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MOUNTAINS!
All the Mountainous Chains of Arda, created by the Valar or by Melkor, or by the effects of disastrous cataclysms, always represent and are deeply part of the long and suffered History of Arda, protecting legendary realms, connecting far regions, or being the lair of terrible enemies.I didn't include all the possible Mountains present in Arda, 'only' the most significant ones, focusing on the World's Shape in the first part of the Second Age, when the Undying Lands were still part of Arda.
I included a Map
BLUE MOUNTAINS (Blue): They once completely divided Beleriand from the rest of Middle Earth, after the War of Wrath they remained still as a broken Chain along the western coast of the World.
It was here that Thorin Oakenshield found refuge and established his own dominion
MISTY MOUNTAINS (Light Green): Created by Melkor to hinder the ride of Oromë the Vala, these Mountains witnessed the founding of the legendary and majestic Halls of Khazad-dûm, but also the sorrows of the Dwarves caused by Darkness and ancient Flames.
GREY MOUNTAINS (Purple): The remnant of the ancient Iron Mountains in the North, often infested by Orcs and Dragons, they were the home of some dwarven settlements in the Third Age.
WHITE MOUNTAINS (Teal): They connect Rohan to Gondor, and were used as a means to ask for the aid of the Horse Lords during the War of the Ring
OROCARNI (Red): Mysterious and ancient Mountains in the Far East of the World, closed to them laid the Bay of Cuiviénen, where the Elves awakened for the first time in the World.
IRON MOUNTAINS (Black): Ancient Mountains in the North of the World, present since the Years of the Lamps.
They were for millennia the lair of the Evil, for in their depths were built both Utumno, in the East, and Angband, in the West of Middle Earth
After the War of Wrath they were completely laid waste and levelled by the furious energy and destruction of the battles (they thus are not present in the Map, but they totally deserve to be mentioned).
EREBOR (You know where it is
): The legendary Lonely Mountain, home of one of the most powerful and richest Dwarven Realms ever present in the World.
PELÓRI (Holy Yellow): The indestructible Wall of Valinor, created by the Valar to fortify their Blessed Realm and protect Aman
Needless to say, my favourite ones are the Pelóri
If you want, leave a comment below to explain why you like one and not the others, or add more information about them, since their summaries are brief