The Echoing Horns — James Haas

The Echoing Horns is the first book in the epic trilogy, Angels On Horseback.
Catastrophe has struck the world and equestrian Rebecca Martin takes on the responsibility for a dozen preteens at her riding stable. They set out on horseback on a journey that will not only transform them, but the world they live in. This is the birth of the Redtail Cavalry and the start of a truly amazing adventure.