English: South Fork of Tantalus Creek, southwestern Back Basin, southern Norris Geyser Basin, Yellowstone Hotspot, northwestern Wyoming, USA on on 9 August 2011. (looking ~S)

The South Fork of Tantalus Creek flows north and drains the Gray Lakes area of Norris Back Basin. The water is warm to hot, acidic, chloride-rich, and sulfate-rich. The green coloration in the channel on the left is from colonies of unicellular, photosynthetic, extremophile organisms - Cyanidium caldarium in this case. Confusingly, despite its green color, Cyanidium caldarium is a red alga (Rhodophyta, Rhodophyceae, Cyanidiales, Cyanidiaceae). This genus of red algae is known to prefer hot, acidic water environments.

The hill in the distance is composed of rhyolitic ash-flow tuff of the Lava Creek Tuff (lower Middle Pleistocene, 640 ka) and covered by Upper Pleistocene glacial till.