The Twin Sisters are without a doubt the most spectacular formations found within the City of Rocks National Reserve, and they deserve mention, as the history here is abundant, both culturally and within the climbing community. Unfortunately for climbers, all climbing activities on the sisters have been banned since 2000, when a court decision decided to uphold the NPS's closure under the reserve's comprehensive management plan.
The Twin Sisters are two dramatic granite spires that mark the high points of a mile-long rock ridge that is separated from the rest of the reserve by about 2 miles. The north spire (~500 feet tall) is composed of the 28 million year old Almo Pluton granite, while the south spire (~600 feet tall) is comprised of the 2.5 billion year old Green Creek Complex granite and represents some of the oldest exposed rock on the continent.
The cultural history of the Twin Sisters is rich. Between 1843 and 1882, nearly 250,000 immigrants traveled through City of Rocks on the California Trail, using the Twin Sisters as a major landmark on their journey. Prior to receiving their current name in 1848, immigrant journals mentioned the formations under various names, including "Steeple Rocks", "Twin Mounds", "Twin Buttes", "Two Dome Mountain", and others. Inconclusive ethnographic information also hints at the formations being of important spiritual value to the area's Native American groups.
The earliest known climbing ascents on the Twin Sisters were completed in the 1960's by the renowned and visionary Lowe party, which included Greg, Jeff, and George Lowe. The Twin Sisters house some of the longest and highest quality rock in the City of Rocks, ranging from historical, Yosemite-esque crack lines to highly technical face routes. I invite others with more (personal) knowledge of these routes to add them here 1) for future reference, in case the climbing ban is ever lifted and 2) for posterity's sake, as I, for one, enjoy following the early history of North American climbing.
The Twin Sisters are impossible to miss and are visible from many vantage points within the reserve. From the main City of Rocks dirt road, take the westward dirt road at the split that occurs just north of Register Rock.
Update to the climbing ban: after working for the reserve last summer and having had discussions with the park's superintendent, it is my understanding that one is still allowed to climb on the Twin Sisters, but only using "historical" methods. That means no ropes or modern gear, although the use (or ban) of sticky rubber was never mentioned to me...
All that being said, I take no responsibility if you get "in trouble" for some reason. While I believe the climbing community has a duty to follow the rules in order to protect the lands we climb on, I also feel that, sometimes, said rules are "irrational". I encourage you to make your own decisions, and I'll leave this discussion with a (likely paraphrased) quote from Jack Turner: "Rules are not about order. They are about obedience."