Volcanic Springs

Geysers, Hot Springs,
Mud Pots, and Fumaroles

Sylvan Springs Group

The beauty of Evening Primrose Spring, which was said to be not inferior to Morning Glory Pool at its best time, once was the reason for the Park Service to maintain a trail into the center of Sylvan Springs Group. Unfortunately, at some unrecorded date between the 1959 earthquake and 1964, Evening Primrose Spring switched from a brilliant cadmium yellow appearance to a murky yellow-brown consistence. This caused the interest in visiting the remote location to fade and the trail deteriorated to such an extent that it is now virtually unrecognizable. Meanwhile the Sylvan Springs Group is a true backcountry area and on a visit everybody for himself has to take care of his safety.

Sylvan Springs seen from the Grand Loop Road:

Sylvan Springs Yellowstone

Sylvan Springs Yellowstone
One possible route starts at the Monument Geyser Basin trailhead and branches off to the right just before the trail begins to climb the steep grade. This route provides the advantage that you don't have to wade through Gibbon River. For some hundred yards it follows the river on its west side through light forest, than it crosses a patchwork of dry and wet meadow areas, bushy sections, and ribbons of standing and fallen trees. Hikers who are ready to experience true wilderness will like it, others who feel uncomfortable if boots and socks get wet and muddy will not be so happy with it. As everywhere else in Yellowstone backcountry be prepared to come across wildlife, especially bison and bears, and take care accordingly. Particularly worth mentioning is the huge variety of wildflowers, such as different types of orchids, Indian paintbrush, fringed gentian, or harebell, both on wet and dry sections of the meadows.

Our destination at this time was the center of Sylvan Springs Group and the prominent features there, so we left out the southern subgroup with Bridge Spring, and we also did not plan to visit the extended northern section, where mostly less impressive springs are located. From the meadows a good access to the southern subgroup is not easy to detect because it is obscured by trees. In contrast, the center area is visible from time to time during the hike and can't be missed. The hiking distance from the Monument Geyser Basin trailhead to the group's center is around 2.2 km (1.3 miles).
We started our exploration tour in the southwest at the small spring GSSGNN020, still surrounded by trees and thus living up to the name sylvan spring. Its pronounced sulfur lining and lack of orange cyanobacteria indicate acidic conditions.

GSSGNN020, a true sylvan spring:

Sylvan Springs GSSGNN020

Spring GSSGNN020 Yellowstone
While GSSGNN020 lies a little bit separated, the large, sulfur lined and constantly bubbling blue pool named Dante's Inferno leads you to the core area. But what has a hot spring in Yellowstone to do with the medieval Italian poet Dante? After all, there is neither a hot spring nor a geyser mentioned in his literary work. Some believe that the name "Dante's Inferno" was originally meant for the entire, vaguely amphitheater shaped group, which would make sense with respect to the nine concentric, descending circles of hell in Dante's poem Inferno from the 14th century. Others have pointed out that the extensive, stepped terraces the spring builds up with its runoff stand for this very levels of hell. See Valle del Diavolo (Valley of the Devil) in Tuscany for the historical connection between geothermal areas and Dante.

Dante's Inferno:

Dante's Inferno Yellowstone

Dante's Inferno Yellowstone
On the surface of Dante's Inferno water domes up every few seconds, sending out concentric, foaming waves almost over the entire pool. This is no effect of boiling since the temperature is only about 80 °C (176 °F), but of a mixture of rising steam and other gases.

Dante's Inferno, big bubble:

Dante's Inferno Yellowstone

Dante's Inferno Yellowstone
Just as remarkable, in particular for an acidic spring, are the terraces or terracettes in the widespread runoff area, exhibiting a pattern of delicate, partially sulfur coated scales on their horizontal surfaces. Even if the following picture looks like it was taken from a position inside the runoff, I actually stood on dry gravel at the edge of the terracettes.

Dante's Inferno, terracettes within runoff:

Dante's Inferno Yellowstone

Dante's Inferno Yellowstone
Dante's Inferno is still separated by a low ridge from the rest of the group. If you cross the ridge or go round it in eastern direction, you meet with the famous Evening Primrose Spring.

Evening Primrose Spring:

Evening Primrose Spring Yellowstone

Evening Primrose Spring Yellowstone
Obviously, the eventful chemical and microbiological history of Evening Primrose Spring is only fragmentarily documented, as to be expected due to the remote location. Around the beginning of the 20th century it was, according to the official Yellowstone Park website, a "deep crystalline spring similar to Morning Glory Pool". This implies that the spring must have been an alkaline pool near boiling temperature with transparent clear water of deep blue appearance. However, for this state the name "Evening Primrose" would not fit, and indeed it was attributed by the scientists Eugene T. Allen and Arthur L. Day not until 1926 (see Lee H. Whittlesey, Yellowstone Placenames).
At some time between the turn of the century and 1926 the chemistry of the spring must have changed from alkaline to acidic and an enrichment by finely dispersed sulfur must have taken place, which caused the appearance to turn from a blue spring with yellow rim into a vibrant yellow pool with a red hue around the edge. More recently, a similar sudden sulfur discharge into a formerly blue hot spring was observed with Crater Hills Geyser. From 1964 onwards (possibly earlier), as clearly recognizable on historical pictures in Yellowstone's Photo Collection, Evening Primrose Spring's surface got covered by thick, yellow-brown froth, consisting of almost pure sulfur. The next change occured between the summer of 1970 and the summer of 1971, when the temperature of the spring dropped from 90 °C (194 °F) to less than 80 °C (176 °F) and almost immediately an invasion of sulfur-oxidizing Sulfolobus bacteria took place (see T.D. Brock, Thermophilic Microorganisms and Life at High Temperatures). In this state the murky spring gave such a repulsive impression that some called it the ugliest pool of Yellowstone. The muddy brown cloudiness of the water continued at least until the year 2000, as documented by a picture in the RCN database. I found the earliest evidence for a noticeable recovery of Evening Primrose Spring from the Sulfolobus invasion for the year 2006, probably caused by a rise in temperatur. Since then the roiling yellow-green water, encircled by an intensely yellow colored rim, offers an impressive sight again. However, the spring remains to be acidic.

On arrival at the center of the Sylvan Springs Group a large canyon with bare ground opens up, winding down the slope of a hill form southwest to northeast until it reaches the level of the Gibbon River meadows. The next photo shows an overview with Sylvan Spring on the left side and Evening Primrose Spring in background on the right side.

View from the top of Sylvan Springs Group northeast towards Gibbon River:

Sylvan Springs Overlook

Sylvan Springs Overview Yellowstone
Moving on from Evening Primrose Spring to further springs is not an easy procedure. It takes both a lot of time and a lot of care to reach positions which provide a sufficient view on single features. Since the stability of the ground in between the springs is unpredictable, one should move as much as possible along the outer edge of the thermal area. A few feet north of Evening Primrose Spring the perpetual spouter GSSGNN036 shows up. With its appealing cliff design and colorful bowl it somehow reminds of a (giant) tabletop water fountain.

Spouter GSSGNN036:

Spouter GSSGNN036 Yellowstone

Immediately north of GSSNN036 the slightly opaque green pool GSSGNN037 adjoins.

GSSGNN037:

GSSGNN037 Yellowstone

Heading west from GSSGNN037 leads upslope alongside the area with the highest activity, where you first come across the muddy spouter GSSGNN045.

Spouter GSSGNN045:

Spouter GSSGNN045 Yellowstone

However, GSSGNN045 is only a premonition of the most vigorously spouting hot spring on location. The splashes of the strongly acidic Sylvan Spring may reach up to 1.5 m (5 feet) height. For this one I also found the descriptive but ambiguous name Lobster Claw Spring in a scientific paper, which seems in its literal meaning quite appropriate with respect to the knobby dentation along the edge.

Sylvan Spring:

Sylvan Spring Yellowstone

Sylvan Spring Yellowstone
The slope west of Sylvan Spring is occupied by some smaller perpetual spouters. Two examples are the extremely fine spraying GSSGNN057 and the cone-shaped GSSGNN059.

Spouter GSSGNN057:

Spouter GSSGNN057 Yellowstone

Spouter GSSGNN059:

Spouter GSSGNN059 Yellowstone

A widely audible thumping announces the next attractions, which are located 250 m (820 feet) northeast down the canyon. The beats are loud enough already to be heard more than 200 m (660 feet) outside the thermal area. They come from two large mud pots near the northern edge of the field. The first one, designated as GSSGNN027, intermittently pushes out bunches of mud droplets, at one time forming somewhat larger blobs, at another time fine spray. There is also a second vent present, lined with a rust-colored coating just as the first one, which primarily emits steam.

Mud pot GSSGNN027:

Mud pot GSSGNN057 Yellowstone

Mud pot GSSGNN057 Yellowstone
The second mud pot lies beyond a small ridge to the northeast. Coffin Spring is slightly larger as GSSGNN027 and even louder. It ejects mud squirts and flakes which often form bizarre eruption patterns. Obviously, the "raised lid of the coffin" grows permanently by adhering mud before it becomes too heavy to support its own weight and a part of the overhang crashes back into the spring.

Coffin Spring:

Coffin Spring Yellowstone

Coffin Spring Yellowstone
If you follow the northern edge of the thermal area farther east, you come across the perpetually spouting blue pool GSSGNN025. It provides a pleasant contrast to the predominantly muddy grey-brown springs on location.

Spouter GSSGNN025:

Spouter GSSGNN025 Yellowstone

Spouter GSSGNN025 Yellowstone
In the east Sylvan Springs Group is passing gently into the Gibbon River meadows. Spring GSSGNN021 was our last stop before we headed back to Monument Geyser Basin trailhead.

Spouter GSSGNN021:

Spouter GSSGNN021 Yellowstone


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