Is this a run-on sentence?
Archaeologists are unable to explain the mysterious structures in Jordan, Syria, and Turkey called "Big Circles," made of stones piled several feet high, the two-thousand-year-old circles are more than 1,300 feet in diameter.
A trick to try is saying the entire sentence out loud at a moderate pace in one breath. If you find yourself running out of breath or it's difficult to finish then it is more than likely a run on sentence.