The mountains of today were formed through cycles of layer deposits, block uplifts, and erosion. As the final uplift began, the Salt River began downcutting, carrying debris to the surrounding lowlands. The coarsest materials were dropped close to the mountains, while finer sediments were carried out into the center of the basin. Natural processes formed the caves, probably starting between 50,000 and 400,000 years ago. The rock alcoves that house the cliff dwellings are located in a layer consisting primarily of siltstone. Water dissolved the minerals binding the rocks together. Spalling formed the caves. During spalling, thin layers of siltstone break loose and fall from the ceiling. The thin layers of siltstone were fashioned into tools and weapons by the prehistoric peoples.
Another geologic feature seen at the monument is the distinctive Gila conglomerate rock unit. Gravel, clay, and silica were cemented together to form these interesting rocks. Some cementing is done with caliche, the natural cement of the Southwest. As groundwater evaporates, it leaves behind a tiny amount of lime, which gradually cements the smaller stones together. Many smaller rocks appear to be stuck together within a larger rock. Weathering and erosion are continuing forces of change in the monument’s landscape of today and tomorrow.
Did You Know?
Tonto National Monument is home to a crested saguaro. Biologists disagree as to why some saguaros grow in this unusual form. Some speculate that it is a genetic mutation. Others say it is the result of lightning or freeze damage. About one in 150,000 saguaros develop this unusual growth.