Hiking Tunnel Mountain

Tunnel Mountain is a remarkable remote geosite within the Tumbler Ridge Global geopark. While it is best done as a day-trip from Windfall Lake, it can just be done in a very long day from the Windfall Lake trailhead.

Anticipate a lot of elevation change and alpine ridge walking, where visibility is limited if there is cloud cover. Route finding and navigational skills are essential.

The easiest way to climb Tunnel Mountain is simply to follow the southeast ridge to the summit, but this omits the enthralling experience of “climbing through the mountain”, entering at the lower entrance of the Tunnel and emerging higher up the mountain at the smaller upper entrance.

The final approach to the lower entrance is across and up a steep scree slope. Use caution here in the loose rock.

There are a number of similar limestone passages in the Tumbler Ridge Global Geopark, but this tunnel is one of the largest and is one of the few that is open at each end (many of the others are choked at one end, or head deep underground). The limestone rocks that form Tunnel Mountain are from the Rundle Formation, and are of Mississippian age.

 

Tunnel Mountain Geology

Kevin Sharman

The major structures that make up Tunnel Mountain are a very tight anticline/syncline pair which are slightly overturned on one limb (photo 1), with the cave in the core of the syncline.  There are many faults on the limbs of the folds.

There are two faults that are responsible for the cave’s formation (photo 2).  The lower of the two faults is slightly above the floor of the upward sloping passage.  The bedding flattens above the cave, and it is cut off by the upper fault.

The cave has been formed in the shattered rock around the faults.  The lower fault plane is visible at the upper entrance (photo 3).  At the lower entrance, there is water seeping along the fault plane, which probably helped to dissolve the rock.  Above the cave there is an intensely fractured fault zone with abundant calcite veining (photos 4 and 5).

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