Lake Allison Erratics

Mima Mounds & Missoula Floods: Table of Contents
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What is an Erratic?

'Erratics' are simply rocks of a type that have been transported to a location that is far from where we would expect to find them, which is usually near the formation of origin. The most common transport mechanisms for erratics are ice and water. A combination of the two, called 'ice rafting', transports a rock first by ice in a glacier, and then by water while still trapped in the ice. This can result, as in the case of Missoula Floods, in transport distances over thousands of kilometers. Erratics can be of any size, but as a general rule, 'larger is better' for increased confidence in the 'forensic evaluation'.

Technically, forensic geology attempts to answer questions raised by the legal system using principles of geological science. In this discussion I will purposefully use the term 'forensic evaluation' in a more facetious sense, meaning 'an effort to determine whether or not the rock specimen in question has been kidnapped and assaulted by a glacier'.

Specifically, the forensic evaluation will examine the composition and condition of the rock to determine if possible the formation of origin, estimate the age of the specimen, and deduce the forces that have acted over time to result in its current condition.

• What is the relative bluntness of the edges?
• Do the edges exhibit uniform bluntness, or are there both smooth and sharp edges?
• Do any of the faces exhibit scratches, gouges, cracking or chipping?
• What are the dimensions (length, width, height) of a box that could contain the specimen?

Another important aspect of the forensic evaluation is the 'context' in which the specimen was found.

• Was it isolated, or in a group?
• On the surface, or buried in some material?
• If buried, what was the nature of the material in which is was found?
• If the specimen was evidently broken, were mating pieces found nearby?
• At what elevation was the specimen found?

Answers to these questions provide the investigator with varying confidence levels for making the determination as to whether or not the specimen was a victim of glacial trauma. These criteria are beyond the scope of this article, but are not terribly complex and will be addressed in a future topic with respect to the samples ultimately collected at Goods Quarries.

Lake Allison

For purposes of this discussion with respect to the Chehalis River watershed, we are most interested in the elevation at which erratics were found in the Willamette Valley of Oregon, as this provides clear indication of the elevation potential for Missoula Floods to spill over from the adjacent Cowlitz River watershed to the south. Given a Columbia River flow restriction at Longview, Washington or beyond, the Cowlitz watershed and the Willamette Valley are 'hydraulically inseparable'.

The lake which formed in the Willamette Valley as a result of Missoula Floods is now called 'Lake Allison' in honor of Prof. Emeritus Ira S. Allison of Oregon State University. The work of Prof. Allison and others to collect the Willamette Valley erratic database and many other interesting facts are elegantly summarized on a color wall chart suitable for framing, available for download from the US Geological Survey and linked to the image here:

USGS Lake Allison Erratics Chart

In studying the 'key' for interpreting the USGS chart, reprinted below, it is immediately clear how the idea of Missoula Floods never exceeding 122m (400 feet) in the Willamette Valley became established in the minds of so many geologists.

Key for Chart Above

But always remember students, to read the text and don't just look at the pictures! In the case of the text on this chart, there are some intriguing statements with respect to erratic elevation:

• The vast majority of erratics lie below 122m above sea level; however, Piper (1942) noted boulders "widely scattered in valleys and on tops of hills and knolls 800-1,500 ft. [244-457m] high" eight miles south of Portland, near Prosser mine, east of Lake Oswego. This location has not been confirmed.
• Allison (1935) described "A half dozen small [erratics], ranging in size up to 8 inches [20 cm] lying on Judkin's Point, Eugene, Oregon, at an altitude of about 650 feet [198m] above sea level, are so far above any other known occurrence that their authenticity is doubtful". Allison attributes the high elevations of these erratics to transport by Native Americans.

These statements clearly indicate that 'some' erratics were reported or found at elevations higher than 122m, but what exactly does the "vast majority" mean, quantitatively? To answer this, we need to examine the actual erratic database, which I have downloaded from USGS, sorted by elevation, and made available via the preceding link. From that data, the following summary information may be extracted:

• One documented erratic was found at 430' in 2001
• Eleven found by Allison in the 1930's were recorded at elevations from 400' to 500',
• Seven found by Allison were recorded at elevations from 500' to 600'
• One was recorded by Allison at a difficult-to-reconcile 827'
• And the Judkin's Point erratics at 650' discounted by Allison are not included

So 19 out of about 400, or roughly 5%, of the erratics were found at an elevation higher than 122m, all but one by Ira Allison himself. Quantitatively then, 95% is what the 'vast majority' means in the USGS context. Surely, even though Allison lacked the modern convenience of precise coordinate and elevation determination via GPS, he could not have over-estimated the elevations of 18 erratic locations, nor were they all likely to have been transported by Native Americans.

After studying this information, I was confident that the 122m 'maximum elevation' of Lake Allison was in fact a statistical simplification, useful for understanding the chronology of flood events and their cumulative impacts in the Willamette Valley.

And by the same token, equally confident in the fact there were 'some' exceptional Missoula Floods with the elevation potential to access the Chehalis watershed.

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