The Hisatsinom Chapter of the Colorado Archaeology Society will present Mary L. Gillam and Lillian D. Wakeley as they discuss “Are Utah’s Sand Island ‘Mammoths’ Late Pleistocene? A Geologic View”, on Tuesday, July 2, at 7 p.m, at the First United Methodist Church, 515 Park Street, Cortez. In their presentation, they will discuss two petroglyphs on a cliff near Bluff that have been interpreted as images of living mammoths and thus late Pleistocene in age. Evidence for erosion of the cliff is not consistent with this age interpretation. Also, partial erosion of younger petroglyphs shows that very old petroglyphs are unlikely to be preserved on such weak stone.
The petroglyphs are on a 20-meter cliff cut into Navajo Sandstone by the San Juan River after the last glaciation. The river first undercut the cliff, causing rockfall. After enough talus accumulated at the cliff base to protect it from the river, cliff erosion continued by fracturing, scaling and grain removal. Slow erosion by surface runoff and by moisture seeping through the rock erode the rock surface continually even though the “mammoths” are beneath a slight overhang. Geologic dating suggests that the cliff face beneath the “mammoths” stabilized during middle to perhaps early Holocene time, long after mammoths and mastodons became extinct around 13,000 to 12,500 years ago.
Some elements of these petroglyphs resemble mammoth anatomy. Yet their interpretation must be based on all available evidence, including the geologic setting. Further work on the petroglyphs and their geo-environment may resolve scientific questions but could take years or be inconclusive, partly due to cost and other limitations.
Mary Gillam is a consulting geologist with specialties in geomorphology, young sediments, and surface soils. She has worked for approximately 25 years on research and applied studies in most western states, including projects concerning archaeology or river geomorphology in the Four Corners region. Her degrees include a Ph.D. from the University of Colorado and an M.S. and B.S. from Stanford University. She has also taught introductory geology at San Juan College and is a “friend of the geosciences” at Fort Lewis College.
Lillian Wakeley is a consulting geologist with world-wide research experience in near-surface soil properties, desert and river geomorphology, specialty geo-materials, interpreting paleo-environments, and science for the public. She holds an M.S. from Utah State University (where she is an adjunct faculty member) and a D.Ed. from Penn State. Previously, she spent 27 years as an engineering and environmental geologist for the Corps of Engineers, where she led a geologic study of the “Kennewick Man” site.
Please contact Diane McBride at (970) 560-1643 for more information.