A compound sentence joins two main clauses with a conjunction. (vault.com)
Use a semi-colon when two main clauses exist in balance, and are closely linked, e.g. Batman was out fighting crime; Alfred stayed home and tidied the Bat Cave. (creativeboom.com)
In the first sentence, the main clause states that «the bridge between the centuries... was found [to be] ongoing.» (religion-online.org)