You have you pull yourself up on a rock using your hands and feet.I'm a little confused with the word "rock'', because sometimes it is
uncountable and sometimes countable. Could you please explain it to me.
Thank you for speanding your time answering my question.
He's climbing on a particular rock. It's not referring to "rock" as a type of substance. So it is a rock. Where people go rock climbing there are usually numerous rocks in the landscape.