Dr. Cílek, the Director of the Czech Republic’s Academy of Sciences Institute of Geology delivers a fascinating talk about the Bohemian Karst region of the Czech Republic, around Beroun, that weaves the human historical, mystical, and mythological elements with the underlying geology and speleology.
(00:36:32, stereo audio, 70.1 mb)
On solid ground — that’s how many of us think of good old, stable earth. So it’s disconcerting when the ground moves out from under us in any way.
Because of our environment, history, and geology, Colorado has conditions where ground movements can costs millions of dollars in annual property damage from repair and remediation, litigation, required investigations, and mitigation. There has been recent attention to swelling clay soils and heaving claystone bedrock, and the media has helped publicize these problems, which are predominant along the Front Range. But that’s only half the story. Geologic hazards in Colorado also include ground that sinks. Ground subsidence and soil settlement pose significant hazards in Colorado in many areas throughout the state. A variety of causes, some human-made and others inherent to the geology and geomorphology of Colorado, cause these sinking problems.
At the end of the 19th and beginning of the 20th Century, some of the first settlers of the plateau region of western Colorado along the Colorado River, and the Uncompahgre and Paonia river basins, looked to fruit crops for their livelihood. The semi-arid but moderate climate was well suited for fruit orchards once irrigation canal systems could be constructed.
But serious problems occurred when certain lands were first broken out for agriculture and wetted by irrigation. They sank, so much in places (up to four feet!) that irrigation-canal flow directions were reversed, ponding occurred, and whole orchards, newly planted with fruit trees imported by rail and wagon at considerable expense, were lost. While not understood, fruit growers and agriculturists began to recognize the hazards of sinking ground. Horticulturists with the Colorado Agricultural College and Experimental Station (the predecessor of Colorado State University) made one of the first references to collapsible soil in their 1910 publication, Fruit-Growing in Arid Regions: An Account of Approved Fruit-Growing Practices in the Inter-Mountain Country of Western United States (pdf download). They warned about sinking ground and in their chapter, Preparation of Land for Planting, made one of the first recommendations for mitigation of the hazard. They stated that when breaking out new land for fruit orchards, the fields should be flood irrigated for a suitable time to induce soil collapse, before final grading of the orchard field, irrigation channels excavation, and planting the fruit tree seedlings.
Many areas of Colorado are underlain by bedrock that is composed of evaporite minerals. Indicative of the word evaporite, these minerals were deposited during the cyclic evaporation of shallow seas that existed in central Colorado millions of years ago. As the water continued to evaporate, the remaining solution became hyperconcentrated with salts: minerals such as gypsum, anhydrite, and halite (rock salt). These minerals precipitate out of solution and accumulate in shallow nearshore basins on the bottom of the sea floor. Depending on the paleoevironment, thinly interbedded fine sandstone, mudstone, and black shales can also occur in the evaporite. Mostly Late Paleozoic and Mesozoic rock formations contain evaporite beds in Colorado. Some are thin and discontinuous — only minor beds within a rock formation. Others are massive, with evaporitic minerals many hundreds of feet thick.
Millions of years of burial, plastic deformation, mountain building, and erosion have forced the evaporite beds to the shallow subsurface and/or ground surface today. Evaporite minerals in Colorado are a valuable mining resource. Historic mining occurred throughout the state where thin gypsum beds were exposed. Active mining continues in the massive deposits near the town of Gypsum.