Sulfide Caves in Devil's Hopyard: Karst Features? EarthCache

Sulfide Caves in Devil’s Hopyard:  Karst features?

The Devil’s Hopyard State Park is underlain by rocks of the Brimfield Formation.  The Brimfield Formation is made up of schist and gneiss that contain significant amounts of sulfide minerals₁, notably iron-sulfides such as pyrite and pyrrhotite.  When pyrite is exposed to water containing dissolved oxygen it weathers according to a chemical reaction₂ and forms rust (iron-oxide minerals limonite, hematite, or goethite) and sulfuric acid.  The rock is rendered weaker because the space where the pyrite formerly resided becomes a hole in the rock.  Sulfuric acid then dissolves other constituents of the rock, further weakening the rock. Precipitition of iron-oxide minerals renders the rocks rusty and rocks of the Brimfield Formation are notorious for their rusty-weathering.  This EarthCache explores some examples of cave formation associated with sulfide weathering of the Brimfield Formation at the Devil’s Hopyard State Park₃.

Location:   East Haddam, CT N. 41^o28.640’, -072^o26.887’

Listed by: CTGeosurvey

Directions:  To get there, follow highway signs on Rte. 82 to the Devil’s Hopyard State Park and take the orange trail₄ south.  You will find two parking areas.  We suggest you park in the upper parking area near Chapman Falls rather than the lower parking area near the picnic grounds.  That is because of another EarthCache located at the Falls₅ and because you will pass an example of sulfidic weathering (Figure 1). 

Follow the trail along the west side of the waterfall to a bridge where it crosses a tributary to Eightmile River. Along the tributary stream just south of N. 41^o29.040’, -072^o20.549’ there is a large overhang on the valley wall that resulted from the erosion of a presumably sulfide rich layer.  Solution of the sulfide minerals and creation of sulfuric acid rendered the rock susceptible to erosion by groundwater seepage.  That is, ground water seeping from the rocks actually eroded the rock one grain at a time.

Figure 1.  Small cave in an overhang on the valley wall of a tributary to Eightmile Brook near Chapman Falls. (Tree trunk is about 6” in diameter for scale) 

Several other factors may also have contributed to this erosion, such as rainwater and ice wedging) of individual grains.  It is also possible that the stream was at this level during cave formation, as it had not eroded its canyon down to its present level. If this was the case, then running water from the stream would also have contributed to the erosion.  The resulting large overhang now extends into the rock 4 or 5 feet.  It is a small cave!

Continue down the trail past some rusty outcrops and eventually to a picnic area where you will find a covered bridge that takes you to the orange blazed trail.  Follow the orange trail south to a point due west of the given location.  A trail goes steeply uphill to the Devil’s Oven which is a small cave developed along a fracture in sulfidic schist and gneiss (Figures 2a and 2b).  It is approximately three feet wide and extends into the cliff about 20 feet, becoming progressively narrower.  The cave is sort of tear-drop shaped in cross section.  Limestone caves often have this shape because of formation at a water table surface sometime in the past.  It is possible that the Devil’s Oven also records the elevation of a former stream level or water table at a time when the valley was not as deep as it is today.

   2a.                                                                        2b.

 Figure 2a. The Devil’s Oven, a cave developed along a fracture in sulfidic schist and gneiss.  Cave is about three feet wide.  2b.  Looking into the cave. 

If you follow the trail around to the left (north) you will come to additional small caves, one of which is illustrated in Figure 3.  This is a tunnel like cave formed in the same rock formation as the Devil’s Oven.  This, however, did not form along a fracture and as a result is not very high.  It is a long tunnel that has a small window (perhaps a chimney?) at its end.  This cave is about 25 feet long. Most of the caves are found at a similar elevation (~250’).  Possibly all the caves formed at the same time when groundwater was more acidic and/or more oxidizing.  For the groundwater to be at this elevation the valley must not have been as deep as it is today.

Figure 3.  Small cave north of Devil’s Oven.  Cave is about 3 feet high at opening.  Note light inside the cave:  that is a small window at rear of cave.  This cave is a tunnel.

Study the topographic map (Figure 4) and notice that the upland surface on both sides of the Eightmile river canyon is roughly about 450+’ in elevation.  The canyon is around 250 deep.  The sides of the canyon contain areas with somewhat hummocky topography.  See particularly the west side between Chapman Falls and the tributary brook near the southern park boundary.

In addition a couple of areas have closed topographic contours (notably just west of Foxtown Road , see magenta circle on Fig. 4).  These features are similar to what is found, on a much larger scale, in karst topography.  Karst topography develops on limestone or marble, rocks that are relatively soluble in acidic groundwater.  The features at Devil’s Hopyard are much more subtle, but we maintain the topography at Devil’s Hopyard was formed by similar processes.  The sulfidic schist is not as soluble as limestone or marble and hence the topographic features are more subdued.

        Figure 4. Topographic map of Devil’s Hopyard State Park and surrounding areas.  Contour interval is 10 feet.  Solid magenta dot marks location of Devil’s Oven.

If you return to the orange blazed trail and continue a short distance south you will be rewarded by a magnificent vista in which “a long stretch of the river is visible,” (Leary, 2004, p. 66-67.)