The Geography of the Young Kingdoms described by Moorcock is somewhat vague, reading the works of Elric.
Chaosium inc. published a map (here a courtesy link for reference) which they admit is a rendition of what the Young Kingdoms may look like, although they changed in depiction through the works of Moorcock. The map is, of course, not a complete depiction of the world and not a planisphere at all.
In Atlas of the Young Kingdoms (published by Chaosium, ISBN 1-56882-021-6), on page 7, it is said:
The Plane of the Young Kingdoms is essentially a flat, oval disk, about 15,000 miles [...] north to south. Because it was created by Chaos, the influence of which remains strong to this day, the earth does not always conform to natural laws. [...]
On this plane, the edge of world is real, and not the product of superstition. It is a terrible place of rushing waters and empty void which only few people have seen.
Richard Watts, in his introduction to the volume published in 1994, says that they based as much as possible of the geography on Moorcock's canon, but filling the gaps for added value to those playing their RPG.
I'm looking for help tracing what of these facts on the Young Kingdoms back to canonical sources.