But I think that in "Much of the state's lands", "much" (uncountable) is the subject, hence "was". (As in "one of the books is missing.")
[Updated]
A friend told me:
a. Much of the state's land was formed.
b. Much of the state's lands were formed.
The second actually works, but "much" for a plural thing is informal. So in formal writing you'd still very much expect to see "many of them were" as the only correct option instead of "much of them".
I replied: So "much" here is countable, meaning "many". Right?
Friend: Yes, it has the same meaning.
Your friend is partly correct, but I have two points: First, using "much" this way is not informal, and second, "many of them were" is not a helpful comparison because that is a standard/straightforward case with nothing unusual about it.
The reason "much of the state's lands were" is unusual is because "land" can be both countable and uncountable. If you're referring to some land somewhere (or all of the state's land) you would use "land" and it would be uncountable. But "lands" is used to refer to specific land areas or types, so "the state's lands" is countable.
So then, you are probably wondering why the sentence uses "much," which is typically used with uncountable nouns. That's because here it means
much of each/many of the lands (countable) that are being referred to. There are many individual "muches" being combined into a larger "much." So when choosing the verb (were), you base it on the fact that "lands" is plural/countable. Earlier, I didn't say that "much" is plural/countable in this sentence because I thought that would be confusing, but in fact it is plural/countable.
Here's a similar example: "Much of the cheeses were...". "Cheese" is uncountable, but "cheeses" refers to multiple
types of cheese and it is countable. However, unlike the "state's lands" sentence, my example here is contrived and confusing, so it would be much, much better to rewrite it as something like this: "Large portions of each of the cheeses were...". Indeed, "portions" is the subject, and in this construction it's much clearer that "portions" is plural/countable.
Also note that your two sentences (a and b) actually have subtly different meanings, because the meanings of "land" and "lands" are different.
How about this?
Sediment was washed down the Mississippi River and it (the sediment) left enormous deltas and vast areas of coastal marsh and swamp. Much of the state's lands were therefore formed.
So I would now say " ... leaving enormous deltas and vast areas of coastal marsh and swamp" modifies "sediment".
I disagree. The noun "sediment" and verb "left" don't work together, because the sediment doesn't leave something behind on its own. So in this context the verb "left" can refer to the river or the process, as I mentioned earlier. For your new sentence you would ideally change the verb to something like "formed into."