Geological Survey Professional Paper 387-A Botanical Evidence of the Modern History of Nisqually Glacier Washington

A prominent end moraine below Nisqually Glacier marks the greatest downvalley advance of the glacier in its modern history. The glacier started to recede from its point of maximum advance about 1840. This estimate is based on the ages of trees growing on the moraine and includes an interval of about 5 years for trees to become established after glaciation. Evidence in the Nisqually and White River valleys shows that trees become established 1 to 14 years after a glacier recedes from a given position. Although much information is available on the positions of the terminus of Nisqually Glacier during its recession from this maximum point, and a few sections of lateral moraines exist on the right valley slope in the vicinity of Tato Falls, no prominent moraines other than the 1840 moraine exist that can be dated from botanical evidence alone.

The relation of the maximum recent advance of Nisqually Glacier, marked by the 1840 moraine, to the next earlier advance is inferred from the age of the forest downvalley from the moraine and from examination of the layers of organic matter and pumice overlying the till representing the last glaciation of the area. The present forest with a maximum age of about 300 years, plus a humus layer at the surface consisting of rotten wood and representing at least 200 years, plus charcoal beneath the humus representing an earlier generation of trees of at least 100 to 200 years of age, represent a total of 600 to 700 years. The several organic matter and pumice layers underneath the humus and charcoal layers certainly represent 300 years or more. Thus the time that elapsed between the last major glaciation of Nisqually Valley and the advance marked by the 1840 moraine was probably at least a thousand years. Further study of the sequences of pumice layers may increase the estimate of the length of this period by several thousand years. However, the maximum advance of Nisqually Glacier in at least a thousand years un doubtedly occurred in about 1840.

Incomplete studies of the moraines below Emmons and Tahoma Glaciers revealed a prominent moraine in each valley corresponding in time to the 1840 terminal moraine of Nisqually Glacier. These studies indicate that Tahoma Glacier started to recede about 1835 and that Emmons Glacier started to recede about 1850. In contrast to Nisqually Glacier, however, prominent moraines below Emmons Glacier and botanical evidence indicate that maximum positions attained by two other advances occurred in 1745 and 1895. The maximum modern advance of Emmons Glacier is marked by the 1745 moraine. At Tahoma Glacier a small segment of a 1635 moraine was discovered which indicates that the advances marked by the 1635 and the 1835 moraines reached about the same extent downvalley. The 1635 moraine found below Tahoma Glacier was not found at Nisqually or Emmons Glaciers but present studies do not permit the conclusion that it does not exist at these places. The following table summarizes the history of these recessions.

Evidence that ice started to recede from prominent moraines below three glaciers during the period 1835—50 suggests that the advance of each glacier during this period was approximately concurrent. No significance is attached to the 15-year spread in the three date determinations. However, the advance that occurred during this period was exceeded at Emmons Glacier by an advance that began to recede from the maximum point about 1745 and was equaled or exceeded by one at Tahoma Glacier that began to recede from its maximum point about 1635. Thus the evidence indicates that because of the probable similarity of climatic trends around Mount Rainier the glaciers on its slopes may advance and retreat in approximate synchronization. However, the data indicate that the relative degree of downvalley advance varies considerably at the different locations in accord with glacier behavior as noted by other investigators (Sharp, 1951b, p. 106).

Thus, only a start has been made in the accumulation of data on glacial history at Mount Rainier from which climatic variations can be inferred within the past several hundred years. The authors expect to continue these studies in order to accumulate additional information on several other glaciers of Mount Rainier and their environs. In addition to the collection of data on specific glacier movements at other locations several related problems require further study: (a) Botanical evidence that can be used to determine the time of past glacier advances as well as the further refinement of evidence that is used to determine the time of the start of a recession; (b) evidence of flooding and ponding along possible marginal drainage channels; (c) data on the rate of wood decay in different environments and the development of methods of measuring the degree of decay so that more positive age determinations can be made on the basis of fallen logs; and (d) botanical evidence that can be used to distinguish mudflow deposits from glacial moraines.