I found a “recipe” here:

http://www.state.tn.us/education/ci/standards/sci/9_12_earth_sci.shtml

From the site:

“Sandstone—Fill half of a small paper cup with sand. Make a mixture of half water and half-white glue. Add mixture to sand, stir, and pour off excess liquid. Mudstone–spread some wet mud in an aluminum pan. Leave undisturbed until the water evaporates. Rock Salt–dissolve salt in warm water until no more will dissolve. Pour the salt water into an aluminum pan and leave undisturbed until the water evaporates. Sandstone–Make a mixture of half water and half-white glue. Half fill a small paper cap with sand, add glue-water mixture, and sit. Pour off excess liquid. Leave undisturbed until the water evaporates. Conglomerate–follow instructions for sandstone, substituting a mixture of sand, gravel, and clay for the sand. Have students describe, sketch, and label each of the models above and compare their models to its sedimentary counterpart.”

So I’d say, use half school glue, half water, and then add your “treasures” and sand and let it dry.

***EDIT***

I found another way to do it:

http://www.childrenparty.com/partythemes/prehistoricparty.html

“Play Archeologist. First, mix two cups of sand, one cup of cornstarch, and one and a half cups of water in a large pan on the stove. Heat the mixture until warm, then shape a handful of the mixture into an egg around a small plastic dinosaur. Allow the egg to dry until firm. Make one egg for each guest, and hide the eggs in the yard or party room. Let the explorers hunt for the eggs and break open their discoveries when they find them!”

I definitely recommend using more sand then anything else – we’ve tried several of the commercial ones for our budding archeologist and more clay based of them took DAYS (I kid you not) of our oldest child and myself scraping away at them. (Probably more true to life but certainly frustrating for a child).

Good Luck!

Addendum: HS Mom – those are fabulous! I foresee some archeological projects in our near future! Thx!