It was October 2, 1966 when my Uncle, the late Dr. Ted Lathrop, stood atop Mt. Thielsen and observed what he believed to be a glacier below on the steep, picturesque northeast face of the mountain. Two years later, we verified the existence of this small glacier, located in a beautiful cirque. It is the southernmost glacier in Oregon (Lathrop, 1968, Nafziger 1971, 1976, 1981, 1987, 1993, 2000, 2004). After Dr. Lathrop’s death in 1979, I was successful in having the Oregon and United States Geographic Names Boards name the glacier in honor of my Uncle, who was an active Mazama. Since 1968, I have led small parties to observe and photograph the glacier. In 2005, I visited it for the 34th time.

The glacier can be accessed by the Mt. Thielsen climbers’ trail, which heads east from highway 138. At the Pacific Crest Trail junction, a turn north guides the hiker along the spectacular west cirque and up onto a ridge, which affords views towards Diamond Peak and the Sisters. A switchbacking descent leads to Thielsen Creek and our annual campsite (either in the trees or on an open area on a knoll with the mountain looming above). I believe that this area is one of the most beautiful in Oregon. A general review of the geology of Mt. Thielsen has been published (Nafziger, 1974).

Lathrop Glacier consists of two ice “arms” extending up adjacent steep chutes. A larger mass of ice is located on top of the irregular terminal moraine, which was formed by a larger, earlier glacier. This ice mass is covered with boulders eroded from the mountain. A small pool at the east edge of the moraine forms the source of Thielsen Creek. A rock scramble up the terminal moraine is required to observe the glacial “arms” close up. There is a steep descent on the moraine from the west to east “arm.”

During the first few trips, I established a network of 26 points from which I could photograph the glacier each year. Early attempts to use standard glacier measuring techniques failed owing to the glacier’s steep angle of repose and relatively rapid ice movement. As the years progressed, a few of the standard photographic points became useless owing to tree growth.

Accordingly, I decided to use the photographs as a basis for measuring the areal extent of the ice on the “arms.” With the exception of one Mazama backpack in 1974, for the first 25 trips, I took one to three people with me, including my Uncle and cousin Jim Lathrop. Since 1997, the trips have been official Chemeketan day hikes and backpacks. Some of these have included Ted’s grandchildren. I feel a personal responsibility, commitment, and passion to continue these trips as long as possible.

A planimeter was used to measure the visible ice mass on each “arm” from projected 35 mm slides. The planimeter measurements were converted to feet using calibrated geologic markers at the edges of both glacial “arms” and the summit. The camera, lenses, projector, projector-screen distance, etc. all were maintained constant over the years to minimize variability. Results from measurements of the areal extent of the ice on the two “arms” from the photographs are summarized in the accompanying graph. Similar measurements on the base ice mass were not possible owing to the thick rock cover.

In the graph, three curves depict the measured areal extent of each “arm” and the sum of both “arms.” Notable are the peaks representing maxima extent of the total ice on the “arms” in 1972, 1974, 1983, 1999, and 2003. Minima occurred in 1970, 1973, 1981, 1992, and 2001. The cycles are irregular in length, ranging from 2 to 16 years over the 35 years of measurements. With the exception of the five maxima, the glacial “arms” have decreased gradually (by about 36%) over this period of time (slightly more than 1% each on average). At this rate, the glacier could disappear by about 2070, thus following the disappearance of Sholes Glacier on Mt. McLoughlin in 1939.

Lafrenz (2001) attributes the glacier’s existence to its preferential topographic position and to an insulating debris cover. He expects that it will persist longer than a bare-ice glacier of similar size. Lathrop Glacier is located in a steep northeast-facing cirque whose width (beneath the terminal moraine) is greater than its length, with a windowed pass, and a peak to the southwest. These factors, together with minimal sunlight and an active rockfall allow the glacier to exist. It is expected that Lathrop Glacier will slowly and steadily decrease in size over the next several decades. This is consistent with observed global warming trends and with the behavior of most of the glaciers in the Cascade Range.

Bibliography

Lafrenz, M. D. 2001. The Neoglacial history of Mt. Thielsen, Southern Oregon Cascades. M.S. Thesis, Portland State University, Portland, OR, 80 pp.

Lathrop, T. G. 1968. Return of the Ice Age? Mazama, 50(13),34-36. Nafziger, R. H. 1971. Oregon’s southernmost glacier: a three year report. Mazama, 53(13),30-33.

Nafziger, R. H. 1976. Mt. Thielsen’s glacierettes: a five-year update. Mazama, 58(13),17-20.

Nafziger, R. H. 1981. Lathrop Glacier revisited: 1976-1981. Mazama, 69(13),50-51.

Nafziger, R. H. 1992. Update on Lathrop Glacier from 1988 to 1992. Mazama, 74(13):106-107.

Nafziger, R. H. 2000. Lathrop Glacier from 1994 to 1999. Unpublished report.

Nafziger, R. H. 2004. Update on Lathrop Glacier: 2000-2004. Unpublished report.