http://www.sciencemag.org/news/2018/10/these-undeciphered-symbols-date-back-roman-times

What’s in a name? For the ancient Picts—a federation of Celtic-speaking tribes that lived in modern-day Scotland—the answer might go something like “salmon-beast” or “fish-flower.” Those are just two combinations of some 30 mysterious symbols that this society of farmers, who lived between the third and 10th centuries C.E., carved into hundreds of freestanding stone monuments and bone tools. But the symbols have not been deciphered, and their meaning has perplexed researchers for centuries. Now, archaeologists may have gotten one step closer by figuring out when the earliest symbols appeared.

Because most Pictish symbols are carved in stone, they can’t be dated using traditional methods that rely on the decay rate of organic materials. Instead, archaeologists have relied on imprecise rules of thumb that suggest symbols incised on unshaped stones in this part of the world usually date to about the fifth century C.E. But that evidence is highly circumstantial and not considered as accurate as direct dating.