Brandywine Falls: A hangover from the Ice Age

In the summer of 1910, Jack Nelson, a surveyor for the Howe Sound and Northern Railway, and his axeman, Bob Mollison came upon a spectacular waterfall plunging into a rocky gorge near their campsite at Daisy Lake. Being surveyors they speculated on the height of the falls, and being betting men Nelson wagered his bottle of brandy against Mollison’s bottle of wine – closest estimate to the measured height takes all. The pre-metric height of the falls turned out to be 195 feet. The axeman got the booze and the name “Brandywine” stuck long after the bottles were drained.

Although Brandywine Falls was first measured and named in 1910 the sequence of events that lead to their formation goes back several thousand years. No one was around back then to witness the series of volcanic eruptions that flooded the ancestral Cheakamus valley with basalt lava flows. The vent and cinder cone from which they came has long ago disappeared – eroded away by ice that later flowed through the valley during the Fraser glaciation. But the lava survived and left a clue to its age – a snippet of charred wood beneath one of the flows gives a carbon 14 age of 34,000 years.

This date is within the Olympic Interstade, a nonglacial period between major episodes of regional glaciation. The lava flowed out across older deposits of glacial till, silt, and river gravel creating a layer of hard, resistant basalt lava on top of easily eroded, unconsolidated glacial deposits – a perfect setup for waterfalls.

Although waterfalls can develop in a variety of geological environments the combination of a resistant cap rock on top of weaker material is the most common scenario, particularly for free-falling types like Brandywine. The waterfall and its plunge-pool define a “nick point” in the stream gradient where most of the erosion takes place. Relatively little erosion occurs on the hard surface of the cap rock above the falls but in the violent turbulence of the plunge-pool the underlying weak material is scoured out until the overhanging lip of cap rock fails and breaks away. A new lip is formed and the fallen material itself is ground away in the plunge-pool.

When the Fraser ice receded from Cheakamus Valley some 10,000 years ago the stream that later became known as Brandywine Creek began to seek out a course across the newly exposed surface of the old lava flows. At some point, well downstream from the present falls, it dropped off the edge of the basalt and began to erode away the underlying till. Over the succeeding millennia the “nick point” that Nelson and Mollison christened Brandywine Falls has slowly and methodically chopped its way upstream to the present lookout at the end of a short trail from Highway 99. If there is anyone who hasn’t been there its well worth a visit.

Written by: Jack Souther

#Geology #Glaciers


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