Formation of the English Channel

If you are plugged in today, you've likely seen the report about a paper in this week's Nature about a ice dam-bursting megaflood scouring event that is thought to have produced the English Channel. I simply don't have the time at the moment to comment on this in any detail....so here's a few blurbs from a New Scientist report.

Half a million years ago, Britain was connected to mainland Europe by a broad chalk ridge that spanned what we now call the Dover Strait (or the Pas de Calais in French). But somehow that ridge was destroyed, forever separating England and France.

The cause was revealed by an ultrahigh-resolution sonar survey of a large chunk of the channel's bedrock. It shows the Weald-Artois Ridge, as it is called, was breached and toppled by a monumental torrent that gushed from an overfilling glacial lake that the ridge had been damming on its northern side.

Not so long ago, I posted about the Channeled Scablands of the northwestern United States.

The Imperial team calculates that 1 million cubic metres of water per second flowed for several months to carve the seafloor valleys, some of which are up to 10 kilometres wide and 50 metres deep. The flow rate was 100 times the average of the Mississippi river today, and 1000 times that of the Rhine, Gupta says.

It turns out a study in 1985 hypothesized these features and inferred processes from some much lower resolution sonar data. Must be kind of cool to see your theory confirmed with higher resolution data like that.

The BBC has a good summary of the Nature paper too.

Mystery of the megaflood

Last night we watched a NOVA special about the unique geomorphic region known as the 'Channeled Scablands' of the northwest United States called Mystery of the Megaflood. During the latest Pleistocene as the continental ice sheets began to retreat, huge lakes (similar in scale to some of the Great Lakes) that were dammed by ice broke free and rushed across the state of Washington creating erosional and depositional features that perplexed geologists and geomorphologists for a long time.

If you don't know about this story, check out the NOVA website (click on image or link above). They have tons of information to help you learn more about it. The program was pretty entertaining. As a geoscientist it's always nice to see the science portrayed in these programs. Yes, they can be a little sensationalized with the scary music and overdramatized reenactments, but that's kind of fun in a way. I recommend it.

Check out this interactive on their website for a tour across the region highlighting the key features.
One of my favorite features are the giant bedforms (ripple marks, essentially) that were formed near where the flood started near Missoula, Montana. I went on GoogleEarth and started poking around the area and lo and behold you can see them fairly easy. They are on the order of 35 ft high and have wavelengths of hundreds of feet. The image below shows an area about 5 miles across (copy/paste these coordinates into GoogleEarth to go there: 47.48N, 114.59W).


And here's a closer shot (this area about a mile across). These suckers are huge!
Anyway...check out the website and rent the show. It also goes into the incredulity the early 20th century geologists had regarding this catastrophist point of view. That's another interesting topic for another time.

Another post forthcoming is -- what happened when these floods reached the ocean? People have done some work on this and it is really interesting...stay tuned.