The Red Orogeny is the earliest piece of Westeros’ geologic history that we can infer with the available data.  From our analysis of the Red Keep Sandstone and the Winterfell Limestone, we know that Westeros has moved gradually north throughout its history.  From this, we can infer that the Red Mountains formed as oceanic crust to the south subducted beneath continental crust to the north.  An oceanic spreading ridge (like the mid-Atlantic ridge) located south of the map likely drives this process.  When the Red Mountains formed, they likely extended into southern Essos as the oceanic plate subducted beneath the entire continent.  Due to their age, and perhaps more dominant weathering and erosion on the eastern flank of the range due to climatic conditions, the Red Mountains in Essos have been lost to time.  The Red Mountains are notably lower in elevation and far easier to traverse, betraying their true age.  The morphology of such weathered mountains is similar to the Appalachian Mountains in the eastern US, which formed around 500 Mya.  We infer a similar age for the Red Mountains.