I'm looking to find a story I read years ago in a paperback of alien invasion shorts.
The story takes place in what seems like North America hundreds if not thousands of years before the Europeans arrived. A tribe of Indians stumble across a growing number of geometric shaped aliens that are - if I remember correctly - multi colored and gelatinous... in my mind they are like violent giant intelligent Jello molds - large triangle shaped I believe. The aliens start to grow in number and the Indian tribe (or just part of it) attack and fight with the invaders. If I remember correctly the Tribe of warriors manage to defeat the aliens in the end - most of the actin takes place in a large grassy area surrounded by forest and maybe mountains.
I remember reading it in the early 1980s.