GSA field trip day 1 - The story begins at the Wallula Gap

            Field Guide Stop 1: Wallula Gap (46.0875 N, 118.9115 W)

During the last ice ages, a gigantic lake formed behind an ice dam called Glacial Lake Missoula in Idaho/Montana. From time to time, the ice dam would fail, and an immense flood of water would come roaring and scouring across the Channeled Scablands of eastern Washington state. Here, at Wallula Gap on the Columbia River, the floodwaters would reach this constriction point and back up to form a temporary lake. The waters backed up and temporarily reversed up the Snake River valley. More slowly than the initial flood, the waters would drain through this gap and down the Columbia River valley. At it's peak, the discharge through Wallula Gap gap was 6 million cubic meters per second.

As the flood waters backed up behind the gap, the sediments settled out in the ponded waters. As the waters slowly drained and receded, the now exposed sediments were blown by winds going from the SW to the NE to create massive loess deposits of the Palouse Hills. This repeated multiple times, with each new set of flood sediments blown back to form more windblown loess deposits. 

This picture was taken where the flood sediments were deposited. This is the source region of initial lake deposits that made the Palouse Hills loess.  (The actual source rock where the flood deposits sediments originated is located far to the NE in the Okanagan Highlands and Rocky Mountains.)

The scale of the megafloods is truly mind-boggling. The floods may have even overtopped the mesas of Wallula Gap, seen here in the distance of this picture.

Taken in: United States / Washington / Wallula (show map)

Taken on: October 18, 2017

Tags:

GSA

Geological Society of America

geology

loess

Palouse Hills

megaflood

coulee

Channeled Scablands « less

Sets appears in: • GSA 2017 field trip Loess deposits of Palouse Hills

Groups appears in: • Geology Rockers • Science Nerd Depot more » • Geologicpedia: learn geology through images. • For The Love Of Science! • The Layered Earth (accepting submissions for publication) « less

Camera: Google Pixel, f/2.0, 1/2513 sec, 4.7mm, ISO 51 (more info) (hide info)

Camera:	Google Pixel
Aperture:	f/2.0
Exposure Time:	1/2513 sec
Focal Length:	4.7mm
ISO:	51
Exposure Bias:	0 EV
Flash:	Off, Did not fire
Exposure Program:	Program AE
Exposure Mode:	Auto
Metering Mode:	Center-weighted average
White Balance:	Auto
Color Space:	sRGB
Software:	HDR+ 1.0.163413028z
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