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Monday, 27 August 2012

Erratic clusters and alignments

The other day I came across a very interesting study of glacial erratic distributions in Washington State, in the NW of the USA. Researchers have plotted the locations and types of thousands of erratic boulders on the flank of Rattlesnake Mountain.  The erratics apparently became visible for the first time following very extensive forest fires which effectively cleaned off the vegetation cover, leaving a bare land surface.  The interesting thing is that few of these erratics have been directly dumped by glacier ice coming from the Cordilleran Ice Sheet.  Instead, they have been carried and emplaced by floating and grounded icebergs in periodic paleofloods and temporary lakes -- with water up to 800 feet deep at times.

The erratics appear to be of many different ages -- suggesting that many have been picked up, carried and dumped on several different occasions, each one coinciding with a new glacial episode.

There are many single erratics, erratic clusters and erratic trails -- but in this case the trails appear to be perpendicular or transverse to the main direction of iceberg movement or water flow. Erratics do indeed behave in a myriad of erratic ways......


The rocks were left after ice dams holding back a huge lake near Missoula, Mont., broke, re-formed and broke again from 1 million to 2 million years ago to as recently as 13,000 years ago. The floodwaters backed up at the downstream end of the Pasco Basin behind Wallula Gap, a narrow ridge opening through which the Columbia River flows today. The lake lapped the gentle slope of Rattlesnake Mountain, northwest of the gap and part of the Hanford Reach National Monument in south-central Washington Rattlesnake Mountain was the highest peak protruding from the 800-foot-deep temporary body of water, dubbed Lake Lewis. The rocks and boulders, so-called erratics, grounded as the waters of Lake Lewis receded after a few days like a slowly draining bathtub.
Bjornstad led a team that surveyed and analyzed the mostly-granite-strewn debris fields over 15 square miles of Rattlesnake Mountain. He said that the ice-rafted debris left deposits of three types: widely scattered rocks and boulders, distinct clusters and "bergmounds" – low, cone-shaped clumps of erratics that, like a moraine left by glaciers, alter the topography. Bjornstad's group discovered rafted rocks as long as 14 feet.
The erratics were concentrated along northeast-running gullies. Bjornstad suggests that the speed of the flowing water varied as it crossed an uneven surface, and that may have created eddies that forced an ice jam in the deeper, quieter waters at the back of these gullies. The erratics and bergmounds decreased as the surveyors worked up the mountainside. Bjornstad attributed the lower number to smaller successive floods.
Bjornstad and his colleagues found that most of the erratics were rounded, showing the effects of weathering and suggesting that they were carried in by older Ice Age floods.



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