Oxford Sinkhole System
To log this earthcache, please don’t stress about answering the questions. Simply send your best attempts in a private message to me, (the cache owner), and then go ahead and log it as found.
You don’t need to wait for my approval. All attempts will be accepted.
Go ahead and have fun learning! 
- [REQUIRED] Please post a photo in your log of yourself or a personal item at one of the curved tree trunks to prove you visited the site.
- Walk around the pond and look at the surrounding terrain and the slope of the land around it. What makes it fairly obvious that this is a sinkhole?
- Look at the trees. Can you find any curved trees that show evidence that this is a sinkhole?
In late July, 2018, a small sinkhole was observed by members of the Oxford Area Lions Club near the western end of the parking lot in the privately-owned Lions Parkland in Oxford, Cumberland County.
A sinkhole is a natural depression or hole in the Earth's surface which may have various causes. Most natural sinkholes are caused by the chemical dissolution of water-soluble carbonate rocks or gypsum. Sinkholes may vary in size from 1 to 600 m both in diameter and depth. Sinkholes may be formed gradually or by sudden collapse, and are found worldwide in karst areas.
Karst:
A landscape that has been eroded by dissolution, producing ridges, towers, fissures, sinkholes, and other characteristic landforms. Karst landscape is not unique to Nova Scotia, it occurs over about a quarter of the earth’s surface.
The term "sinkholes" has also been used to describe ground subsidence caused by anthropogenic activities, for example, the collapse of old mine workings close to the surface, also known as abandoned mine openings.
Generalized model for karst topography and sinkhole development in Nova Scotia
The sinkhole in Oxford, Nova Scotia was described as approximately 60 cm deep by 100 cm wide. On August 9, 2018, approximately 12 tonnes of shale were used to fill the depression back to surface grade. On August 10, the area again subsided, resulting in a square-shaped depression approximately 1.5 m2 and 30 cm deep. The area was cordoned off and monitored until August 20, when a resident witnessed the development of the sinkhole. At that time, emergency responders were called to the scene and the park was closed to the public. From August 20 to early September, 2018, the sinkhole grew rapidly from a few metres to approximately 40 m in diameter. The underground extent of the collapsed cavern is presently unknown.
Lion's Head Club sinkhole
In Nova Scotia, most natural sinkholes or karst topography are formed over areas where Windsor Group gypsum occurs in the near-surface. The geological boundary between the gypsum- and salt-bearing Windsor Group and the underlying sandstones and shales of the Horton Group or older basement rocks is particularly prone to sinkhole development.
In areas of sinkhole development, the primary geohazard is related to sudden catastrophic subsidence due to the collapse of cavities in the bedrock created by the dissolution of the gypsum and/or limestone. Caution should be exercised in these areas prior to the construction of buildings or roads.
It is important to note that 90% of new sinkholes in populated areas are induced by human activity. The two main causes of human-induced sinkholes are water table decline and changes to water drainage patterns. Avoiding or limiting these activities can reduce the risk of future subsidence. Excessive groundwater pumping from water wells in karst areas can lower the water table, which increases the depth of vertical drainage in the soil above the bedrock. The increased vertical drainage promotes soil erosion and transport in the subsurface, leading to void development and subsidence. Any changes to natural surface water run-off and infiltration patterns can lead to localized increases in water input, which increases the risk of sinkhole development. Surface water run-off from houses, roads, parking lots, and other developments should be piped away to established surface water drainage pathways.
For a typical sinkhole, the ground is slowly slumping downhill over time. A stake in the ground would eventually lean over further and further, with the direction of the lean pointing downhill, toward the sinkhole. However, trees growing near a sinkhole will continue to grow straight upward. So after a time, the base of the tree will curve upwards from its base. For older sinkholes that have started to fill in, there are also frequently very high “walls” around the sinkhole showing that there was a dramatic drop in ground level.
Curving trees near the Oxford Sinkhole area.
The geology of the Oxford area is structurally complex, and there are few outcrops, and few historical wells and exploratory drill records to supply subsurface information. The northern and western limits of the Windsor Group in the Oxford area are interpreted to be in fault contact with the Upper Carboniferous Cumberland Group. The nature of the southern and eastern contacts is unknown. Based on the presence of interbedded evaporites, slates, and mudstones, the Windsor Group strata are interpreted to be folded in diapiric structures, regionally expressed in the east-northeast-trending Claremont-Malagash anticline, with localized zones of increased deformation (Boehner, 1986). Regional geophysical surveys help define the extent of the Windsor Group and diapiric structures in the area, but the information is too limited in scale to model individual voids in the subsurface (see Nova Scotia Department of Energy, 2017, and references therein).
Aerial view of the Oxford Lion's Head sinkhole (circled in pink) and the surrounding area with sinkhole chain.
The sinkhole at the Lions Club is part of a sinkhole chain that spreads across the area to the southwest. You can see from the following Lidar map that the chain covers a much larger area than most people think. There is a trail that runs parallel to the highway that you can follow to the earthcache location. This trail has already been populated with geocaches so will be easy to find.
Map by: Sean Smith (halepauhana153)
Map by: Sean Smith (halepauhana153)
Since the Lions Club sinkhole is not accessible to the public and is currently being filled in by the new owners, we will be studying one of the additional sinkholes in this chain.
Map by: Sean Smith (halepauhana153)
