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Cordilleran Ice Sheet 15-20,000 years ago – 4000 feet thick.

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Presentation on theme: "Cordilleran Ice Sheet 15-20,000 years ago – 4000 feet thick."— Presentation transcript:

1 Cordilleran Ice Sheet 15-20,000 years ago – 4000 feet thick

2 Flood Routes (basalt lava or water)

3 CORDILLERAN ICE SHEET LOBES 1Purcell Lobe blocked the Clark Fork River forming Lake Missoula Channeled Scabland 2Okanogan Lobe blocked the Columbia River (at Grand Coulee Dam) forming Glacial Lake Columbia (Grand Coulee, Banks Lake, Steamboat Rock, Dry Falls, & Moses Coulee) 3 The Puget Lobe scoured the Puget Sound


5 PURCELL LOBE ICE DAM Blocked Clark Fork River (Idaho-Montana border )

6 Created Glacial Lake Missoula Covering 7,800 square kilometers PURCELL LOBE

7 Ice dam, merely a small section of the lobe three miles long ten miles across (broad and flat) 2,000 feet tall – thus holding back 2000 feet of hydraulic head which EVENTUALLY Burst through the Clark Fork Canyon Ten times combined flow of all the riversof the world PURCELL LOBE

8 2nd Burst through the Clark Fork Canyon Ten times combined flow of all the rivers of the world PURCELL LOBE

9 Such catastrophic floods etched coulees now known as the Channeled S cablands in eastern Washington where water velocities were highest – average velocity was 65 mph. “Constricted Burst velocities” in excess of 100 mph  instantaneous erosion. PURCELL LOBE

10 First Effects of Flood Inundating 16,000 sq. miles hundreds of feet deep Quickly stripped 200 feet of soil Left scabs or erosion remnants of Basalt STOPPED AT WALLULA GAP (constriction point) – creates Lake Lewis Several weeks 200 cubic miles of water per day (pressure in) to a gap that could discharge less than 40 cubic miles per day (pressure out). Velocity increases by square root of 5

11 Wallula Gap

12 Kalama (WA) Constriction (Carrolls Bluffs)

13 Restriction at Kalama Creates Lake Allison which filled the Willamette Valley all the way to Eugene Estimated depth was 380 feet at Eugene Flood waters delivered most of the silt to produce the most fertile agricultural soils in the country

14 Estimate Flood Levels 1250 feet at Wallula Gap 1000 feet at Dalles 925 feet at Hood River 400 Feet at Portland 275 feet at Clatskanie

15 The Draining of Glacial Lake Missoula


17 Coulees are gouged-out canyons in the high desert of central Washington that now carry little or no flowing water. The two most spectacular are Moses Coulee and, just to the east, Grand Coulee. Both photos are of Grand Coulee and show the region known as Dry Falls. During the height of the Ice Age Floods, a torrent of water plucked the basalt bedrock away, forming a waterfall 400 feet tall and 3.5 miles wide. (Niagara Falls, in comparison, measures one-mile wide, with a 165-foot drop.) THE GREAT COULEES OF CENTRAL WASHINGTON

18 This is an area near Sprague Lake that shows pool formations that remain today as evidence of the great lakes flow across this area.

19 Dramatic evidence remains West Bar, along the Columbia River near Quincy, exhibits “mega- ripples” -- ripples generated by the massive flood First Noticed by Harlen Bretz in 1922 and published in 1923. Bretz was not warmly received as the scientific community hates catastrophism

20 Erosion Feature



23 Photo compliments of the National Park Service The flood ripped away huge boulders from the underlying lava rock and carried or floated them – constriction points literally “shoot” these rocks out Flood Debris

24 The Dalles constriction point makes Lake Condon Columbia Gorge significantly deepened by flood Kalama Narrows back up floods Willamette Valley

25 FINAL STAGES OF THE FLOOD Formed a new dam Causing the lake to refill Resulting in a new flood Average of every 55 years or so for 2,000 years! Each Flood event 2-3 weeks Each time Lake Missoula emptied the Purcell lobe continued its southerly progression

26 The highest known shorelines are found at an elevation of 4,200 feet. Ancient shorelines on Mt. Jumbo Missoula, MT

27 13-30 feet these ripple marks would dwarf any ordinary ripple mark Camas Prairie ripple marks

28 Lake Columbia -- –across Spokane Cut deep canyons, or coulees in bedrock

29 OKANOGAN LOBE Dry Falls is 3.5 miles wide with a drop of over 400 ft.

30 Okanogan Lobe Palouse Falls Canyon Carved out over 2000 year period


32 15,000 y.a. Olympic Mountains help squeeze the ice sheet down the Puget Lowlands 1 mile thick Gouged/Scarred Puget Sound lowlands –Creates/defines Straits of Juan de Fuca –Cascades on east –Olympics and Vancouver Island west

33 Puget Lobe 13, 500 y.a. receded Melting snow/ice = water runoff Caused –Pacific Ocean to rise –Flooded Puget Sound Trough –North South Hills (Drumlins) in Seattle –Created very irregular coastline –Numerous islands remain

34 Mt. Erie (WA)

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