Some email-discussions already emerged concerning the essay Mapping Arda before the online-publication of OtherMinds 01. I will now use this public forum to summarize them for the broader public.

a) Could the Battle of the Powers lead to a Bay of Utum?

Ambarkanta-map-V shows the landscapes of Middle-earth after the Battle of the Powers as Tolkien saw it in 1936/37. Although Utumno has already been destroyed, the Ered Engrin still builds a great and unbroken curve from west of east. There is no trace of any Bay.

“But the symmetry of the ancient Earth was changed and broken in the first Battle of the Gods, when Valinor went out against Utumno, which was Melko's stronghold, and Melko was chained. Then the sea of Helkar (which was the northern lamp) became an inland sea or great lake, but the sea of Ringil (which was the southern lamp) became a great sea flowing north-eastward and joining by straits both the Western and Eastern Seas.” (HoME 04: The Ambarkanta; 1936)

Tolkien wrote further descriptions of this event during the 1930s which sounds quite similar:

“But first Morgoth was overcome with war and bound and led captive and imprisoned under Mandos. In that war of the Gods the lands were rent anew.” (HoME 05: Later Annals of Valinor; mid-1930s)

“§21 Thus it came to pass that after long council the Gods resolved to make an assault upon the fortress of Morgoth in the North. Morgoth did not forget that the Elves were the cause of his downfall. Yet they had no part in it; and little do they know of the riding of the power of the West against the North in the beginning of their days, and of the war and tumult of the first Battle of the Gods. In those days the shape of the Middle-earth was changed and broken and the seas were moved. It was Tulkas who at last wrestled with Morgoth and overthrew him, and bound him with the chain Angainor, and led him captive; and the world had peace for a long age. But the fortress of Morgoth had many vaults and caverns hidden with deceit far under earth, and these the Gods did not utterly destroy, and many evil things still lingered there; and others were dispersed and fled into the dark and roamed in the waste places of the world.” (HoME 05: Quenta Silmarillion; 1937)

In all these quotes (and on the accompanied Ambarkanta-map-V) nowhere any Bay of Utum is mentioned. Nonetheless Pete Fenlon drew the Bay of Utum onto his map. The reason for this can be found somewhere else:

“Then, seeing that his hosts were overthrown and his power dispersed, Morgoth quailed, and he dared not to come forth himself. But he loosed upon his foes the last desperate assault that he had prepared, and out of the pits of Angband there issued the winged dragons, that had not before been seen; for until that day no creatures of his cruel thought had yet assailed the air. ... Then all the pits of Morgoth were broken and unroofed, and the might of Fionwe descended into the deeps of the earth. And there Morgoth stood at last at bay, and yet unvaliant. He fled into the deepest of his mines and sued for peace and pardon; but his feet were hewn from under him and he was hurled upon his face. ... Thus an end was made of the power of Angband in the North, and the evil realm was brought to nought; and out of the pits and deep prisons a multitude of thralls came forth beyond all hope into the light of day, and they looked upon a world all changed. For so great was the fury of those adversaries that the northern regions of the western world were rent asunder, and the sea roared in through many chasms, and there was confusion and great noise; and rivers perished or found new paths, and the valleys were upheaved and the hills trod down; and Sirion was no more. Then Men, such as had not perished in the ruin of those days, fled far away, and it was long ere any came back over Eredlindon to the places where Beleriand had been.” (HoME 05: Quenta Simlarillion; 1937)

Of course this is a not a description of the Battle of the Powers, but of the War of Wrath. Nonetheless this description is very crucial for the understanding of the results of the Battle of the Powers as Tolkien might have seen them in his later years:

Ҥ49 That siege [of the Battle of the Powers] was long and grievous, and many battles were fought before its gates of which naught but the rumour is known to the Quendi. Middle-earth was sorely shaken in that time, and the Great Sea that sundered it from Aman grew wide and deep.
And the lands of the far North were all made desolate in those days, and so have ever remained; for there Utumno was delved exceeding deep, and its pits and caverns reached out far beneath the earth, and they were filled with fires and with great hosts of the servants of Melkor.
§50 It came to pass that at last the gates of Utumno were broken and its halls unroofed, and Melkor took refuge in the uttermost pit. Thence, seeing that all was lost (for that time), he sent forth his Balrogs, the last of his servants that remained faithful to him, and they assailed the standard of Manwe, as it were a tide of flame. But they were withered in the wind of his wrath and slain with the lightning of his sword; and Melkor stood at last alone. Then, since he was but one against many, Tulkas stood forth as champion of the Valar and wrestled with him and cast him upon his face, and bound him with the chain Angainor. Thus ended the first war of the West upon the North.” (HoME 10: Annals of Aman respectively AAm*; 1950)

In 1975-77 Christopher Tolkien integrated parts of these paragraphs into his version of The Silmarillion. There they were read by Pete Fenlon in 1980/81:

“The lands of the far north were all made desolate in those days; for there Utumno was delved exceeding deep, and its pits were filled with fires and with great hosts of the servants of Melkor.
But at the last the gates of Utumno were broken and the halls unroofed, and Melkor took refuge in the uttermost pit.” (Sil: Of the Coming of the Elves and the Captivity of Melkor)

The very central word here is “unroofed”. In the 1930s Tolkien used “unroofed” to describe the events of the War of Wrath, but not of the Battle of the Powers.
Thus “unroofed” was not part of Tolkien’s choice of words for the Battle of the Powers as he drew the Ambarkanta-maps in 1936/37. But this changed in the 1950s.

A. Morgoth in Beleriand in 1937 and the 1950s
---------------------------------------------
1. Morgoth rebuilds Angband and raises the three great peaks of Thangorodrim.
2. In the War of Wrath Morgoth releases the Winged Dragons and hides in the deepest cavern of Angband.
3. Angband is “unroofed”, the sea draws in and swallows Beleriand.

B. Melkor in Utumno in the 1950s
-------------------
1. Melkor builds Utumno and raises the great curve of Ered Engrin.
2. In the Battle of the Powers Melkor hides in the deepest cavern of Utumno and releases the Balrogs.
3. Utumno is “unroofed”, but not much further is reported from the North.

To complete the parallels, Pete Fenlon consequently drew the Bay of Utum. And thus it found its way onto our maps, too.

b) Was Utumno located exactly at the northern centre of the Bay of Utum?

If you measure carefully the exact middle between the western and eastern end of the old great curve of Ered Engrin, the midmost point is not in the centre of the Bay of Utum, but a little west of it (although still clearly under the sea in front of the fjords of Barl Syrnac).

Hence Utumno should not be considered under the exact northern middle of the Bay of Utum.

If you want to have the middle of the Bay of Utum exactly above the centre of old Utumno, you would have to shift the whole Bay westward. This would mean to shift the Barl Syrnac westward, too. But this is impossible, because right west to the foothills of Barl Syrnac begin the areas mapped by Tolkien himself, which do not show any trace of the Barl Syrnac, of course.