Places in Middle-earth
Gates of Beleriand
Type: Valleys, Caves, Tunnels
Region: Beleriand & North
Location: The three strategic gateways into Beleriand from the North, used by Morgoth's forces in their invasions; from west to east, they are: the Pass of Sirion, the Pass of Aglon, and Maglor's Gap.
[After the Dagor Bragollach:] The March of Maidros1 was no more. The fell sons of Fëanor were broken and wandered far away.... The Gorge of Aglon was filled with Orcs, and the Hill of Himring was garrisoned by soldiers of Angband; the pass of Sirion was pierced and Tol-sirion1 retaken and its dread towers rebuilt. All the gates of Beleriand were in the power of Morgoth.
The War of the Jewels, HoME Vol 11, Part 1, The Grey Annals
Although Maglor's Gap is not explicitly named above as a gate of Beleriand, it is, in fact, the alternate route through the March of Maedhros when the Pass of Aglon is unavailable:
[For] a while [Maedhros] closed once more the Pass of Aglon, so that the Orcs could not enter Beleriand by that road. But they... passed through Maglor's Gap, and destroyed all the land between the arms2 of Gelion.
The Silmarillion, Quenta Silmarillion, Ch 18, Of the Ruin of Beleriand and the Fall of Fingolfin
1 This text is from one of Tolkien's early drafts; the name is an earlier version of the proper name as published in The Silmarillion.
2 Gelion... rose in two sources and had at first two branches; Little Gelion that came from the Hill of Himring, and Greater Gelion that came from Mount Rerir.
The Silmarillion, Quenta Silmarillion, Ch 14, Of Beleriand and its Realms
Contributors: Elena Tiriel 19Dec09