The Lush solid shampoo that I use occasionally is solid sodium laurel sulphate compressed into a cake like soap. It works pretty well as a shampoo, better than soap on long hair because soap can leave scum and also the alkalinity damages hair.The lush stuff is talc and climbing chalk.
On a trip though I really don't bother with hair washing other than in plain water. A bit of grease makes it less prone to tangle and easier to comb!