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Re: 1st Inland sea incrsion

<the production of the seaway probably took over 20Ma to finally separate
eastern and western North America.>

I ask because I do not know: isn't the abbreviation Ma (for Mega annum) used
as an absolute temporal reference, as in "20 million years ago"--rather than
a relative reference, as in "it took 20 million years to accomplish?"

Patrick Norton