These rocks are formed by the phenomena know as columnar jointing and are defined as such:
Columnar Jointing:
A structure that forms in rocks (most commonly in basalt) that consists of columns (mostly commonly hexagonal in shape) that are separated by joints or fractures in the rock that formed when the rock contracted, most often during cooling.
This columnar jointing happens as lava cools underground so, as a result, there is not much columnar basalt exposed for us to see. In fact the Columbia River Gorge is one of the few places on earth where this can be seen so readily.
These basalt #boulders are frequently used in landscaping in this area as they are both useful and nice to look at.
This one is located about 100 feet from my house so I get to see it often.