The reason Thangorodrim and the Iron mountains were not on the Silmarillion map was because CT was not sure exactly where to place them. The source map he used (HoME V) didn't agree with the description found in later texts. Angband was built behind the Iron mountains as was Utumno. The location of Utumno is shown in one of the Ambarkanta maps where it is north of the Blue mountains.
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Ailios
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