Geology Quicklinks. Smaller lakes still covered the valley floor, including two broad lakes in the northeastern side of the valley. Large amounts of sediment from the volcanic San Juan Mountains continued to wash down into these lakes, along with some sand from the Sangre de Cristo Mountains. Dramatic natural climate change later significantly reduced these lakes, leaving behind the sand sheet. Remnants of these lakes are still found today, in the form of sabkha wetlands. While smaller individual dune ridges show changes over 138 years, larger dune forms have remained fairly constant. Thus, either the Great Sand Dunes grew and changed very slowly and uniformly over vast time - or they formed relatively quickly during an era of extreme drought, and are now relatively stable. William Henry Jackson - 1873Kris Illenberger - 2011.
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