When you cannot use an internal fire, heating rocks are a great way to heat your shelter and stay warm. Never select rocks from near a water source, or any porous rocks for that matter, dense rocks are safer. Rocks found near water sources could have water trapped inside them. As the water heats, the rock will turn into a pressure bomb and explode.
One method of using heated rocks to warm your shelter is to place several large rocks in the fire and heat them for an hour or more. Dig a hole about a foot wide and 4-6 inches deep inside your shelter. Use sticks and such to move the heated rocks from your fire into the hole in your shelter.
Another method is for warming your sleeping area. Dig a trench in the area you will sleep in, again about 4-6 inches deep and body length. After heating the rocks place them inside the trench and cover them with dirt. You can experiment with the number of rocks to regulate the heat. When the rocks have baked the ground and add heated moisture to the area, place your bedding materials down and sleep warmly.