What are those small fresh-water aquatic creatures with cone-shaped "shells" made froma collection of pebbles?

So I was in Shenandoah National Park and saw these creatures about 1/4 of an inch to an inch long in still pools of a creek and I wanted to know what they were called because they were very interesting. I didn't see what their inner bodies looked like exactly b/c they were inside little rod/cone-shaped shells that seemed to be made of little pebbles and sediment fused together, but their little heads would peak out of the little "trap door" of their shells and they looked crustacean/insect nymph-like. They would move around like snails and create little paths in the bottom sediment of the water pools or attach themselves to rocks. Anyway, they were very interesting and so I was just curious about what they actually were. thank you!