Why won't my patio base (crushed stone) drain water?

I found a lot of shale rock on a pasture and thought it would work well for a crushed stone base under a flagstone walkway we are refurbishing. It crushed quite easily, and when I compacted it made a very firm, stable base. I tested the base to see how well it would drain, and the water just sat on top. After the small amount soaked through it left the surface looking slimy/muddy. Did I do something wrong? Should I not have crushed the stone so finely?