NEW EVIDENCE FOR LATE-GLACIAL ADVANCE OF THE OKANOGAN LOBE INTO UPPER GRAND COULEE

Until recently, confirming evidence for glaciation within upper Grand Coulee (UGC) has been lacking except for glacial deposits atop the Steamboat Rock monolith, perched >200m above the coulee floor. Presented here is new evidence to indicate UGC was indeed temporarily filled with Okanogan-Lobe ice near the end of the last glaciation.

Glacial polish, striae, and grooves were recently discovered on exhumed granite inselbergs along Banks Lake via kayak reconnaissance ~2.6 km northeast of Steamboat Rock. Here, a near-vertical exposure trends N35W - parallel to the coulee axis; striations dip southeast 10-25 degrees. The down-coulee dipping striae are clearly visible on the lichen-free surface below the summer waterline. A subsequent reconnaissance identified two additional sites nearby with suspected glacial polish and grooves but without striations.

Previous lack of evidence for ice contact along the coulee floor is attributed to masking by a dense cover of lichens, except for within a meter of the waterline for Banks Lake. Most granite outcrops were also previously buried beneath Lake Columbia silt until the early 1950’s. Subsequently, much of the protective silt was washed away by wave action along Banks Lake. Comparison of photos taken 9 years apart indicate even short-term weathering can degrade and mask glaciated surfaces.

Other supporting evidence for late-stage occupation by the Okanogan Lobe is a layer of diamict south of Steamboat Rock, also near the level of Banks Lake. The massive, poorly sorted, clast- to matrix-supported mixture consists of mostly angular basalt clasts in a dense and compacted, dark-gray mixture of finer material – previously interpreted as lodgement till (Atwater 1987). Directly overlying the till are graded sandy beds from a series of small outburst-flood events followed by dozens of lacustrine varves deposited within glacial Lake Columbia until about 14 ka.