In adults: Red marrow is found mainly in the flat bones hip bone, breast bone, skull, ribs, vertebrae and shoulder blades In the cancellous (spongy) material at the proximal ends of the long bones femur and humerus. Yellow marrow is found in the medullary cavity the hollow interior of the middle portion of long bones.In a child, all bone marrow is red bone marrow and can be found in nearly all flat and long bones, including the epiphysis and diaphysis.As the child ages the red bone marrow in the diaphysis of the long bones will be replaced with yellow bone marrow.ellow marrow is found in the medullary cavity the hollow interior of the middle portion of long bones. In a child, all bone marrow is red bone marrow and can be found in nearly all flat and long bones, including the epiphysis and diaphysis.