Interestingly, the Grand Canyon, for all its press, is not terrifically
useful for determining specifics about a particular section in time.
The Redwall Limestone, from the Mississippian period, is about 250
meters of rock representing around 60 million years, or about 4mm of
rock record for every 1,000 years. That's not much to go on, at all.
Now, geologists can still obtain a great deal of information from these
short-spanning outcrops, but it's not very good resolution. There are
sections elsewhere in the world that record much more information per
unit time - a random strat column from Pakistan reveals a section with a
rate of 1m per 1,000 years - 2 orders of magnitude more information
about. It's neat that the canyon samples such a large time frame -
spanning billions of years, but that only means that there are heaps
upon heaps of data gaps.
Gaps in the geological record are caused by periods of either loss of
material or non-deposition - grains are being removed as or more quickly
as they are being deposited. Such holes in the record are known as
"unconformities," in that one layer does not conform properly with the
adjacent layer - there's not a fluid recording of the flow of time.
To end on a less lighthearted note, here is a joke I made up on one of the car rides:
What do you call a volcano in the Vatican?
A cardinal cinder.