I didn't take this photo. There were other people in the spring, so it would have been uncool to take a picture.
Yes, that's an outcropping of travertine marble. The hottest pool was very hot today, but there are several pools, each less hot than the other.
There's a ranger station on US395 on the south side of Bridgeport. Turn east on the road just north of the ranger station. Continue on the dirt road marked with an official sign. Part of the road requires some adventurous driving.