Chalk is a sedimentary rock of biochemical origin. It is soft, white and porous. It formed in the deep ocean far from land via the gradual accumulation of the calcite shells of coccolithophores. Layers of chalk are known that are hundreds of feet thick, indicating extremely long periods of accumulation.
See Steve's video interview about asteroid capture at Moonandback:|
See Steve's blog
On The Future
A Project Plan for Space Based Solar Power