Want to see more photos of Mt. McKinley glaciers?  I forgot to define a few terms relating to mountain and tidewater glaciers a couple of posts back.  So, this post will attempt to rectify that omission.

Tarns are small lakes that fill cirques, the circular snow accumulation bowls on mountain slopes.  An arête is a sharp ridge of rock that is left between two cirques that are being eroded toward each other.  Arêtes may be seen on many peaks along the Icefields Parkway in Banff and Jasper NP and on the slopes of Mt. McKinley and other mountains in the Alaska Range.  My first 2000 photo below shows a classic bowl-shaped cirque on Mt. McKinley surrounded on three sides by arêtes.  The second photo, taken from slightly above the ridge, shows how just narrow and steep one of Mt. McKinley’s arêtes can become.  The photo also clearly shows the fracture (“joint”) pattern of the very hard, erosion-resistant granite that forms Mt. McKinley.

 

 

A horn is a sharp-pointed, pyramidal mountain peak formed by the“headward erosion” of several cirques. The Matterhorn in Switzerland is the most obvious and most famous example.  But my wife’s 2007 photo of cloud-shrouded Mount Saint Nicolas in Glacier NP, Montana is also a great example, with four cirques, one even containing a tarn, surrounding its peak and creating its classic pyramid shape.

 

I mentioned earlier that glaciers are very effective bulldozers and can excavate large volumes of rock as they move.  When a glacier flows down a “V”-shaped mountain valley originally formed by stream erosion over many thousands, or even millions of years, it can quickly widen and deepen it into a classic “U”-shaped glacial valley, sometimes in only a few hundred to thousand years.  This gouging out of rock by ice is exactly like using a piece of coarse sandpaper on a newly cut 2x4.  The sandpaper quickly smoothes the end of the lumber, but, due to the coarseness of its grit, it may also knock splinters off the corners.  And, just like coarse sandpaper produces large quantities of sawdust, the glacier produces huge volumes of “rock dust” (actually called “rock flour,” but known to geologists as “silt”).

The glacier picks up everything from sand to boulders and carries them along, frozen into the bottom ice.  The sand and gravel may smooth and polish or groove ("striate") the bedrock surface over which the glacier is flowing, but the boulders may knock off huge chunks where they hit an ice-wedged fracture (joint).  The glacier can also erode by “ice plucking,” where water freezes in those joints and, over time, loosens the rock sufficiently that it can fall onto, or is picked up by, the glacier.

What happens to all that rock debris that the glacier creates, incorporates into its ice, or pushes ahead as it moves downslope?  That debris is partly carried along by the ice and partly carried away from the ice by glacial meltwater streams.  The part carried by the ice is a random mix of every size rock particle.  The part carried by the streams is “sorted” by the running water (just like a sluice box sorts out placer gold from sand).  First, the boulders are likely not moved much by the water, the cobbles are dropped out a short distance downstream from the toe of the glacier, and the sand is carried greater distances downstream. 

Obviously, a tidewater glacier can drop all these particle sizes directly into the ocean.  But, in a mountain glacier terminating some distance from the coast, only the very fine “rock flour” may reach tidewater.  Because this silt-sized sediment is so fine and so easily agitated by waves and currents, it may take years to completely settle out, so may move far out to sea.  My 2000 photo below shows a tan-colored “rock flour” plume being discharged into the deep green water of the Lynn Canal near Haines.  The plume is coming from the Katzehin River, which originates as meltwater from Meade Glacier.  This glacier is actually located east of the Lynn Canal in the Coast Range, about 25 miles southeast of Skagway along the Alaska-British Columbia border.

 

This same rock flour/silt is found in Lake Louise and Moraine Lake in Banff NP and at many glacier-fed lakes like Portage in Alaska.  In those lakes, however, the water is a turquoise blue that attracts every photographer who sees it on a sunny day.  My photo is probably unusual in that regard, as few photographers are attracted to "dirty water," even if they know the cause!

Next time: How do plate tectonics and continental drift really work?  A kitchen experiment involving hot cocoa (hot chocolate), marshmallows, and melted butter provides an analogy that is easy to understand, but difficult to swallow!