I've been working on a sideproject for the setting recently, and I've noticed a bit of oddness. It's known that the layers of the Abyss are numbered according to the order of their discovery by the Guvners, and that the plane itself was discovered when the Plain of Infinite Portals was. However, there's some oddities with the timeline of the early layer discoveries here.
First off, the discovery of the Plain is accompanied on the Abyss poster in Planes of Chaos by a small excerpt from the party that discovered it, where they mention spending time in Broken Reach. However, Broken Reach was only founded just barely over two centuries before the point that Planes of Chaos was set. Secondly, the seventh layer was recorded on the same poster as having been discovered by "Mordenkainen the Archmage", who reported his discoveryto the Guvners while commissioning a new spellbook. The implication is obviously that this is the Mordenkainen of Greyhawk, but that character is only in his 90s, and wouldn't have been known as an archmage until his 30s or so.
There's a few ways I could see to rectify this. Perhaps the layers were only numbered by when the Guvners recorded them (similarly to how they consider themselves as the first faction since they were the first to incorporate under the rules they created), and for some reason they didn't begin recording layers until a couple centuries ago. This would be especially strange given how late the numbering of the major layers are, and how the entry on the first layer really sounds like they're talking about sending parties out to find things no one had seen before, but it could work with some effort. It could be that the Lexicon of the Abyss (the IC book that the Guvners record the layer discoveries in as they occur) was just a total piece of propoganda put out by the Guvners to try and promote themselves as a faction, and the Guvners don't even buy it themselves. You could even just say it was talking about an older Broken Reach and an older Mordenkainen, neither of which are the ones commonly known today, although this explanation reaks of being a copout to me.
What do the rest of you all think? And do any of you have any explanations beyond the ones I've thought of so far?
Using the poster map of the Abyss in Planes of Chaos and the information in The Factol's Manifesto, here's a timeline indicating the reigns of the Guvner factols and when each layer of the Abyss was discovered. Very, very roughly. The dates are all Anno Domini, the dating system most of us use, for convenience and clarity (and because there isn't really a Planescape dating system apart from the reigns of the Guvner factols).
No, I don't know why I took the time to figure this out. Just a whim, I guess.
The methodology worked like this: The Factol's Manifesto was published in 1995. Planes of Chaos came out in 1994. The Fraternity of Order was founded 982 years before the publication of The Factol's Manifesto. Assuming the first factol was Factol Clarille (a reasonable theory, I think - how long could the Fraternity of Order have possibly existed without knowing anything about the Abyss?), and assuming that new Abyssal layers were discovered at a more or less even pace, I figured out these dates algebraically when I didn't have a better reason. I arbitrarily gave Factol Lariset, Hashkar's immediate predecessor, the same reign as Pope John Paul II (29 years).
The discovery of the Phantom Plane is notable because the discoverer was the archmage Mordenkainen. Mordenkainen is only 80 some years old in the Greyhawk campaign, so either Planescape takes place in Oerth's far future or there's some time travel afoot. Or there was more than one archmage called Mordenkainen. Or another reason. Maybe it was an error on the part of the scribe. And time can do funny things when Sigil's portals are involved. I'd actually rather assume it's an error (on the part of Wolfgang Baur and Lester Smith rather than the Guvners, and straight-up retcon it), and replace Mordenkainen with Ahm (of the Black Book of Ahm described in Fiendish Codex I) or Vilhara, or some other famous demonologist who might have lived almost a thousand years ago. However, note that time travel isn't that unlikely an explanation. Mordenkainen is close friends with Heward and Murlynd, both of whom have traveled through time on numerous locations - Heward lives in a nexus that leads to many planes and times. Also, Mordenkainen's apprentice Bigby has been spotted centuries in Sigil's past; the Bigby Academy in Sigil was seized by the Fated centuries ago, and it's not clear how long the statue of Bigby in Sigil has been up. So there's canonical precedent for Mordenkainen and Bigby traveling together to the City of Doors long ago.
As far as Broken Reach goes, the present-day Broken Reach was founded a few centuries ago on a portal to the Outlands, but the portal to the Outlands has moved over the centuries as the various Plague-Morts have slid. Presumedly the present Broken Reach is built on the site of the previous Plague-Mort shortly after it was delivered to the Abyss by its Arch-Lictor.
So some of the previous Plague-Morts might also have been previous Broken Reaches. Far from being a cop-out, it makes sense to me. A gate-town can only exist as long as there's a gate under it. When the gate moves, the town moves with it, and it's logical for the inhabitants to stop bothering to rename the town after a while (there's only so many iterations of "New New New Broken Reach" before it gets old, and the chaotic inhabitants of the Abyss are unlikely to bother with such details when the people of Plague-Mort aren't bothering to rename their town either most of the time). So every time Plague-Mort slides, the people of Broken Reach pick up stakes and colonize the ruins on the Abyssal side of the portal. And on and on and on.
1013 Factol Clarille the Munificent takes office as the first Guvner factol.
1017 Plain of Infinite Portals discovered.
1018 Driller's Hives discovered.
1025 Forgotten Land discovered.
1026 Factol Jarkopple the Inflexible takes office.
1026 Grand Abyss discovered.
1033 Wormblood discoverd.
1035 Realm of a Million Eyes discovered.
1036 Phantom Plane (discovered by Mordenkainen - 970 years ago!)
1036 Skin-shredder discovered.
1036 Burningwater discovered.
1037 That Hellhole discovered.
1047 Molrat discovered.
1047 Twelvetrees discovered.
1042 Blood Tor discovered.
1064 Iron Wastes discovered.
1082 Azzagrat discovered.
(passing of Alonthas the Dark, compilation continued by Orrota the Lich, of the Steady Hand)
1090 Factol Zander the Explorer takes office.
1112 Demonweb Pits discovered.
1113 Heaving Hills discovered.
1113 Unnamed (The Swallowed Void) discovered.
1113 The Crushing Plain discovered.
1114 The Ice Floe discovered.
1118 Factol Darvik takes office.
1119 Spirac discovered.
1121 Darklight discovered.
1122 The Child-Factol Andalon takes office.
1124 Wells of Darkness discovered.
1125 Factol Soretti the Spider takes office.
1126 Smaragd discovered.
1134 Gaping Maw discovered.
1180 Factol Masol the Just takes office.
1181 Thanatos discovered - Orcus died during the time of Archimandrite Saki
1276 Factol Jura takes office.
1278 The Rotting Plain discovered.
1295 Factol Jaretta - 700 years ago (FM, 114)
1337 Shedaklah discovered.
1412 Durao discovered.
(Disintegration of Orrota, compilation continued by Archimandrite Saki)
1454 The Sulfanorum discovered.
1561 Plains of Gallenshu discovered.
1573 Factol Karakor the Lawgiver assumes power.
1593 The Worm Realm discovered.
1594 The Woeful Escarand discovered.
1626 Yeenoghu's Realm discovered.
1743 Torremor discovered.
1785 Death of Orcus according to original H4 timeline. This was retconned in later FR products, but Dead Gods and Fiendish Codex I use the earlier dates. I arrived at the date by taking the Dale Reckoning date of H4: Throne of Bloodstone, converting it to Common Year (Greyhawk), and then assuming 591 CY=2001 AD. That's somewhat roundabout, but I think that's right within a year or two (because 591 CY was also 1998 AD).
1841 Factol Lariset the Inescapable assumes power.
1863 The Prison of the Mad God discovered.
1868 Factol Hashkar, reigns 130 years
1945 The Caverns of the Skull (Kali's realm was discovered during World War II!)
1994 Abyssal lexicon records 679 layers
1995 Factol's Manifesto completed
1998 Faction War
1998 Factol Jamis, reigns 10 years (so far)
2008 The present day