Complex sentence: They were to proceed to the Bayou Catalan (or De Pecheurs) at the head of the lake, and 60 miles from the troopship anchorage, which was to be the disembarkation point for the attack on New Orleans.
Split into simpler sentences: They were to proceed to the Bayou Catalan (or De Pecheurs) at the head of the lake-60 miles from the troopship anchorage. This was to be the disembarkation point for the attack on New Orleans.