Angular Gravel, full range of size sand to fist size on floor, is what a raw tunnel has on the floor. Geologist,Caver and Miner in my past. Your tunnel appears either carved out of SOFT stone or it is water made. If soft stone needs tool marks and debris(see above). If water made, needs more dimensional irregularity, perhaps with tool marks showing modification. Great work so far ,esp textures. I've beat this dead horse enough. Keep on Mike