Abstract

A complex sequence of river terraces in the Nenana Valley was examined to determine whether the surfaces were depositional or erosional in origin. Of prime concern was the identification of outwash derived from the Healy (early Wisconsin) and Riley Creek I (late Wisconsin) glaciations.

Two terraces were formed during the Healy glacial cycle. The higher level represents the depositional surface of the Healy outwash. The lower level was formed during recession of the Healy ice when approximately 18 metres of that outwash was removed. The erosion was initiated when glacial Lake Moody, dammed behind the Healy moraine, spilled over the drainage divide and began excavation of the Nenana gorge. The surface of Riley Creek [] outwash has been offset by faulting near the gorge mouth. Isolated remnants of that surface stand at different elevations, complicating the correlation of downvalley outwash with the Riley Creek I moraine.

The evolution of the Nenana gorge is associated with spasmodic development of erosional terraces throughout post-Healy time as the Nenana River attempted to establish a new equilibrium condition. This erosional trend has been interrupted only by out-wash deposition during episodic expansion of the Riley Creek ice.

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