CLASSROOM EARTH
Figure 4. Found along and embedded within the Cispus River depositional terraces are boulders over two meters in diameter. While contradict- ing the traditional outwash interpretation of the poorly sorted terrace deposits, these boulders corroborate a lahar origin from Mount Adams. A. Over two meters long, this boulder is embedded in the top level of the highest terrace on the northern flank of the Cispus River north of the Cispus Learning Center. Rock hammer (circled) for scale. B. The author stands atop a boulder stranded on the highest depositional terrace level along the Cispus River north of the Cispus Learning center. (Photograph courtesy of Autumn Pope.)
field visits showed that the poorly sorted gravels were ubiq- uitous across the terraces and suggested a minimum depth of thirty-two meters above the current river. We quickly noticed that the terraces extended slightly up Adams Creek towards Mount Adams, indicating that my hypothesized debris flow likely originated as a lahar from Mount Adams. Though my source had been identified, I puzzled over its end: to where did the lahar continue? Further west, the Cispus River entered a narrow gorge with few deposits but bordered by strath terraces extending over 35 m above the river. There were no obvious sedimentary remnants, yet a lahar should leave some record of its presence, especially one as large as the one I was study- ing. I pondered this dilemma as I travelled along the river.
Though nearly invisible in the field, Washington State Geological Survey’s LiDAR database revealed that the evi- dence was meters above my head. Gently incised into the strath terraces were anastomosing channels over thirty-five meters above the river. Not only were these channels gentler than those hosting the current river, but the braided chan- nels were found above what is now a meandering stream. This supported my lahar interpretation: far above the current river, anastomosing channels were etched into strath terraces along a narrow gorge, forming features quite unlike anything being produced by the current river. Indeed, as seen at other volcanoes, it is only natural for a lahar or any debris flow to produce anastomosing channels! From there, the lahar would continue through the widening river valley as shown by the continuation of depositional terraces down to where the Cispus River enters the Cowlitz River Valley where they appear to have been obliterated by a younger lahar from Mount Rainier.
At the entrance to the narrow gorge along the Cispus River, the lahar appears to have been slowed by a hydraulic dam, causing the lahar to temporarily pool upriver. This provides the perfect opportunity to estimate the volume of the flow. By analyzing the area inundated contempora-
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neously compared to the cross-sectional area, I calculated a minimum volume of 0.31 cubic kilometers, more than four times greater than the largest previously identified lahar from Mount Adams (the Holocene Trout Lake Lahar of Vallance, 1999). Though less easily calculated than the volume, the age is bracketed above by a layer of 1.2 ka pumiceous tephra that overlies the terraces and below by the Evans Creek Glaciation (21 to 18 ka) (Schuster, 1973), which must predate the lahar because of the lack of moraines in the area. The weathering evidenced by clast rinds in the deposit noted by Swanson (1991) suggest that the lahar may not be much younger than the Evans Creek Glaciation, making this lahar not only the largest but potentially the oldest lahar from Mount Adams yet identified. By broadening the range of Mount Adams lahars beyond the pioneering work of Vallance (1999) on Holocene lahars on the southern flanks, this research stirred a number of questions for continued study, yet my gaze would soon again turn northwest to the Puget Lowland.
Procuring Permits: Sampling the Mima Mounds
By May, my professor and I were deep in research permit applications. With continued travel restrictions in Washington State, the Department of Natural Resources was limited on the number of awardable research permits. A beautiful spring made fieldwork all the more tempting, but in due course we received our permits to collect pebbles from mounds at Mima Prairie Natural Area Preserve, Rocky Prairie Natural Area Preserve, and West Rocky Prairie Wildlife Area Unit. Adding masks and social distancing skills to our usual geologic tools, we prepared to enter the field for what would turn out to be a particularly warm spring season – a poor combination with masks! Over the course of the following months, I spent five days at Mima Prairie and one day at Rocky Prairie collecting samples, while Prof. Pringle investigated West Rocky Prairie.
While I mapped curious cobbles and boulders common in some intermounds, we conducted point counts of the underlying coarse-bedded gravels and in the cross section of one mound at the DNR pit at Mima Prairie, we collected well over several hundred samples of gravel.
www.aipg.org Jan.Feb.Mar 2021 • TPG 17
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