The Las Guijas Mountains are a small northwest–southeast trending

mountain range A mountain range or hill range is a series of mountains or hills arranged in a line and connected by high ground. A mountain system or mountain belt is a group of mountain ranges with similarity in form, structure, and alignment that have arise ...

in southern

Pima County, Arizona Pima County ( ) is a county in the south central region of the U.S. state of Arizona. As of the 2020 census, the population was 1,043,433, making it Arizona's second-most populous county. The county seat is Tucson, where most of the population ...

. The range is approximately long by .''Sells, Arizona-Sonora,'' 30x60 topographic quadrangle, USGS, 1994 Surrounding ranges includes the

Cerro Colorado Mountains The Cerro Colorado Mountains are a low mountain range in southern Pima County, Arizona, USA. The highest point of the range is Colorado Peak (). The range consists of a NNW–SSE trending ridge with several shorter ridges extending off the main ...

to the northeast, the

Tumacacori Mountains The Tumacacori Mountains is a mountain range in Santa Cruz and Pima counties in southern Arizona, United States. The highest peak in the range is an unnamed summit with an elevation of . Tumacacori Highlands The Tumacacori Highlands is a series ...

of Santa Cruz County to the east, the

San Luis Mountains The San Luis Mountains are a small, lower elevation mountain range of central-southern Pima County Arizona adjacent to the U.S.-Mexico border, northeast of Sasabe, Arizona–Sasabe, Sonora. The range is northwest-southeast trending, about in len ...

to the south and the Baboquivari Mountains across the Altar Valley to the west. Arivaca is immediately to the southeast and the old mining townsite of Las Guijas is in the wash just north of the range. The highest peak of the range with elevation of lies northwest of Arivaca which is at . Las Guijas Peak at lies just 4500 feet south of the highest.''Cerro Colorado, Arizona,'' 7.5 minute quad., USGS, 1979

Name

The name of the range came from 19th century Spanish miners referring to ''las guijas'' for ''the rubble'' as the placer gold they were working occurred in the

gravel Gravel is a loose aggregation of rock fragments. Gravel occurs naturally throughout the world as a result of sedimentary and erosive geologic processes; it is also produced in large quantities commercially as crushed stone. Gravel is classifi ...

s or conglomerates along the stream valleys and gulches draining the range.

Geology

The southwestern flank of the range in underlain by up to of

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...

age ash fall

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...

rhyodacite Rhyodacite is a volcanic rock intermediate in composition between dacite and rhyolite. It is the extrusive equivalent of those plutonic rocks that are intermediate in composition between monzogranite and granodiorite. Rhyodacites form from rapid ...

composition known as the ''tuff of Pajarito'' named for the Pajarito Mountains of Santa Cruz County to the southeast, later referred to as the Cobre Ridge Tuff.''Geologic units in Pima county, Arizona, Jurassic volcanic rocks,'' USGS
/ref> This tuff was sourced from the Cobre Ridge caldera. The northwest end of Cobre Ridge lies around south of the Las Guijas range. Younger Jurassic or Cretaceous sandstone and other sediments occur above the tuff.Riggs, Nancy R. and Cathy J. Busby-Spera, ''Facies Analysis of an Ancient, Dismembered, Large Cauldera Complex and Implications for Intr-arc Subsidence: Middle Jurassic Strata of Cobre Ridge, Southern Arizona, USA'' Sedimentary Geology, 1991, v.74 pp. 39–68 The northeastern flank of the range is underlain by an intrusive

granite Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...

also of Jurassic age.

References

{{Mountains of Arizona Mountain ranges of Pima County, Arizona Mountain ranges of Arizona