Ky. 15 about 2 miles north of Jackson remained blocked Friday morning after a boulder fell about 5 p.m. Thursday, according to authorities.
The boulder fell during blasting in a construction zone in Breathitt County at Jackson, said Kentucky State Police dispatcher Levi Grubbs said.
Police aren't sure when the highway will be reopened, but it might be clear by 3 p.m. Friday, he said.
Detours are being posted until the boulder can be cleared.
Associated Press contributed to this report.