Your Guide must have been referring to the North Rosillos Ranch, not to be confused with the Rosillos Ranch. The Harte Ranch section of the North Rosillos Mountains, was added to the park in 1987. Property owners Houston H. Harte and Edward H. Harte donated the 67,000 acre ranch to the Texas Nature Conservancy in 1985 with the understanding that it would eventually become part of the park. This portion of the park remains largely undeveloped, and yes, our science & resource management staff do research work there, but it is not closed to the public.
The Rosillos Ranch remains privately held property inside the park's congressionally authorized boundary.