Complex sentence: Mr. Rockefeller was so impressed by the area's beauty that he arranged to buy up most of the island of St. John then he donated most of it to the U.S. government for the creation of the Virgin Islands National Park, our 29th national park.
Split into simpler sentences: Mr. Rockefeller was so impressed by the area's beauty that he arranged to buy up most of the island of St. John. He then donated most of it to the U.S. government for the creation of the Virgin Islands National Park, our 29th national park.