The Time Required for Podzol Profile Formation as Evidenced by the Mendenhall Glacial Deposits Near Juneau, Alaska

MENDENHALL GLACIAL DEPOSITS NEAR JUNEAU, ALASKA ROBERT F. CHANDLER, JR. BECAUSE of the uncertainty and great length of time that has elapsed since most soils started to form, it is rather difficult to evaluate time accurately as a soil-forming factor. The deposits left by glaciers that are now receding provide an opportunity to obtain relatively accurate estimates of the rate of soil development. The Mendenhall Glacier, about 12 miles north of Juneau, Alaska, has been receding at various rates for a considerable period. The age of the trees growing on the deposits left by the receding glacier give one a reliable estimate of the age of the soils. In addition, the actual position of the glacier during the past 35 or 40 years has been indicated by markers. A sketch map of the region around the Mendenhall glacier is presented in Fig. i. The positions at which samples were obtained and the approximate age of the soils are indicated on this map. The Mendenhall glacier is 7 to 9 miles long. It arises in a large ice field in the mountains to the northeast, and is one of the many glaciers that flow out of the region. The glacier is about 2 miles wide. A photograph of the glacier taken from a bridge across the Mendenhall River is shown in Fig. 2. The history of the glacier is not known for any great period, but from the age of the trees it apparently has been receding up the valley a distance of 2 to 3 miles during the past 250 to 300 years. It is known that the ice halted during the period between 1740 and 1850 and formed a moraine about I YZ miles in front of the present glacier. The go-year old samples were obtained from thismoraine. Between 1850 and 1906 the glacier retreated about Y* mile. Since 1906 it has been receding more rapidly averaging about 70 feet a year. Vegetation becomes established quickly on the newsoil material. Within 2 years lichens, mosses, and fireweed are growing. In 3 years grasses and red alder come in, and in 15 years Sitka spruce seedlings are present. The age of the soil at the various locations was ascertained by determining the tree age by ring counts and adding 15 years. This, of course, worked well when only one generation of trees had grown on the site. In the case of the mature podzol profile designated by an age of ± 1,000 years, the age could not be accurately determined. The fallen logs and decaying wood in the humus layer indicated that at least two and perhaps four generations of trees had grown on the site. Mature trees in that region vary from 250 to 500 years of age. The vegetation on the 15-year-old soil was principally red alder with a few small Sitka spruce seedlings. At the go-year-old site the vegetation was largely Sitka spruce with an abundance of mosses as ground cover. On the 25o-year-old site Sitka spruce and a few aspen dominated, while the ± 1,000-yearold site was covered with the climax vegetation of the region, Sitka spruce and western hemlock. The 15-year-old profile showed no visible horizon differentiation. The go-year-old profile appeared very similar except that a 2-inch A0 horizon had accumulated on top. Both profiles were a greenish-yellowish gray color, exhibiting no visible evidence of weathering. The 25O-year-old sample showed a thicker A0 horizon (5 inches) and the upper 5 or 6 inches of soil showed a little brownish color, indicating that the soil material was weathering and free iron oxide was forming. The ± i,ooo-year-old profile had an 8-inch accumulation of organic matter on the surface. It was well humified and would be classed as a greasy mor humus layer (3). The A2 horizon was an ash-gray color and ranged from 2 to 4 inches in thickness. The B horizon was dark coffee brown and showed a tremendous accumulation of humus and iron. It was shallow, being only 6 to 8 inches in thickness. The parent material was an olive green to gray color but much heavier textured than the original soil near the glacier. It is unfortunate that this study could not be made of soils developed from uniform parent material. However, it can be logically assumed that a part of the extra clay in the mature soil resulted from weathering over a period of time, even though the material probably contained considerable silt and clay when deposited. The geology of the region is somewhat complex and is thoroughly treated by Knopf (4). The glacier