They are bed heads. Putting them in a garden bed made sense. They'd be left for us in the shed by the previous owner. This bed will provide rocket, silverbeet, lettuce, peas, butter beans, purple sprouting broccoli, golden beetroot and, hopefully, purple carrots. The soil is clay, and carrots don't usually do well in it.
And this is what we're left with - a gigantic pile of weeds. There are only so many we can throw over the fence to the neighbour's chooks. Hopefully a healthy dose of racehorse poo will help it compost down quickly.