Thaw

All males born in The Tharn's Lands are twins. One adventurous twin, Rist, travels with sold ice to see how it is used by Warmlanders. After trouble with the Solar Priests, he has to travel farther downriver, where he encounters a two-thousand-foot waterfall and a vast lowland below. Making good his escape, he finds himself in the Sisterdom of ShadowFall, a domain within Motherland, a yet more advanced matriarchal civilization. Through demonstrated skills and persistence, he overcomes prejudice and barriers, distinguishing himself as a heroic warrior.
Rist's twin, Rusk, is present back home when others uncover a large green cylinder from The Ice. This eventually is discovered to be an ancient flying craft operated by a Developing Intelligence Rusk will call Una. Rusk goes downriver to search and find Rist. Adventures ensue, resulting in a return to The Tharn's Lands and a revolution there.
From their primitive, mist-bound homeland at the foot of receding glaciers, to warmer and more civilized lands downriver and below, twin diminutive "bird-rider" warriors venture southward, taking with them the seeds of revelation and revolution. On the Moon, human colonies have likewise evolved. Those two worlds after the next Ice Age are very different places from today, but human emotions, personalities, and reactions remain unchanged.
The Thaw Trilogy is a tale of future conflicts-the clash of ancient institutions and ancient technologies on an unfamiliar Earth, and remnant human colonies on Earth's Moon.
The Thaw Trilogy, incorporates three stories that first appeared in Analog-"Thaw" (July/August 2013), "Flow" (November 2014 - Hugo nominated), and "Fall" (July/August 2016).