[meritt]

If the markings were formerly large wooden posts, those would have decomposed over time and the enriched organic matter simply retains more moisture.

Separately, if they had dug trenches and lined with clay or something that retains water, it would still retain more water today. But as these markings are separated, I think it's more likely the first.

[NegativeLatency]

Even unlined trenches do this, because the underlying soil holds less moisture than to topsoil. Dig a trench, wait a long time, and smooth it out and you’ll end up with an area where the trench was with better soil.