Strange Maps link explains the Land Ordinance of 1785, which is why one Colorado blog is called Square State.
Actually, it is rectangular, not square, and a little narrower on the north side due to converging lines of longitude. But why quibble?
According to Thomas Jefferson-impersonator Clay Jenkinson, President Jefferson imagined a West full of rectangular states. Jenkinson once solicited names for these imaginary states: I suggested that at least one of them should be called Artemisia, for Artemisia tridentata. (Just look at the distribution maps.)
Once I was on a London-to-Denver flight sitting ahead of two English guys who had booked a ski trip in Breckenridge, their first visit to Colorado. The airplane began its slow descent towards DIA, and somewhere over Sterling I heard one of them exclaim. I turned around, and he was staring down at some road, maybe Colorado 71 north of Stoneham, as it ran ruler-straight towards Wyoming. Miles of straight road--not a common sight in the UK.
You can blame the mapping of the American West on the post-revolutionary French, with their mania for straight lines and "reason." France itself almost ended up looking like this.