fossil record, which would tell us much more
about what was really there.
This exercise also reminds us that we can miss entire chunks of the
dinosaurian fauna until we happen to stumble upon the right fossil
deposits, like Incisorsaurus. -- Jeff Hecht