Skip to 0 minutes and 7 secondsWe've already talked about the Missoula Floods as a classic example of a megaflood that occurred before people were around to record it. But how do we know what happened during an event that we didn't observe? And what techniques can we use to learn about other prehistoric megafloods? The story of J Harlen Bretz provides an ideal illustration of how we can successfully make the case that mega-events did occur in the past, even though we've not observed any direct equivalence in our modern history. The hypothesis that Missoula Floods created the Channelled Scablands was first put forward by American geologist J Harlan Bretz in the 1920s. He was a field geologist who studied the dramatic and extremely unusual landscapes of Eastern Washington state.

Skip to 0 minutes and 59 secondsHis experiences of looking at river and glacier environments in other places led him to believe that these things weren't normal versions of those environments. They did have familiar features, such as waterfalls and potholes, but they looked like giant versions of these landforms that he was more familiar with. Bretz looked at maps that detailed the shape and the features of the area and formed the hypothesis that these landscapes couldn't have been caused by conventional floods but only by much larger megafloods. He knew that what he was proposing would cause controversy amongst other scientists, who had reasonable doubts that it was impossible because no one had ever witnessed a flood so large.

Skip to 1 minute and 44 secondsThe idea that so much change at the surface of the Earth could happen so fast also went against some very deeply held geological ideas in the earliest 20th century. Various scientists of the time actually described his hypothesis as outrageous, preposterous, and incompetent. However, the key objection of many scientists was not the effects of his hypothesised flood but the fact that he was unable to give a convincing cause for why the floods occurred. It eventually took more than 15 years from when Bretz first raised the idea for a scientist called Joseph Pardee to describe evidence for a sudden draining of the glacial Lake Missoula.

Skip to 2 minutes and 28 secondsThis enormous lake was trapped behind the tongue of a glacier that was once east of the Channelled Scablands. As the last ice age ended, this glacier began to melt and release the water held in the lake. All of this would have flowed out through Eastern Washington. But there was still one missing piece of evidence needed to truly prove the draining of the lake carved the Channelled Scablands. This only finally came when enough aerial and satellite imagery existed to show giant ripple fields linking the lake and the Scablands and proving conclusively that vast amounts of water must have flowed towards this area from the lake.

Skip to 3 minutes and 13 secondsOnce both cause and effect of this megaflood had been well-documented, the wider community of scientists began to be won over. Finally, in 1979, more than 50 years after he first suggested the idea, when Bretz was 96, the Geological Society of America awarded him the prestigious Penrose Medal.

Discovering evidence for past events

In this video, Dr Dan Hobley explains the story of how evidence for the Missoula megaflood was discovered.

J Harlen Bretz became interested in Eastern Washington State in 1910 when a new map was published showing its natural, physical features, and in particularly some large potholes. He began visiting and conducting field research in the area some years later.

What he observed was remarkable and unusual.

He observed evidence for waterfalls at the cliffs of Dry Falls but of a scale and size much larger than had been seen previously.

Dry Falls is estimated to be 3.5 miles wide, which is five times the width of Niagara falls in North America. It drops 400 to 600 vertical feet compared with 165 vertical feet of Niagara Falls.

This historical waterfall would have had torrents of water flowing through it, at scale that is hard for us to fully appreciate.

This and other observations, led him to the conclusion that these landscapes could only have been created by an extreme event of a megaflood.

But as we heard in the video, going from these observations to providing convincing evidence, and that evidence being accepted was challenging.

In the next step we’ll discuss this.


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Extreme Geological Events

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