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Proving past events

We’ve considered the story of the geologist J Harlen Bretz and discussed why he wasn’t believed and how he was eventually able to convince people.

Not only is this a great story from history, it highlights an important idea. There are only two ways of finding out if extreme geological events have occurred.

Either you discover the effect or you document the cause. People are only convinced if you do both.

Let’s look again briefly at two extreme events we’ve considered previously to reinforce this idea.

Missoula megafloods

In the case of the Missoula megafloods, J Harlen Bretz documented the effects of the floods in terms of their sculpting of the landscape of northern Washington state.

However, it was the addition of Pardee’s science describing the release of water from glacial lake Missoula sufficient to cause a megaflood, that began to convince people.

Storegga slide

In the case of the megatsunami following the Storegga slide, the cause is obvious today as a large slumped mass of material on the seabed.

But, scientists needed to document the patterns and timings of ancient coastal flooding and sediment deposits associated with it in order to make the case for the slide definitely triggering a megatsunami.

This has been done by confirming that the tsunami deposits are all of exactly the same age using geological dating, and by confirming that the magnitude of coastal flooding increases as you approach the source of the waves. Meaning the megatsunami died away as it travelled, as we see in modern tsunamis.


What we’ve considered shows how our understanding of extreme geological events has developed.

In the case of the Missoula megafloods the sheer scale and impact of such an event was one of the reasons why it wasn’t believed. Geologists of the time didn’t like the idea that a sudden catastrophic event could cause such significant change to the planet.

In the next step we’ll look at another extreme event from the past. We’ll see how the response to it played an important role in humans beginning to develop our understanding of such events.

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This article is from the free online course:

Extreme Geological Events

Cardiff University

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