The South Buttes is one butte with two peaks, each with its very own transmission tower. According to the geology map from the Sutter Buttes USGS publication on the South Buttes is an andesite / dacite dome.
There is no public access to the South Buttes. A big sign at the base of the access road tells you that you are being watched and you will be arrested if you trespass.
In 2009 a fire cleared a portion of the south face of the eastern butte. A huge blocky structure was partially exposed containing many lines of rectangular stones. This area is about 500 feet tall, a 100 feet wide and extends perpendicular from about 20 feet to 40 feet from the butte's surface. It is on a slope around 80%+ grade and contains many stones weighing tens of tons and larger.
There no geological explanation for these rows rectangular stones. The South Buttes is a dome of intrusive igneous andesite and dacite. Intrusive means the rock cooled while still in the earth and did not reach the surface flow at the surface.