“Geologists have a saying—rocks remember.” —Neil Armstrong
It is true that a record of the earth’s formation is held in the rock around us. We are fortunate living on the slope of the Santa Catalina Mountains, having a geohistorical landscape exposed before us. The stories are locked in the stones we walk on. If we only understand the language.
Nowhere is the story more obvious and yet more curious than in a local landscape stone we see every day. This distinctive stone is placed in yards and on the roadsides in SaddleBrooke due to its attractive mosaic of colors and shapes of embedded river stones. Clastic conglomerate is a sedimentary rock* combined with stones* or clasts.
Just how did those water-polished stones end up in this relatively young sedimentary rock? This unusual-looking stone reveals the details of its life story, starting with the upheavals that created the Catalina Mountain Range. Portions of primordial rock were initially loosened from outcroppings and underwent an extended process of weathering and erosion (over many thousands of years). Energetic water action within the Canada del Oro Watershed transported and rounded these fragments tumbling downhill and downstream (for additional hundreds of years), ending up in a basin or depression where layers of sediment are subsequently deposited.
A sediment binding the clasts together may be a mixture of sand, mud, and chemical cement. While this sediment is deeply buried, it becomes compacted and hardened (hundreds to thousands of years more) forming this unique sedimentary rock. The matrix made up of different mixtures gives the stones we see their overall gray, brown, or red color, contrasting with the various colors and rock types of the rounded pebbles, hence the unmistakable mosaic appearance.
Once formed, layers of this conglomerate rock can be unearthed and again reduced by weathering (once more eons pass), exposing the now encased stones. In spite of the extreme age of the rock precursors to the river stones, the marriage of river stones and rock is a relatively recent event, a blink of an eye in geologic history.
Several facts are obvious: The formation of clastic conglomerates requires time and the movement of water. Who knew that the wide dispersal of these stones around the SaddleBrooke area provides evidence of tens of thousands of years of abundant waterflow and flooding throughout the Canada del Oro Watershed? Rarely is the geology so clearly demonstrated, giving us this beautiful result we all see and enjoy.
Yet a surprise connection is the prehistoric human presence in this same part of the Gila River Drainage. Attracted to the same lifegiving water and sedimentary soils essential to the formation of clastic conglomerates, some of the earliest Arizona human civilizations are confirmed by a rich geoarchaeology record of the Western Catalinas.
Our understanding of these ongoing natural processes, the geology, is part of Arizona history, just as is the physical history of our ancient predecessors, the archelogy. It is fascinating to me that each of these earth records are “set in stone.” So, you see why I agree with Neil Armstrong—“Rocks Remember”!
* Though often used in English interchangeably, “rock” is the mineral mass as formed by nature, whereas “stone” refers to rock that has been broken up or worked.