How do I create a mounded border ?

I would like to create a mounded border. This may not be the correct term, but what I am trying to do (have already started) is create a mound (no higher than approx 2-1/2 feet at its highest point) that sits between two gravel paths. It will be about 20 - 25' long when I am finished. What I have done so far is put down a base of several inches of a sandy loam, then load up with a mixture of my native clay mixed with lots and lots of compost on top. However, when I try to water my mound the water immediately runs down the sides of my mound,taking the soil with it, and into the gravel path. This tells me maybe my mound is too steep and I should lessen the angle, but .... I thought I'd just ask if anyone has any experience with doing something like this. I would sure appreciate some advice !!