There is much I don't remember, and there were no witnesses. I got on a green horse, but I don't know what happened next. Obviously something scared the mare. Hoofprints indicate she bucked and ran away. I think I may have gotten a foot caught in the stirrup. The ground was muddy and there were two impressions that probably were made by the helmet. It's also possible I was kicked in the head or stepped on.
I lost consciousness for 30-60 minutes. When I came to the helmet was on the ground still buckled. I went to the ER by ambulance. I was diagnosed with a concussion. I had a lot of pain in my left shoulder, left ribs and right thigh, but no broken bones. I went home the same day.
Except for sore ribs (improving) and a very impressive bruise on my leg, there seems to be no lasting damage. And the mare seems completely unaffected.