Flooring for a horse shelter?

I have a 30x30 shelter up right now for my horses and and starting to notice that when it rains the water starts to slope into the shelter.

Any suggestions on what I should use to built up the inside of the shelter to keep the water out.

I was thinking Aglime, or Mason and Torpedo Sand or a mixture of the three.

Thanks for your help.

I have aglime (I call it limestone screenings) in my dry lot area and in the run in sheds. We had almost 80 tons brought in this last spring. We have had several pretty good rains and its so nice not to have to deal with the mud. Its great stuff, after it packs down and gets wet a few times its like concrete.