I kept trekking until I reached a vast plateau.

[alpacinou]

Is this correct and natural?

I kept trekking until I reached a vast plateau. To my right, an endless flat land stretched all the way to infinity. To my left, two rows of mountains rose, converging at the horizon, which was tinted with a copper glow by the setting sun. The mountains were sharp silhouettes against the sky where black and orange mingled into the twilight.

 

[tedmc]

I think, land, being uncountable, does not take the indefinite article.

I prefer "colours mingled in the twilight".

 

[Tdol]

An endless flat land works for me- it's not always uncountable.

 

[Charlie Bernstein]

Is this correct and natural?

Good start! You might try trimming it down a little. For example:

I kept trekking until I reached a vast plateau. To my right, an endless flatland stretched [STRIKE]all the way[/STRIKE] to infinity. To my left, two rows of mountains rose, converging at a horizon [STRIKE], which was[/STRIKE] tinted with a copper glow by the setting sun. The mountains were sharp silhouettes against the sky, where black and orange mingled in the twilight.

Three things:

- If the horizon is glowing, the mountains on the horizon aren't silhouettes.

- And think about whether you need both "endless" and "infinity." If you just like the sound of them, that's fine. Keep in mind that, by definition, anything endless stretches to infinity.

- Vocabulary note: There's nothing wrong with "rows of mountains," but "mountain ranges" is more likely.

- Style challenge: Can you think of more concise ways to say "tinted with a copper glow by the setting sun"?

 

[Tdol]

Wouldn't this plateau be a flat highland?

A bit like this: https://en.wikipedia.org/wiki/Bolaven_Plateau