2015 Winter Lecture Series
Recent geologic history of the northern San Luis Valley and the Great Sand Dunes
by Rich Madole
USGS, Scientist Emeritus
7 PM, Wednesday, March 25, 2015
Room 130 Porter Hall
Adams State University
The talk will focus on new information about the geology and geologic history of the Great Sand Dunes National Park. This information was developed in the course of constructing the first detailed geologic map ever made of the area. The map is scheduled for release later this year. About 80 percent of the 143 square miles encompassed by the park is blanketed by windblown sand. This sand is part of a much larger body of windblown sand that extends along the west edge of the Sangre de Cristo Range for nearly 40 miles and covers about 240 square miles. At least five different ages of windblown sand are present at the surface; however, deposits of Holocene age (the time between 11,700 yr ago and the present) cover most of the area. Although the sand at the surface of the Great Sand Dunes is presently active, most of this massive body of sand (volume estimated to be about 10 billion cubic meters) began to accumulate prior to 130,000 yr ago.
The key to the geologic history of the Great Sand Dunes area lies not in deposits of windblown sand, but rather in ancient marsh and lake deposits in the westernmost part of the park and adjoining lowland. Groundwater flows westward from the edge of the Sangre de Cristo Range through or beneath windblown sand to discharge points (springs and marshes) west of the Great Sand Dunes. Several radiocarbon ages obtained from marsh and lake deposits indicate that twice during the past 10,000 years groundwater level rose several feet (possibly as much as 20) above present-day levels. During these times, marshes formed in many places and playas became lakes. However, when groundwater level lowered, as it did between 7000 and 3500 years ago and again between 1500 years ago and the present, lakes, ponds, and marshes dried up. During times of low groundwater level, wind eroded and deposited sand over large areas. Most of the sand in the sharply defined dunes that flank the Great Sand Dunes was deposited episodically during the past 800 years.
About the Speaker
Richard Madole: is a graduate of Case-Western Reserve University and earned an M.S. and Ph. D. in Geology at The Ohio State University. Dr. Madole has enjoyed a rich career in the private sector, academica, and with the U.S. Geological Survey. Well respected by his profession, he is now an Emeritus Scientist with the U.S.G.S. in Denver, CO. Dr. Madole's scientific specialties include surficial geology, geomorphology, Quaternary stratigraphy and dating techniques, and the application of these disciplines to determining recurrence intervals of natural hazards, such as landslides, floods, droughts, paleoseismicity, and wildfires.