Another property of rock, granite and like is that they "eat" RF. Experienced
that when driving around and operating from areas in OK3 and VE3 (Kingston).
Just like ocean gives you 10 -15 dB plus, seems that rocky areas take away
about as much. Like driving into an anechoic chamber vs. into amplifier at the
ocean beach.
Looks like perhaps absence of good reflecting?
As far as modeling, use it to compare various designs in similar conditions and
perhaps try them in real life, see how they reflect the modeling.
I would use radials or ground screens under antennas, perhaps good verticals
will perform better than horizontals?

Yuri, oK3BU, VE3BMV, VE1BY