METHODS OF STUDY
TOPOGRAPHIC AND BOTANICAL EVIDENCE SURFACE AGE
In this study two kinds of evidence of recent positions of the ice front were used: Topographic and botanical. The maximum downvalley position reached by a glacier in a period of advance may be marked by a prominent ridge of rock debris, a terminal moraine. Positions at which the glacier terminus halted for an appreciable time during a period of recession also may be marked by a ridge, a recessional moraine (fig. 2). All such positions, however, are not marked by debris, because the glacier may not have carried a sufficient load or may not have remained in a given position long enough for a moraine to form. At some locations in the valley, the slopes may be so steep and unstable that previously existing moraines have been removed by erosion.
Where topographic evidence for the recent ice position is absent or masked, the position is marked by a line of sharp difference in age between forests. As a glacier advances it destroys all trees in its path. As a glacier recedes it leaves a trimline between the pre-existing forest beyond its margin and the fresh glacial debris on its bed. After the glacier's recession, trees again begin to grow on the scoured areas and, in time, a forest becomes well established. However, the location of the old ice margin remains clearly delimited by the difference in size and age of the adjacent forests.
The primary method of dating a historical ice position used here consisted of studying adjacent forests of different ages. Where moraines exist they were used as evidence of an ice position. The age of a tree is interpreted to be the minimum period that has elapsed since time ice left the position occupied by the tree. In order to establish a minimum age of a given surface, an attempt is made to determine the age of the oldest tree on that surface.
In order to explain the significance of the ages of trees found on moraines in terms of the regimen of glaciers, a brief description of glacier shrinkage may be helpful. Glaciers shrink in two different ways, each resulting in a particular topographic expression (Sharp, 1951b, p. 108109). The terminus of a receding glacier is distinct and may be relatively free of surface debris, because the ice in the terminus is moving forward while the leading edge is receding. The terminus of a stagnant glacier is quite different, because a larger loss in volume results from vertical shrinkage than from terminal recession. Consequently, the leading edge and surface are covered with superglacial debris derived from englacial material and from avalanched material from the valley walls. The chaotic surface of the glacier in the zone of stagnation consists of knobs and ridges separating ponds and streams. The decayed condition of the ice is further indicated by numerous arches, caverns, and tunnels. Such a stagnant reach results from the ice becoming too thin to show appreciable motion.
The superglacial debris on a stagnant glacier, ranging from clay-sized particles to large angular blocks (Sharp, 1949, p. 294), is subject to extensive reworking as the underlying ice melts, causing the overlying debris to slump. The finer fractions in the debris are partly removed by superglacial streams. However, modification of the superglacial features decreases as the amount of debris increases relative to the amount of ice (Sharp, 1949, p. 295312).
Trees become established on superglacial debris that is not completely stabilized or that overlies a slowly moving glacier (Matthes and Phillips, 1943, p. 1819, 23). Trees as old as 100 years grow on thick super glacial deposits on Malaspina Glacier (Sharp, 1951a, p. 726). Trees ranging from 1 to 3 feet tall and from 47 to 56 years old were found growing on an ice-cored moraine at the approximate position of Enimons Glacier terminus in 191013 (fig. 2) as represented on the quadrangle map of Mount Rainier National Park. Although seedlings start to grow on superglacial debris on moving glaciers, most of them are believed to survive only after debris ceases to collapse repeatedly.
The age of the oldest tree at a given place, therefore, is equivalent to the time that has elapsed since the glacial debris upon which the tree is growing became stable enough for seedlings to survive. The time required for superglacial debris to accumulate and become stable along the margins of a glacier or on stagnant ice must be determined and added to the age of the oldest tree in order to estimate the length of time since cessation of active scouring. Determination of this interval is done by sampling trees and seedlings growing at known past positions of the glaciers.
The availability of tree seed is also of primary importance because it dictates which species become established and, along with the physical suitability of the seed bed, the rate at which the forest increases in density. The presence of seedlings on the youngest moraines of Emmons and Nisqually Glaciers formed in the last 10 years lends validity to the assumption that at least some seed is available in all years.
When the older modern moraines, such as those downvalley from Nisqually Glacier upon which the trees are less than 120 years old and those downvalley from Emmons Glacier upon which the trees are less than 210 years old, first became stabilized, they were much closer to a seed source than are the younger surfaces upon which trees are now starting to grow. Mature forest trees grew on surfaces adjoining and partly surrounding these older moraines (figs. 7, 9). Therefore, abundant quantities of tree seed were available and seedlings undoubtedly became established as soon as the morainic debris became stabilized.
Data are presented later (p. A11) to show that the period between the stabilization of the seed bed and the establishment of tree seedlings is only a few years. The short time interval illustrates the fact that the tree species that make up the mature forest are among the first plants to become established on new surfaces. Willow species, alder, and heaths form dense thickets in places; tree species are scattered. Thus the evidence discloses that growth of the shrubs, including alder, is coincident with the establishment of tree species. Insufficient time between the formation of new surfaces and the start of tree seedlings rules out any sort of plant succession before the germination and growth of tree species that make up the mature forest. Any change in vegetation through time, which might be inferred from differences in vegetation on surfaces of different age, seems to be related to the death of shorter lived species and the continued growth of trees.
The oldest trees whose age is determined by counting the annual-growth rings, provide the basis for estimating the minimum age of the surface upon which they grow. These rings can be seen in a stump section or in a core sample removed from the trunk. The number of annual rings that can be counted in stumps is believed to be close to the true age of the trees, because nearly all stumps examined measured less than 6 inches high. Trees are probably older than the age indicated by core counts, because most cores were taken more than 12 inches above the ground, which is above the height of trees in the seedling stage. However, error from this source is negligible, because the ages determined from cores are comparable to those determined from stumps. In fact, the highest count of rings in a stump in a young forest was 112, whereas the highest count of a core in the same forest was 117.
The diameter growth of trees and the characteristics of wood, including the identifcation of annual rings are discussed by Brown, Panshin, and Forsaith (1949, p. 12-27, 35, 52, 96-111, 126-163, 196-238), Eames and McDaniels (1947, p. 175-231), and Esan (1953, p. 125-136, 338-411).
Last Updated: 01-Mar-2005