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

MAXIMUM ADVANCE IN LAST THOUSAND YEARS

An old forest grows downstream from the terminal moraine and upslope from the lateral moraines. The age of this forest is a minimum estimate of the age of the drift-covered surface on which it grows. The appearance of the forest, the humus, and the soil profile, suggest that the surface is at least a thousand years old; probably it is much older.

BOTANICAL EVIDENCE

The living trees in this part of the Nisqually River valley range in size from 2 to 4 feet in diameter; some of the recently cut stumps in the old forest are as large as 6 feet in diameter. Annual ring counts of 21 of these older stumps were made, and the age of the trees at the end of their last growing season (1957) ranged from 285 to 303 years. In addition, 23 trees in the old forest were cored, but the borer was long enough to obtain a sample of the entire radius from only 3 of them.

Throughout the old forest are many fallen logs and broken stumps (fig. 3). The wood in some of the logs is solid and appears to be sound; in others, the bark is intact and little decayed, but it conceals a punky, partly decayed trunk. Some logs are almost completely decayed and consist solely of rotten wood. Evidence of still older rotten logs can be found in places on the forest floor by digging in barely perceptible elongate ridges that are completely covered with mosses and forest litter. The humus consists of layers of rotten wood in which the grain of the parent trunks is still apparent. A plane section through these layers cuts along and across the grain at all angles. This decayed wood almost completely carpets the forest and comprises the upper 0.5 to 1.0 foot of the soil profile. Because the sound and partly decayed logs are the same size as the standing trees it is assumed that they were as old as the oldest standing trees, or about 300 years old when they fell. Although the mounds of rotten logs are considerably smaller than the sound fallen logs and standing trees, it is assumed that they, too, were about this old when they fell. The evidence, then, consists of living trees that are approximately 300 years old and logs that are assumed also to have been 300 years old when they fell.

The different degrees of wood decay suggest that the falling of trees is a continuing thing and show, furthermore, that trees have fallen throughout a long period of time. Clear-cut evidence of the minimum amount of time that has elapsed since the death of the earliest generation of trees was not found. Evidence was found, however, that permits a rough estimate of the rate of wood decay in this area. The ages of two trees growing on the pit wall formed when a 4-foot noble fir was uprooted indicate that at least 50 years have elapsed since the tree fell. The wood in the fallen tree is sound. As will be discussed in more detail on page A—14, trees ranging in age from 111 to 117 years are growing on the Nisqually Glacier end moraine. Beneath the boulders comprising the moraine is an only partly decayed stump, 4 feet in diameter, which has been dead for at least 117 years. The partial decay in this length of time demonstrates that much time is required for logs to rot completely. In the Kautz Creek drainage basin, a western hemlock, 142 years old, was found growing on a fallen log of western redcedar in which the heart wood had the appearance of freshly cut lumber. These data, scanty though they are, indicate that at least 200 years was required to form the humus layers composed of decayed wood.

The fact that all fallen logs, whose original diameters still can be measured with accuracy, are similar in size to the 300-year old living trees, indicates that perhaps most of the rotten wood in the humus was formed from trees that were of this size and age when the fell. No evidence was found to suggest that any appreciable amount of the humus was formed by smaller trees. Therefore, if these two assumptions are valid, that at least 200 years was required for the wood to rot and the trees which fell were 300 years old, then these trees must have begun to grow at least 500 years ago.

A fire occurred before the death of the first trees, which are represented by the oldest humus. Evidence of this fire consists of large fragments of charcoal and charred wood, 2 to 5 inches in diameter and 8 to 12 inches long, between the humus layers on the surface and mineral soil below. Measurement of the annual-ring curvature in two larger fragments shows that one has a radius of about 1 foot and the other about 1.5 feet. These fragments came from logs with minimum diameters ranging from 2 to 3 feet, thus the minimum ages of the trees represented by these fragments ranges from 100 to 200 years. The fire required to char wood of this size probably destroyed all humus and killed the contemporary trees. Therefore, the fallen logs and humus layers postdate the fire which formed the charcoal. The charcoal represents an additional interval during which trees grew on the drift-covered surface. Thus, the botanical evidence, consisting of the age of living trees, fallen logs in all stages of decay, humus layers, and charcoal beneath the humus suggests that the subsurface has been forested for at least 600 to 700 years. This evidence is summarized as follows:

---

¹The result of a carbon-14 analysis of a wood sample (unit 2, p. A-11, collected by R. D. Miller nearby at a level between the humus and the underlying pyroclastics was received subsequent to this study. The age is 1640±250 years Before Present (B.P.) (sample W—922, Rubin and Berthold, 1961).

---

These characteristics of old age are present in all parts of the old forest that were studied except on the southeast side of the valley. Here, a recent fire destroyed much of the evidence of the older forest. However, the presence of a few charred snags suggests that the forest downvalley from the moraine in this part of the valley is comparable in age to that on the northwest side. Charred logs found beneath the moraine boulders indicate that the fire, which largely destroyed the old forest, occurred before the last glacial advance which formed the end moraine.

PYROCLASTICS AND HUMUS SEQUENCE

From available botanical evidence alone, extrapolation of the age of the surface upslope from the 1840 moraine beyond 600 to 700 years is impossible. However, the material below the modern humus layer and charcoal suggests that more time, perhaps much more than 600 to 700 years, has elapsed since the surface beneath the old forest was last overridden by ice. The location of a section illustrating this is labeled pumice on figure 6, and the description (R. D. Miller, written communication, July, 1959) is given on page A—11.

The following section does not provide a basis for an age estimate, because the ages of the ash falls are not yet known. The till, designated as unit 10, represents the last glaciation of the valley outside the 1840 moraine. The essential question being discussed here is what length of time is represented by the pyroclastics, charcoal, humus, and living forest that now lie above the till. Arguments have been presented to support the conclusion that the present forest plus the preceding generations of trees represented by the rotten logs in the humus layer (unit 1) and charcoal (unit 2) represent a period of at least 600 to 700 years. The duff, or organic matter, units 4, 6, and 9, between layers of pumice is evidence of still earlier forests and may easily represent several hundred years. If the organic matter and pumice layers represent only 300 years, which seems to be a minimum, at least 900 to 1,000 years have elapsed since the surface downvalley from the 1840 moraine was last glaciated.

---

¹Carbon 14 age is 1640±250 years B. P. see footnote p. A—10.

---

Although detailed studies of the pyroclastic sequences in this area have not been made, the sketchy data indicate that the age of this surface is much older than one thousand years. For example, R. D. Miller and D. R. Crandell reported that the sequence found under the surface immediately downvalley from the 1840 moraine corresponds closely to one that they examined, located approximately 900 feet higher in altitude than the moraine (oral communication, July, 1959). The similarity of the pyroclastic sequences indicates that the age of the two surfaces is probably comparable and much older than one thousand years.