Consider the following statements:

1. Chalk is a comparatively soft, white and very pure form of limestone.
2. Chalk country may contain dry valleys known as coombes and has little surface drainage.
3. Extensive swallow holes and underground cave networks are generally less developed in chalk because of its friable nature.
4. Chalk invariably forms rugged, barren highlands unsuitable for either grazing or arable farming.

Which of the statements given above are correct?