Subject: What type of soil filters water best?

I am doing a science project based on the above question.  I am familiar with the 
earths natural filtration.  I have also built a model using sand and gravel. Is 
sand considered a type of soil? If so, can it be said that sand filters water 
best?  I've used sample soil from different areas: from the park, the wood, and 
the garden.  Any recommendations on what else I could try?  Sand and gravel 
worked best, I'm just not sure if sand is considered a type of soil.

Re: What type of soil filters water best?