From South Pass, the Pony Express Trail followed the Sweetwater River northeastward to Split Rock Station which was located 65 miles north of Rawlins on US287 heading for Lander. Located 12 miles from Plant’s Station and 14 miles from Three Crossings Station. The Station Tender was Plant or Plante. Split Rock Relay Station was a crude log structure with a pole corral and was located on the south side of the Sweetwater River.  

Split Rock is a massive, cleft up-thrust of igneous material. In this vicinity, Russell, Majors & Waddell erected a stage station in 1859. It can be seen as a cleft in the top of the Rattlesnake Range. This was an important landmark to early travelers, since it can be seen for a full day’s journey from the east, and for 2 days behind as they continued westward. There are 35 highway miles between Split Rock and Devil’s Gate. 

Pony Express lore tells that William “Buffalo Bill’ Cody exchanged horses at Split Rock Station on a record ride from Red Buttes Station to Rocky Ridge Station and back. Due to another rider’s untimely death, Cody was forced to do an extra leg to his normal relay and eventually covered a total distance of 322 miles in 21 hours and 40 minutes, using 21 horses in the process.  

http://www.expeditionutah.com/featured-trails/pony-express-trail/wyoming-pony-express-stations/

Most sources generally agree on the identity of this station, probably because it appears on the 1861 Overland Mail Company contract.  A French-Canadian named Plante managed operations at the station, known by the stationkeeper's name or the natural formation off to the northeast.  Gregory Franzwa does not specifically list Split Rock Station as a Pony Express site, although he does mark it on his maps.  As late as 1988, a marker stood on the north side of U.S. 287/789, which read: 

Split Rock. A famous natural landmark! used by Indians, Trappers, and! emigrants on the Oregon Trail.! Site of Split Rock Pony Express! 1860-1861, stage, and telegraph! station is on the south side of the Sweetwater.! Split Rock can be seen as a! cleft in the top of the Rattlesnake Range.

http://www.nps.gov/parkhistory/online_books/poex/hrs/hrs6a.htm#75