Why does Perlin noise produce horizontal lines?

Here are a few images of my terrain which I generated using perlin noise.

All three images contain horizontal lines which are obviously not very natural. My tile system is very barebones, and just uses its position to get a value of the perlin noise and from there determines what color the tile should be.

Of course the error could be in my code, but I figured I would ask here because I have so far not found anything that could be causing this problem.

I will add that I am currently only using 1 layer of perlin noise, so perhaps the issue could be solved by using multiple layers (not sure what the correct terminology is).