The scandalous wedding of young Robb Stark to Jeyne Westerling isn’t the only thing to have been stained red in the history of Westeros (spoiler!).  The Red Keep, home to the Iron Throne in King’s Landing, appears to be made of red-hued sandstone.  We again assume that the quarry is near King’s Landing to facilitate construction, and that the sand was likely eroded from the aptly named Red Mountains to the south.  The crimson coloration might come from a variety of sources, including ferric iron oxide hematite (an iron mineral) that also binds together the New Red Sandstone found in the UK on Earth.  This mineral is indicative of terrestrial origin, suggesting formation within a desert environment.  From our analysis of the size of the Game of Thrones planet, we know that deserts form within a narrow latitude range near 30°.  And our analysis of the Winterfell Limestone suggests that Winterfell was located near 25° north 300 Mya.  We therefore conclude that the Red Keep Sandstone originated in a terrestrial desert environment on southern Westeros around 450 Mya, when King’s Landing, and the entirety of southern Westeros, was within the southern hemisphere.  This approximate age is constrained by the eons required for tectonic forces to translate southern Westeros to its eventual collision with northern Westeros 350 million years later.