The Editor,
The Journal of Glaciology
Sir,
Small spheroidal, silt-packed, moss cushions, or polsters, are abundant on the terminus of Matanuska Glacier (lat. 61° N., long. 147° 41″ W.) in the Chugach Mountains of south central Alaska. In outward appearance, these resemble “jökla mys” (glacier mice), described and named from observations and collections on Hrútárjökull, Iceland, by Dr. Jón Eythórsson (Journal of Glaciology, Vol. 1, No. 9, p. 503, 1951). The jökla mis described were moss-covered stories, whereas specimens from Matanuska Glacier are concentric moss layers in which sandy silt and a few small pebbles have been incorporated during growth.
On 10 September 1950, L. L. Ray, F. C. Whitmore, Jr., A. C. Orvedal and I made a reconnaissance of the broad terminus of Matanuska Glacier. The terminus, at an altitude of approximately 1700 ft., is a gently sloping surface mantled, in places, by mounds and ridges of cobbly ablation moraine with relief as much as 50 ft. The polsters were called to my attention by Dr. Ray, who first noticed them; others of the party aided in determining their widespread occurrence.
Polsters occur on gentle slopes of bare melting ice in about the same abundance as in waters of supraglacial streams; a few were collected on patches of ablation moraine. Where they rested on exposed ice surfaces, they sometimes lay in shallow wells thawed by their insolation. There was no apparent preferred orientation of the polsters in the thaw wells.
The dominant shape is a distinctly oblate spheroidFootnote *; the shortest axis does not exceed one-half the longest, which ranges from 1 to 6 in. (2.5 to 15.3 cm.). Irregular shapes are less common and of similar size. Surfaces are firmly bound with living moss, although on spheroidal types one of the two flatter sides, usually that on which it has rested most recently, has a somewhat less luxuriant growth. A diameter of 6 in. (15.3 cm.) appears to be the maximum limit for spheroidal polsters; many of the larger ones were broken or eroded. A size limit may be imposed by tensile strength of moss stems.
Specimens were sent to Professor W. C. Steere, Stanford University, who identified five species, Ditrichum flexicaule, Andreaea rupestris, Pohlia nutans, Ceratodon purpureus, and Polytrichum juniperinum. In ordinary habitats the last four of these form a loose mass, whereas Ditrichum tends to form compact, close-meshed polsters and is thus a primary factor in the formation of spheroidal bodies.
Internally, both types of polsters have poorly defined cores about one-quarter to one-half inch (0.6–1.3 cm.) in diameter, composed of intermixed dead moss and sandy silt. Concentric layers of moss and enmeshed mineral matrix are discernible either partially or completely enveloping the core. Variations in layer thickness and lack of definite boundaries between layers indicate absence of distinctly cyclic accretion. The included mineral matter is poorly sorted, with grains ranging from clay sizes up to one-quarter inch, although particles of silt and very fine sand are dominant.
Polsters develop from a young plant in the vegetative phase or from a fragment of an older plant. Once a Ditrichum plant, or colony, assumes a polster-like form, presumably through normal branching, conditions on the melting surface of a glacier favor development of a spheroidal mass. No material on the surface is sufficiently stable to serve as a substrate; instead, each polster is jostled about and alternately washed by flowing melt water charged with fine rock debris, rolled along melt water streams, tumbled down oversteepened ablation slopes, and slowly rotated as it melts into the ice surface. Fine-grained mineral matter is trapped among the minute surficial leaves. Continued movement molds the spheroidal shape and encourages nearly equal growth on all sides. Mosses temporarily on the underside would conceivably obtain sufficient reflected light for continued photosynthesis.
As mosses are capable of surviving relatively long periods of drought and of reviving to full growth activity within a few hours with light and water available, freezing temperatures or temporary stranding on dry surfaces merely suspends growth.
Moss polsters on Matanuska Glacier furnish an extreme example of plants adapted for, and, in a sense, a product of changing habitat conditions. The growth form, especially of Ditrichum, and ability to survive periods of drought, make growth of these mosses possible on the melting glacier surface, where the substrate is too unstable to allow normal plant colonization unless plants can carry with them in their mobile environment a soil-like substrate.
Dr. Eythórsson pointed out that his jökla mys are contrary to the adage about rolling stones gathering no moss. I would bring to the Editor’s attention that the related jökla mýs of Alaska indicate a further paradox—rolling moss does gather stones!
