|Snake River gorge cutting the|
Flood Basalt layers, through which
the Bonneville flood discharged
|Detail of the boulder field. Note preferent tilt |
of boulders towards the left, in agreement with
flow direction. More pics here and here.
|Rounded boulders left by the high waters of the|
Bonneville flood among a Neogene basalt ridge
spillway. Flood flow in the direction of the picture.
More pics here.
One thing that makes this scenario key to understand the importance of large flooding events in shaping landscape is the ubiquitous Columbia River Basalt formation (one of the largest flood basalt regions) and the Yellostone-Snake River volcanic zone. Both covered many of the areas we visited with a nearly flat layer of very fluid basalt, during the Neogene, long before the Pleistocene floods (see e.g. Fouch, Geology, 2011). The simple relief and lithological setting left by the flood basalts enhances the erosional features related to the Pleistocene water floods and the presence of vertical tectonic motions.