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That construction "is of a" means "is in the area of" or "in the realm of".
This sentence is saying that the actual age of lakes cannot be determined but they are older than recorded history.
Yes. "Geological" is used like that; another word with a related (but not the same) meaning is "astronomical", which can be used just to refer to very big numbers or can be used in general-purpose hyperbolic expressions such as "the costs were astronomical".
I knew that the word "astronomical" can mean "very large" or "very expensive", but I didn't notice "geological" can be used in a similar way.
Your explanation is easy to understand.
So, can "geological" mean "very old"?
Just to make sure...because I can't find a dictionary which states so.
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So, can "geological" mean "very old"?
Just to make sure...because I can't find a dictionary which states so.
PD
Pretty much - but not just very old; so old that its duration can only be measured with the use of expressions such as those used by geologists, like 'era' or 'eon'.
"is of a geological time frame" 
Linear Mode
