The U.S. took a significant step toward acquiring its 57th national park yesterday when the Nature Conservancy announced that it had signed an agreement to purchase historic Baca Ranch, which borders Colorado’s Great Sand Dunes National Monument and Preserve. Conservationists have long lobbied for national park status for the area, which includes the tallest sand dunes in North America. In 2000, the Clinton administration authorized purchase of the 151-square-mile ranch and creation of the national park. The Nature Conservancy paid $31.3 million — its most expensive acquisition ever in the Rocky Mountain West — and will turn the deed over to the federal government. The area is expected to become a national park by 2005.