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More than one case... As a member of the Massachusetts Archaeological Society, I had the opportunity to examine several large, beautifully-crafted bifaces that were made of what was called, "marine flint".
Some were supposedly dredged up well beyond the present MA shoreline. Others were found in situ with burials at "Wapanucket", a famous, and well-documented site near Middleborough, MA.
They were particularly striking, because most knapped ("flaked" or "chipped") stone artifacts in MA are so crudely made that the Texas Indians would have sneered at them. However, the crudeness was not due to lack of skill, but, rather, to the tough, grainy nature of the metamorphic materials that were all that was available for use. All of MA was heavily glaciated, and there is no exposed limestone -- which is the source of f1ne-grade, knappable silicates (chert or flint).
In such an area, those fine "marine flint" artifacts really do stand out as anomalies.
AFAIK, the source of that fine, glassy, "marine flint" remains a mystery...
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I agree with you; exploration of the ca 10-11,000 YBP shoreline should be very enlightening!
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(P.S. & FWIW, tomorrow, in my role as Texas Archeological Steward, I will be doing lectures on archeological methods and flintknapping demonstrations for one of our local schools...)
(Yes, Texas officially spells it, "archeology" -- no "æ" diphthong...)
I've seen some estimates that say the world's oceans were lowered during the Ice Age by as much as 500 feet.
