I started in an isolated corner of the map. It comprises less than one 16th of the total map space, and in order to get stone, I have to mine around the local obeliskians It's a dangerous job, but someones gotta do it. EDIT: Turns out obeliskians like to lull you into a false sense of security. Also, it wasn't just one assemblage, it was several assemblages stacked on top of each other. Thankfully, the obeleskians were incompetent, allowing my squad of rookies to defeat them.
There isn't any way to do so at the moment. But wow, that is scary! That lakeside view wasn't all it was advertised to be.
In the past I've actually flattened terrain such that water turned into land. However there's two caveats: The "water" which is actually the map border can't be used, obviously At some point colonists cannot go out into the water (too deep), so you can reclaim up to a point, then past that it's no-go
