Geological History of Saguaro National Park

Saguaro National Park, as you may have noticed by now, is in Tucson, Arizona – in fact, Tucson Arizona actually splits the park into two separate districts. Stands to reason then, that the geological history of Saguaro National Park is actually a large part of the geological history of the Tucson Basin.

The Tucson Basin is comprised of a section of the upper Santa Cruz Basin, a valley which drains towards the northwest bordered by rugged mountain ranges. The northeastern boundary of the Tucson Basin is formed by the Santa Catalina and Rincon Mountains (penny beginning to drop?) and the boundary on the west is formed by the Tucson Mountains (got it now?)

The City of Tucson, Arizona, is right slap bang in the middle of the Tucson Basin (1,000 square miles of it) at an elevation of around 2,300 feet. The Rincon Mountains have a maximum elevation of just over 8,500 feet, and the Tucson Mountains don’t even manage to reach 5,000 feet (they stop at around 4,687).

The Rincon Mountains are encompassed by the Coronado National Forest and the Rincon Mountain District. They are composed of foliated and folded banded gneiss, as well as schist and the granite which can be dated back to the Precambrian age, with younger colluvial and alluvial deposits draped over the base, often forming a thin cover. The Tucson Mountains, on the other hand, are dominated by Cretaceous and Permian limestone, as well as red beds, arkose and volcanics. There are also some quaterney gravels present.

The Tucson Basin, however, is filled with the lacustrine, fluvial and debris deposits from the erosion of surrounding mountains and the ranges further up in the drainage. Alluvial fan deposits can be found at the perimeter of the Tucson Basin, and the center of the basin has flood plain and river channel deposits. Typically, these are Tertiary and Quaternay in age, up to an amazing 8,000 feet thick in places.

Saguaro National Park is home of the “green giant” . . . the Saguaro Cactus, which has an incredibly slow growth rate. Growth actually occurs in fits, starts and spurts (a bit like my youngest, I swear he’s grown a full 6 inches in as many months) – most of the growth of the Saguaro takes place each year in the summer rainy season. A saguaro seedling might measure only 13 or 14 inches after one year yet may be over seven feet tall by the time it reaches 50 years of age. The first branches (or “arms”) don’t generally sprout until the Saguaro cactus is 75 years old, starting life as prickly balls before extending outwards and upwards. It’s not until this stage is reached that the saguaro begins to flower, producing seeds and fruit.

He seems to be just chillin’ and enjoying the view.

By 100 years of age, the Saguaro cactus may have reached 25 feet tall, some live 150 years or more growing to an astonishing 50 feet high and weighing in at around 8 tons . . . how’s that for impressive?

Some Saguaros die from old age, some quite frankly never reach a ripe old age. The seeds and seedlings may be eaten by animals, large saguaros are killed by winds and lightning, severe drought can weaken and kill saguaros of all ages. To put it plainly, the saguaro cactus lives in a constant state of vulnerability.

Okay, you’re not permitted to harvest the fruit in the Saguaro National Park without permission, but I thought you might like to see it anyway!