Welcome to this EarthCache. This educational page guides you through examining a large geological specimen: a massive, pinkish-orange boulder prominently displayed in a public space. While it now serves an aesthetic purpose, this boulder originated deep within the foothills of the Front Range and tells a story of an ancient, arid landscape that existed millions of years ago.
The Origin: The Lyons Formation
This boulder is composed of Lyons Sandstone, a distinct geological unit deposited during the Permian period, approximately 250 to 280 million years ago. At that time, long before the modern Rocky Mountains were uplifted, the region was part of a vast, dry desert landscape located near the equator
The ancestral mountains to the west were eroding, shedding immense amounts of sediment. Strong, persistent winds picked up these eroded grains, transporting and sorting them across a massive dune field known as a paleo-erg (an ancient sand sea)
Geological Features to Observe
By looking closely at the texture, layers, and coloration of this massive boulder, you can read the environmental conditions of the ancient Earth.
1. Eolian Textures and Sorting
Because this rock was formed by eolian (wind-blown) processes, the individual grains are unique. Wind is an exceptionally efficient sorting agent. It leaves behind fine-to-medium quartz grains while blowing away lighter dust and failing to move heavier pebbles . If you look closely at the boulder, you will notice that the sand grains are nearly uniform in size—a characteristic known as excellent sorting Furthermore, as these grains bounced along the ancient desert floor, their sharp edges were knocked off, resulting in highly rounded grains.
2. Cross-Bedding (Fossilized Dunes)
Look at the face of the boulder for sloping, intersecting geometric lines. These are cross-beds, the literal cross-sections of ancient sand dunes
As the wind blew across the desert, it pushed sand grains up the shallow, windward slope of a dune until they reached the crest. From there, the sand avalanched down the steep leeward side (the slipface), creating inclined layers . Over time, as changing wind directions shifted and sliced off the tops of older dunes, new dunes migrated over them, leaving behind stacks of intersecting, angled rock layers
3. Coloration and Mineral Cementation
The striking pink-to-reddish hue of the boulder is caused by the presence of hematite (an iron oxide mineral) coating the individual quartz grains This iron rusted over millions of years under highly oxygenated, arid conditions.
The sand grains are tightly held together by a natural cement—primarily secondary silica (quartz) and calcite—which fills the tiny spaces between the grains and makes this stone incredibly durable and resistant to weathering
EarthCache Logging Educational Questions
To log this EarthCache, please examine the boulder at your location and send your answers to the cache owner. Do not post the answers in your public online log.
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Grain Examination: Look very closely at the surface of the boulder (use a magnifying glass if you have one). Describe the size of the sand grains. Are they uniform, or do you see a mix of large pebbles and fine silt?
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Cross-Bedding Analysis: Locate the prominent layered lines (cross-beds) on the boulder face. Measure or estimate the angle at which these layers tilt relative to the flat ground. Do all the sets of lines tilt in the exact same direction, or do they intersect at different angles?
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Texture and Feel: Run your hand along a section of the stone. Does it feel smooth, or abrasive like sandpaper? Based on the heavy compaction and natural cementation explained above, do any grains rub off easily into your hand?
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Color Variation: Is the pink/red color perfectly uniform across the entire boulder, or do you notice lighter streaks, white dots, or darker bands? What do you think caused these variations?
References
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Kendigelen, O. (2016). Facies description, depositional environment and petroleum potential of the Lyons Sandstone, Colorado Front Range and Northeastern Denver Basin. AAPG Search and Discovery. https://www.searchanddiscovery.com/abstracts/html/2016/90260ice/abstracts/2474784.htm
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Ross, M. R., Hoesch, W. A., Austin, S. A., Whitmore, J. H., & Clarey, T. L. (2010). Garden of the Gods at Colorado Springs: Paleozoic and Mesozoic sedimentation and tectonics. Through the Generations: Geologic and Anthropogenic Field Excursions in the Rocky Mountains from Modern to Ancient, 77-93. https://digitalcommons.liberty.edu/cgi/viewcontent.cgi?article=1113&context=bio_chem_fac_pub
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Walker, G. (2004). Untitled [Facies Models: Eolian Sandstones]. Accounting, Auditing & Accountability Journal, 17. (Reprinted from Geoscience Canada). https://journals.lib.unb.ca/index.php/GC/article/download/3043/3560/6077
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Peter Barkmann proffesor at colorado school of mines
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a tad bit of gemnai ai