What is the origin of the large nitrogen glacier on Pluto revealed by NASA’s New Horizons spacecraft in 2015?

What is the origin of the large nitrogen glacier on Pluto revealed by NASA’s New Horizons spacecraft in 2015?
The nitrogen ice glaciers on Pluto appear to carry an intriguing cargo: numerous, isolated hills that may be fragments of water ice
New data points to more prevalent water ice on the dwarf planet’s surface
The newest high-resolution shots of the dwarf planet are both dazzling and mystifying
Just 15 minutes after its closest approach to the dwarf planet, the spacecraft looked back toward the Sun and captured this stunning near-sunset view
Pluto has been revealed to be a true icy world of wonders in the latest images