Entered either from Parrish Road or the south side of Holder Park, this sanctuary covers 147 acres of scrub, pine flatwoods, and forested wetlands. On the south side of Parrish Road, just to the west of the sanctuary boundary, is a prominent ancient mound from which the sanctuary takes its name. This large oval-shaped shell mound is now mostly obscured by vegetation, but it was partially excavated in the nineteenth and twentieth centuries and seems to have been used by the native people of the area up to the time of the arrival of Europeans. The mound was still a prominent landmark at the time of the construction of the St. John and Indian River Railroad in the 1880s. A station was established nearby, giving the present sanctuary its full name.