GLACIAL AND VOLCANIC HISTORY OF THE NOOKSACK MIDDLE FORK, WASHINGTON

During the last glacial maximum (LGM), the Cordilleran Ice Sheet (CIS) covered the Nooksack Middle Fork and nearby North Cascades with ice to at least 1800 m above sea level. The CIS melted down below the level of ridge tops in the North Cascades by ~12,300 14C yrs. B.P. and alpine glaciers extended 23 km down the Middle Fork Valley from Mt. Baker and the Twin Sisters Range to the vicinity of Mosquito Lake where deposits of four glacial phases remain. The Middle Fork glacier then receded ~18 km upvalley where it built a lateral moraine containing logs dated at 10,600 14C yrs. B.P.

During the mid–Holocene, Mazama ash and Rocky Creek ash were deposited in the drainage and later buried by the massive Middle Fork I lahar from Mt. Baker, which flowed down the entire length of the valley and extended well downstream from Deming. The lahar mantled terraces with as much as 15 m of deposits up to ~100 m above the valley floor and flowed up into Clearwater Creek, a tributary of the Middle Fork, to elevations at least 40 m above the present river level. About 0.4 km up Clearwater Creek, the lahar rests on Rocky Creek and Mazama ashes. Organic material beneath Rocky Creek ash is dated at 5785 ± 55 14C yrs. B.P. and 5730 ± 170 14C yrs. B.P. above the ash. Wood in the lahar has been dated at 5650 ± 110 and 5710 ± 110 14C yrs. B.P. A younger lahar in the Middle Fork has been dated at 3100 14C yrs. B.P.

In the upper Middle Fork valley, the Deming glacier fluctuated during the late Holocene, building lateral moraines high along the valley sides. One such moraine contains buried logs dated at 2970 ± 35 and 2960 ± 30 14C yrs. B.P. 1.8 km downvalley from the present Deming terminus and 2440 ± 30 and 2205 ± 30 2.2 km downvalley. The Deming glacier terminus fluctuated several hundred meters during the past century.