Thursday, August 18, 2011

Glacial Erratics Near Grand Coulee Dam

Basalt erratic with underlying lake silts west of Grand Coulee, Washington

David Tucker has posted some great write ups on glacial erratics including a write up of a huge erratic near Lake Stevens a-monster-erratic-discovered-in-lake-stevens/ by the Puget Ice Lobe during the last glacial period approximately 18,000 years ago. Shortly after reading his write up I was in the Grand Coulee area checking out the basal contact between the Columbia River basalts and the much older gneiss when I realized what I initially took as outcrops of basalt were ice rafted erratics.

Glacial ice advanced into eastern Washington as far south as the Waterville Plateau just north of U.S. Highway 2 between Banks Lake and Waterville. The ice in this area came from the Okanogan ice lobe as the Okanogan Valley (Okonogon in Canada) provided a north-south conduit for ice flow. The Waterville Plateau is a great place to see moraines, kettles, eskers and erratics. The area has been designated a National Natural Landmark because of the outstanding examples of glacial features. 

After the ice retreated from the plateau, the Okanogan Ice Lobe still blocked the Columbia River down stream of present day Grand Coulee forming a lake with the northern edge of the Waterville Plateau as the south shore and margin of the Okanogan lobe the north shore. 

Ice age lake terraces west of Coulee City
Modern erosion exposing glacial lake silts

The glacial ice readily plucked off blocks of basalt from the upland areas to the north as well as blocks of gneiss. The ice then broke off as bergs into the lake and as the bergs melted the basalt erratics were dropped into the lake. These erratics did not travel very far so they lack some of the exotic nature that more far traveled erratics have. And their short travel distance and lack of time being ground with other rocks means there is a fair collection of really big erratics.

Erratics and houses. A couple of big erratics can be seen between houses on the lake terrace

Blowup of a portion of above photo showing much larger than house sized erratic

Another blow up of photo with erratic and houses 

Basalt erratic protruding from the slope above the Columbia River

Erratics underlain by lake silts with ridge of bedrock behind.
The ridge is capped with basalt with bedrock on the lower slopes consisting of much older gneiss   


JoelGombiner said...

Are you sure these aren't blocks that were plucked by kolk-type erosion during the Missoula Floods?

Dan McShane said...

If kolk, they would be rolled into place and should have significant course grained material at their base. The ones I saw up close are sitting on top of fine silts consistent with lake sediments. It is possible some of the bocks were kolk-type, but the kolks I am familiar with would have been from flow away from the lake and not into the lake. The erosive flows of the Missoula flood were dirceted south as the Columbi Lake over topped from the surge of water.