Entrances are connections between an underlying cave passage and the surface above. Most cave entrances are statistical accidents where the breakthrough to the surface is caused by collapse, by valley deepening, or by human activities such as road cuts, quarries, and other excavations. The number of caves with a certain number of entrances decreases rapidly with the number of entrances, leading to the prediction of a large number of caves that have no entrances. Entrances range in size over several orders of magnitude but there is no relationship between the size of the entrance and the size of the cave.

The Northwest Territories of Canada are ~1.2 million km2 in area and appear to contain a greater extent and diversity of karst landforms than has been described in any other region of the Arctic or sub-Arctic. The Mackenzie River drains most of the area. West of the River, the Mackenzie Mountains contain spectacular highland karsts such as Nahanni (Lat. 62° N) and Canol Road (Lat. 65° N) that the author has described at previous International Speleological Congresses. This paper summarizes samples of the mountain and lowland karst between Lats. 64–67° N that are located east of the River. The Franklin Mountains there are east-facing cuestas created by over-thrusting from the west. Maximum elevations are ~1,000 m a.s.l., diminishing eastwards where the cuestas are replaced by undeformed plateaus of dolomite at 300–400 m asl that overlook Great Bear Lake. In contrast to the Mackenzie Mountains (which are generally higher) all of this terrain was covered repeatedly by Laurentide Continental glacier ice flowing from the east and southeast. The thickness of the last ice sheet was >1,200 m. It receded c.10,000 years ago. Today permafrost is mapped as “widespread but discontinuous” below 350 m a.s.l. throughout the region, and “continuous” above that elevation. The vegetation is mixed taiga and wetlands at lower elevations, becoming tundra higher up. Access is via Norman Wells (population 1,200), a river port at 65° 37’N, 126° 48’W, 67 m a.s.l.: its mean annual temperature is -6.4 °C (January mean -20 °C, July +14 °C) and average precipitation is ~330 mm.y-1, 40 % falling as snow. In the eastern extremities a glacial spillway divides the largest dolomite plateau into “Mahony Dome” and “Tunago Dome”. The former (~800 km2) has a central alvar draining peripherally into lakes with overflow sinkholes, turloughs, dessicated turloughs, and stream sinks, all developed post-glacially in regular karst hydrologic sequences. Tunago Dome is similar in extent but was reduced to scablands by a sub-glacial mega-flood from the Great Bear basin; it is a mixture of remnant mesas with epikarst, and wetlands with turloughs in flood scours. Both domes are largely holokarstic, draining chiefly to springs at 160–180 m a.s.l. in the spillway. The eastern limit of overthrusting is marked by narrow ridges created by late-glacial hydration of anhydrite at shallow depth in interbedded dolostones and sulphate rocks. Individual ridges are up to 60 km long, 500–1,000 m wide, 50–250 m in height. They impound Lac Belot (300 km2), Tunago Lake (120 km2) and many lesser lakes, all of which are drained underground through them. In the main overthrust structures, the Norman Range (Franklin Mountains) is oriented parallel with the direction of Laurentide ice flow. It displays strongly scoured morphology with elongate sinkholes on its carbonate benches. In contrast, the Bear Rock Range is oriented across the ice flow, has multiple cuestas, is deeply furrowed and holokarstic but preserves pinnacle karst on higher ground due to karst-induced polar thermal (frozen-down) conditions at the glacier base there.