Photo Credit: Norma Kerby
Unseen Labyrinth: Northern BC’s amazing limestone karst topography
The dog slips farther into the hole and it becomes suddenly apparent that we’ve found exactly what we were expecting.
After yanking up the dog and carefully tiptoeing across snow patches, unsure of what lies beneath, we take GPS readings and put out some discrete flagging tape. This is a sensitive area and too much human traffic could damage the fine-featured surface for which karst topography is known. It is a secret best left to a few.
Karst topography is formed through the erosion of a specific type of bedrock: limestone. Made up of deposited calcium carbonate seashells and casts, limestone is unusual in that, once dissolved in water, it leaves only the impurities. In comparison, the weathering of rocks such as granites and sandstones generates mineral and sediment fragments. For limestone, weathering by water leaves behind cavities, caves and a myriad of other spaces and depressions that generate a landscape of significant ecological richness.
In early September, Campbell River-based karst expert Paul Griffiths and co-researcher Carol Ramsey, accompanied by two colleagues from the Karst Research Institute in Europe, toured northwest British Columbia, examining karst features found on Haida Gwaii and the adjacent coastal mainland. In particular, they gathered information regarding forest dolines (karst sinkholes) found in the region’s wet, windward mountain areas.
Considered an important resource, karst is closely managed in European countries such as Austria and Switzerland. But, according to Griffiths, karst caves and other limestone erosion features are not adequately protected in BC. Although debated during adoption of the Water Sustainability Act, the role of karst topography as an underground water transportation system has not been fully recognized.
The reason for limestone’s solubility is its chemical composition: calcium carbonate. Upon reaction with acidity in water, limestone dissolves into calcium and carbonate ions, which are carried away in surface or subsurface water. Acidity in groundwater can be formed as simply as having carbon dioxide in the soil dissolve into the water, forming a mild acid known as carbonic acid, which then eats away at the limestone. In the coniferous-forest-covered coastal zones, surface and groundwater can become very acidic when carbonic acid is combined with coniferous leaf litter. With large amounts of annual precipitation along BC’s north coast, limestone’s rate of chemical weathering can be rapid.
Over time, as acidic water moves over and through limestone bedrock, it begins to shape these landforms. For surface-water movement, features known as epikarst develop in the upper layers. Epikarst features include surface rills, crevices, small holes and even extended platform cavities, protected only by a thin surface cover of bedrock.
In the area we’re searching, the entire top of the bedrock ridge is covered with micro-karst features. Considering the amount of rainfall on the north coast, many of these small cavities form important dry niches for a myriad of spiders, beetles, centipedes and other invertebrates, plus well-protected burrows for rodents and small predators. Pocket caves and vertical surface crevices can become important bat habitat. The tiny caves we’re examining team with life that uses the piles of mammalian droppings.
The value of epikarst surface features is not only for animals and water movements. Most soils in northern BC, except for floodplains and other alluvial environments, tend to be acidic and nutrient deficient. Soils on rock containing calcium carbonate, though, are not acidic and the impurities in the ocean-deposited limestone make the soils that form in these environments nutrient rich, producing special plants, fungi and lichens such as spleenwort fern, fragile fern and Menzies’ neckera moss. Often, a limestone deposit’s original forest before harvesting had enormous trees that were bigger and healthier due to the rich soils and complex hydrological subsurface environments.
Epikarst is fragile and susceptible to damage from movement of people and equipment, as well as from vegetation removal. Soil cover is often thin and the fine limestone bedrock features are very close to or exposed at the ground’s surface. What has taken at least 9,000 years to develop can be destroyed in minutes, along with the unique ecology associated with it. Any type of activity that crushes the delicate limestone erosion, such as road construction, trails and even livestock movement, can have devastating impacts. This complex habitat of holes and cavities, once lost, will take thousands of years to form again.
But the value of karst topography does not stop at the surface. The most spectacular features are the caves and underground streams and rivers that flow well below the visible landscape. In the Northern Rockies east of Prince George, Haida Gwaii and other portions of the north coast, the Atlin area and the Mount Pope area near Fort St. James, well-documented cave systems give witness to this amazing underworld of stalactites and stalagmites. Some in the Northern Rockies are amongst the deepest in Canada.
Many of the caves, which are defined as a bedrock cavity big enough to allow the entry of a person, date back to the Pleistocene, a geological epoch ending with the last ice age, and can contain plant and animal fossils from that time and later. They also can be important archaeological sites with cultural significance for many First Nations. Most limestone deposits in northern BC are late Paleozoic to mid-Mesozoic in age (over 200 million years old) and caves can provide access to important fossils embedded in the limestone.
Where these underground caves interact with the surface, dolines, sometimes referred to as sinkholes, form a connection between cavities and channels and the above-ground environment. The ecology of these sinkholes or other karst surface depressions can be very unique. Dense, cold winter air from surrounding slopes can drain into these depressions and, during the growing season, direct sunlight might never reach the sinkhole bottom. The plants and animals residing in limestone depressions often can be from biogeoclimatic zones higher in elevation or further north than the surrounding forests.
In addition to caves’ spectacular geological features, their value for bat colonies, winter dens, shelter for wildlife and unique habitat for species specially adapted to caves (troglobites) or those that prefer caves but can live elsewhere (troglophiles) make limestone caves important for their ecological, recreational and aesthetic values.
We show Griffiths and Ramsey some of the karst topography in our region and their consensus is that it is as valuable as any other karst areas found in Western Canada, despite MLA Scott Fraser twice failing to pass an act protecting the values of limestone caves in this province. Combined with that is the potential for limestone removal near proposed industrial development along the north coast.
What the future will be for our northern limestone deposits remains to be seen. As for the little section of eroded limestone that almost swallowed our dog, we removed the flagging tape and erased all evidence of our passage, hoping that the fragile epikarst of this ridge would be there for future generations of wolf spiders hiding in the cracks.