The publication Miller-McCune wonders this and has some scientific info on this interesting question.
In the recent studies that MM cites, a nature setting provided relaxation and mental clarity for a signficant number of participants in a variety of studies. As they note, it stands to reason that if you have less noise and distractions around you, your mind is less likely to be cluttered and you're less likely to feel pressured and distracted.
I can definitely attest that after a nice ocean swim, my mind unspools and I find myself able to calmly reflect on things going on in my life (and wish that I had more time to do that).
What do you think? Do you find that nature does indeed have a calming effect on you? We'd love to hear some of your own stories about this.