Re(Te) natural, slightly unnatural and very unnatural


Is there a good intuition for what counts as natural and the different shades of unnatural?

Is moving earth in a way that could be done with labor and tools always natural?
Is raising a riverbed to make a ford natural or unnatural?
Why is 'Earth Shock' base 3 and hence very unnatural? Earthquakes were know in medieval times, right?

Thanks, and best

Regarding Earth Shock:
If you're causing an earthquake the natural way, you'd need to either move huge landmasses (probably not feasible), or cause shear/stress fractures in the bedrock (but that requires a geological understanding which Europe didn't have until discovered by Robert Hook in 1705). Another option is that you're literally carving out and lifting a big slab of rock a few inches into the air and shaking it. Flight is pretty unnatural for rock, so I could see someone making the case for it being unnatural.

An alternative paradigm for Earth Shock would be that the spell causes a huge amount of force to slam into the ground (think something like an invisible meteorite). That doesn't cause seismic movements, but it would cause similar-looking effects - close enough that people of that age wouldn't be able to tell them apart. And while that amount of force would require a pretty high level spell, it would be perfectly natural for rocks to crack and buckle under it.

Not sure if this helps at all.

As for the other questions, that seems to be extremely subjective. E.g. what Rego actually does has caused some pretty heated debates here in the past. So ultimately, it'll depend on the player and SG.