They were angling up the hill, going from one outcropping of rock to another, because the ground above the outcroppings provides a small level spot where the horses could rest a little (elevation is around 8,000'). While the uphill side of these outcroppings is somewhat level, the downhill side tends to have a cliff of up to 20 feet or so. It is essentially a hunk of rock sticking out of the hillside with dirt piled up on top of it.
About half-way up the hillside, it started spitting rain and thundering a little. Dad instructed everybody to get off their horse and don their rain gear, which consisted of a yellow slicker or poncho tied behind each boy's saddle. One bright young lad untied his slicker and immediately shook it out to unroll it, shaking it wildly right under the nose of his trusty mount. The old gelding, normally as sturdy as a rock, hearing thunder and seeing this horse-killing yellow banshee waving in front of him, promptly reared straight up, turned 90 degrees, and jumped right off the trail on one of these outcroppings. Luckily...there was this large juniper tree just off the side of the trail, which caught the gelding like an outfielder. The gelding landed right in the main fork of the tree, ending up with the fork right about where the girth was, with his front legs about 4-6 feet off the ground and his hind feet about 2 feet off the ground. It probably knocked the wind out of the horse a little, because he fought a little for a minute and settled right down. He just hung there.
Well, while this was happening, another horse (happened to be my mother's mare...her baby) got excited and pulled away from her rider, also stepping off the trail at a lower spot. She fell and began to roll down the hill as she struggled for footing. My dad dove down the hill after her, seeing that she was rolling toward the next outcropping down the hill, with a drop-off of nearly 20 feet. Just before she made that last roll that would have taken her over the cliff, Dad made a dive and landed on her neck, holding her head to stop her from struggling, so she would not roll over. He held her there and began to yell at the boys to bring him a rope to tie her off with. The boys just looked at him with terror on their faces, never before having experienced anything anywhere near what was happening. The lack of immediate response brought a string of language out of my father that I'm sure most of the boys had never heard before either, and it snapped them out of their stupor. Once Dad got a rope around the mare's neck, he ran it around an uphill tree and was able to support the mare while she regained her feet. She was able to get up and made it back up to the trail without further incident.
Dad, then turned back to the horse in the tree. By this time it was raining pretty good, but the thunder was still somewhat distant. Dad took a young pack horse and pointed it straight up the remainder of the hill, slapped it on the rump and up he went. One by one, he did the same for the other horses, with the scouts following on foot. He told them to get to the top, picket the horses, and get camp set up.
Now, how to get the horse out of the tree? How to cut the branch off without it impaling and gutting the horse? How to keep the horse from falling on down the hill once it drops from the tree? It is good he was with Boy Scouts, because one bright young man suddenly appeared with a handy little thing called a "rope saw". It is a piece of wire with small teeth twisted into it and a ring on each end. Dad was able to thread this wire saw under the horse's chest, and by working it back and forth, cut through the branch, about 6" in diameter, from the top of the fork, leaving no splinters to stab or injure the horse. Eventually the branch gave way and the horse fell to the ground, landing squarely on all four feet. He shook himself a little and turned back up to the trail. They headed him right up the hill after the rest of herd, showing no ill effects from the experience at all. He was fine the rest of the trip.
In fact, the only lasting ill effect of the wreck is that my dad lost his camera during the wreck and didn't realize it for a couple days. I think it was hanging from the saddle horn of my mom's mare. It is still somewhere on that hillside east of Greer, AZ.
Only the fact that there were no injuries to man or beast differentiates this experience from a tragedy. I have been through many such wrecks on the trail. Luckily, neither myself, my horses, nor anyone with me, has yet suffered any injuries, other than a few bumps, scrapes, and bruises. In fact, I have never had to cut short a trip due to a wreck. I've been lucky.