First of all, I'm glad we're finally on to something recognizable as a religion in the modern sense. We know enough about the Midwestern Native Americans of the past couple millennia to be able to make reasonable assumptions about what their artifacts mean, and they were biologically modern so we know they had the capacity for symbolic thought, etc. Also oral tradition of the tribes involved (the Ho-Chunk were mentioned) seems to me to be a very valuable source of interpretation.
In most Wisconsin public schools, children in later elementary school do a unit in Social Studies about state history, covering (generally) the ice age, Native Americans including the mound builders, the arrival of the French fur traders, STATEHOOD, and modern goings-on. I was no exception, and my class actually took a field trip to a park in the Milwaukee-esque area that had some mounds preserved. [As a side note, the effigy mounds (and Indian culture in general) are so prevalent in the history of the Milwaukee area that one of the major roads near my house (in Wauwatosa, which is derived from a Potawatomi word) is named for them - Blue Mound Road.] Effigy mounds are very unusual things -- they are quite large in the horizontal dimensions, and tall enough and with steep enough sides that you know something's going on, even if it's an area that was visited by The Glacier [on another side note, I sometimes wonder if glacial features like moraines are ever mistaken for mounds ... though probably not, since the sediment would be very different]. I didn't comprehend it fully at the time, but this is an artifact of a people whose religion was very much a central part of their lives (though perhaps not to the extent of the ancient Egyptians), and they had a very well-developed and complex mythology that motivated this construction.
One of the things I found particularly intriguing about the things revealed by the effigy mounds and their patterns was the related categorization of groups based (primarily, it seems) on geography, and the hierarchy implied by this, since the spirits from which they derived their names had specific temperaments and powers. To me, this implies also a stratification between groups and their "classes," and it seems like an almost unintuitive system. Of course the Warrior group will be happy with their lot, they're the best and most powerful. But what about the lower classes? I mean, if you were a Worm, but you could've been born a Thunderbird, would you be happy/proud? I'm sure there was some conflict between tribes of different status -- the chapter specifically mentions signs of violent death in some exhumed bones. But at the same time, this divisionism persisted. It seems to me that (given my somewhat cynical view of human nature) this indicates an appropriately full understanding of the importance of balance, something that was mentioned frequently in the chapter (particularly regarding the use of non-site-type effigies) and that seems to be somewhat lacking recently (cynicism again).
In general, the unity over large areas is a very impressive characteristic of this religion, though it's not strictly "community" in this sense since they probably rarely met. And in those cases there was killing.