Complex sentence: The site may also yield useful evidence of the historical period of the town of Umatilla, founded over the prehistoric remains in the 1860s as a gold mining supply base and relocated in 1967 in connection with the construction of the John Day Dam.
Split into simpler sentences: The site may also yield useful evidence of the historical period of the town of Umatilla, founded in the 1860s as a gold mining supply base. The town was relocated in 1967 in connection with the construction of the John Day Dam.