Apr 3, 2009

Mima Mounds: mystery solved?

Turns out they're not the gods' basketball "nubbins" after all. The Mima Mounds, grippy as they might make the earth around these parts, probably have a banal origin, if new laser-made maps are to be trusted.
The new maps clearly show that all of the mounds formed near the margins of retreating glaciers, supporting an idea first proposed nearly a century ago, said Robert Logan, chief of geological mapping for the Washington Department of Natural Resources....

As glacial lobes melted, Logan explained, dammed-up water occasionally burst free, gouging channels. The new maps show all of the mounded tracts are adjacent to outburst channels. The surging floodwaters would have carried gravel, which underlies the mounds. The topography also shows evidence that the meltwater pooled up, perhaps dammed by chunks of ice.

Cold winds blowing off the glacier would have frozen the ponded water, Logan said.

That's where the sun cups enter the picture. As anyone who's trekked across glaciers or snowfields in warm weather knows, these scalloped depressions form naturally.

Meanwhile, water would have continued to pour from the melting glaciers, washing across the sun cups and dropping sediment. When all the ice was gone, sediment collected in the sun cups could have been left behind, forming the mounds that remain today.
Those who promulgate the Pocket Gopher Theory aren't packing their tents just yet, though. Read the article to find out why.


Lloyd DeKay said...

This is a great theory for areas adjacent to a glacial front, but "mima mounds" are found in several areas of the Columbia Gorge, often atop highly elevated benches (notably at Tom McCall Preserve, Rowena, OR). The Tom McCall Preserve plateau sits 500' above the Columbia River and was likely overtopped by several Bretz Flood events. However, it's difficult to impossible to imagine those roaring torrents freezing, then sun-cupping and accumulating gravel deposits that invert as the ice melts away. Erosional features, gopher constructs or other hypotheses may seem unlikely, but the inverted outwash mounds hypothesis is even more unlikely.

Anonymous said...

Absolutely agree with Lloyd. Will this be forever a mystery?