This updated text replaces the first three paragraphs of my First Edition's Sub-section B of
Chapter III's Section 6, starting on page 161 and running until the top of page 163:
Either Sakyamuni or Amibtha Buddha (scholars
aren't sure) rising up from a stone cliff-face,
from around 700 CE, at
Sangseon-am [Upper-
Immortal Hermitage], above
[Three Tombs Valley] on the west side of
Nam-san [South Mountain] in Korea's
ancient capital Gyeongju.
Another photo of this Buddha-carving is
B.  Absorption of San-Shin by Korean Buddhism

There were at least three elements similar to Korea's San-shin traditions already present in the
Mahayana Buddhism that took root in Korea, and thus made it easy for the imported religion to absorb
the native spirit:   protection by devas, meditation-practice in caves, and carving stone Buddhas.

Protection by Devas:
In the Hwa-eom-gyeong Eighteen "Mountain Gods" are said to serve as guardians of the dharma
[Buddhist law/ truth/ teachings].  These are considered devas, demi-gods or nature/animal spirits,
ranking lower than the Buddhas, Bodhisattvas and Na-han but ranking higher than any human being.  
Each San-shin of every mountain that hosts a Buddhist temple is said to protect the temple buildings
themselves, and the local sangha [community of monks].  Protection of the nation in general, always a
part of Korea's San-shin traditions, became a major theme of Korean Buddhism throughout history.  
Still today, monks and common people both pray to San-shin for spiritual protection from ill-fortune.

Meditation-practice in caves:  
In the Seon sect which became very important in Korea from the ninth century onwards, it was
standard from the beginning for monks (in both China and Korea) to seek enlightenment without
distractions by extended meditation practice conducted in mountain caves.  Both beneficent and
harmful spirits were encountered there as part of the monk's development;  there are several old
stories about Korean monks and other heroic seekers meeting and being helped or advised by
San-shin while meditating or deeply praying in caves.  There are many Tantric Buddhist texts which
mention the importance of mountains and the spirits which reside there as objects of faith, and also as
an environment conducive to the meditative absorption (samadhi) necessary to attain enlightenment.  
In addition to the power of the Buddha and Bodhisattvas, the mediator must rely on the protection and
assistance of the mountain deities.    Even today, monks bothered by fatigue or mental distraction
while doing intensive meditation sessions frequently go over to the San-shin-gak and pray to the local
San-shin for endurance, energy, purity of mind, willpower and so on.
The power and effectiveness of meditation and devotional chanting/prayer on mountains is also
enhanced by the
ji-ki [telluric energies, or gui] of that mountain -- a less personalized way of viewing
the San-shin relationship.  Seated on the hard cool stone, the monk or adept connects his
"root-chakra" directly to the
eum [yin] of the Earth's bones.  Practice in caves surrounds him with the
i-filled stone, as if in a womb from which he will emerge "reborn".  Practice in front of cliffs offers
inspirational soaring-upwards energy.  Practice on peaks or high ridges brings him as close as
possible to the
cheon-ki [Heavenly energy, or gui] spiraling downwards, allowing him to serve as a
conduit (like an antenna), becoming fully "charged" by the interaction of
ji-ki with cheon-ki.

Carving stone Buddhas:
Since its very early days in India, and as it spread through Central Asia into China, Buddhism inspired
its devotees to carve reliefs and statues out of stone, free-standing or on cliffs or in caves, as
permanent icons.  When Buddhism came to Korea, it found a whole nation made of granite, which is
excellent carving material.  It also encountered the native belief that mountains are manifestations of
spirits (shin).  These factors combined, and flowered into one of the world's most striking artistic
traditions, Korea's ma-ae-bul (and Seok-kul-am / Sokkuram).
For 1500 years now, anonymous Korean monks have carved granite Buddha figures on cliffs, in
niches and up on peaks. They are always nearby a hermitage and often in quite remote locations;
sometimes hidden back in a gorge and other times near the edge of a cliff with a fantastic view.  The
sites were chosen according to pung-su-jiri theories, to help balance the telluric energies [
ji-ki] of that
area.  The best examples of this amazingly powerful art are found at Toham-san, Nam-san,
Seondo-san and Danseok-san around Kyeongju, Kaya-san, Unju-sa in Jeolla-namdo, Kat-bawi on
Palgong-san, Beobju-sa and Sanggo-am in Sogni-san, Deok-san in Chung-nam, Bukhan-san and
Dobong-san in Seoul, and of course Geumgang-san in North Korea.
These artworks are based on the idea that a great mountain is itself a Buddha, an enlightened spiritual
being.  The ancient Buddhas from Shilla and Koryeo are often flat and carved only in intaglio or relief
at their bottom (feet and legs), and become progressively more three-dimensional as you look up, until
the shoulders and head are mostly or fully realistic.  They thus seem to be rising up out of their Earthly
stone base towards Heaven.  They capture in a frozen moment the point at which the incipient natural
Buddha-hood of the crags manifests into conscious reality.  This style of statue-icon is unique to Korea,
and demonstrates well the Korean religious-artistic genius.
Towering rocky peaks and crags, as found all over
Korea, are viewed by those with insight in the same
way, as yet-uncarved Bodhisattvas thrusting upwards
to realization.  Koreans believe that monks and
adepts who meditate on these Buddha-mountains --
in front of cliffs, in caves and on peaks -- will
themselves become part of this process, more easily
attaining enlightenment by sharing in their continuous
stone-rising-into-spirit self-realization.

These three pre-existing factors both tied in very well
with Korea's traditions of San-shin as protective spirit,
spiritual advisor to sincere seekers, provider of
assistance to virtuous people with problems, and
spiritual essence of the mountain itself.