Complete this phrase small as a mouse, big as a

I am very big and made of ice. I form in cold places where snow piles up for many years. I move slowly down mountains like a frozen river. What am I?

A crown of bone that falls to the floor, to branch and grow just like a tree once more. I am cast aside when the cold winds blow.

In parks and forests, you are encouraged to only take photos and leave nothing but us. What are we, that follow your every move?

I once took flight upon the air, now ink my thoughts with careful care. On beasts I stand with pointed will, both word and ward I serve them still.