Soza Mesa is one of the most remarkable landmarks and habitats of our San Pedro Valley. Viewed from the south toward its furthest Eastern up-slope end from across Hot Springs Canyon, as in the banner photo above, it forms a striking horizontal edge to the Canyon itself. Viewed as below, again from the south across-Canyon looking toward the northwest, it gradually begins to decay more strongly at its Western edges, dropping down toward the San Pedro River, with the Catalinas standing far behind it on the other side of the River:
Viewed from overhead (see map below), it appears to spread out from its headlands like a long flat hand with many pointed fingers, approximating ridges which descend for some 5 miles toward the San Pedro River, well out of view to the left:
From the air, flying near the Cascabel Road and looking Eastward toward the Galiuros in January 2003 (below), Soza Mesa's perennial tabletop carpet of green (overwhelmingly composed of Creosotebush!) sets it off from the more broken array of hills located on each side of it. The "root" of the great (now remnant) alluvial fan may be seen at the center of the upper-left quadrant of the photo, where the massive Quiburis-era deposition of sedimentary gravels distributed the Bajada gradually downward and outward from the Galiuro Mountains of 7.7 to 5.5 Million years ago. Later, the upstream cutting of the San Pedro River drainage from the Gila River produced massive erosion of the fan at its edges. For more detail on this time period, see Geology Walk: More recent patterns.
Here, below, is a view of it from a different angle and considerably lower aerial elevation, taken in January 2003. Here the mesa top is a strikingly horizontal line running across the middle of the image, standing well above the Yellow Cliffs and other visible features of Hot Springs Canyon (lower-center). The cutting of this Canyon several million years ago tore away the superficial layers of Quiburis Formation to expose the Galiuro Volcanics, San Manuel, and Willow Canyon Formations you can see flanking the wash below:
We of Saguaro Juniper however much more typically view Soza Mesa from below and/or among its fingers, as the images which follow illustrate. (These next three images were taken in February of 2004, hence their overall drabness.)
From the standpoint of the bottoms of Soza Mesa Washes, each wash is relatively wide and sandy, and this holds true at the upper reaches as well as the lower ones. Below, we are looking upstream from a position near the Western bottom of the Soza Mesa fingers (not very far from the San Pedro River). At this point, two large washes, running out of the upper-left and upper-central part of the photo, converge toward the right, meeting at the lower right of the photo, with a low (but very long) ridge dividing them upstream (marked with a small roadway running up it from below). These are very wide washes indeed.
Below, a view from a more northern finger, the Jack Murphy Wash, which is huge even at this very substantial distance from the San Pedro River confluence (the very light green strip barely visible in the center of the upper-left quadrant of the photo). This is one of the washes whose runoff devasted parts of Cascabel north of Hot Springs Canyon during the July 2003 floods. We photographed here in February 2004 while repairing fences.
Below, a smaller wash also at a long distance from the San Pedro, running down from the Mesa toward the viewer, then curving sharply to the right at the bottom of the photo. Even here the wash is wide and sandy. Note that at right-center its flow has exposed bedrock of the Quiburis Formation, which is present all along these mesa fingers. Note also the large old Saguaro at middle-left, standing right in the middle of the wash in February 2004.
Following the strong summer rains of 2006, we found this massive saguaro entirely toppled:
A significant ecological feature of the Soza Mesa drainage is the prevalence of east-west-running washes, as you can see clearly in the map below, where the giant tableland adopts the aspect of a great octopus extending its tentacles out towards the river valley below it:
Along these washes, the ridge slopes are either south-facing or north-facing, which conditions very different vegetation growth on these two types. Below left, looking upstream, both kinds of slopes are evident, with the south-facing slope at upper left and the north-facing one at mid-right. The main contrast you can see in this image is a darker reddish color on the south-facing side. Below right, a view of this particular south-facing slope from a standpoint directly opposite to it. Note the virtual absence of grasses, with only scattered shrubs and trees established along the slope. (Click on each image to enlarge it.)...
Below left, the north-facing slope at the same location is packed with various annual plants and perennial grasses (including clumps of Bush Muhly). A short distance further downstream, below right, grasses continue, along with a Yucca elata plant, found in our uplands only where moisture is relatively good. (Click on each image to enlarge it.)...
This contrast in vegetation is consistent throughout these east-west drainages, though the differences are more striking following good monsoon rains than they are after drought.
These relatively large (and big-flooding) washes are not the only formations in the lower reaches of Soza Mesa. Some of the smaller side-washes may boast what becomes in good monsoon seasons a rather lush vegetation. For example, in August of 2004, parts of the West Wash (a tributary of Hot Springs Canyon) display the kind of richness seen below -- at left, in the foreground, a substantial stand of Bush Muhly; at right, a variety of perennial grasses share the ground with a rich array of flowering annuals and cacti: (Click on each image to enlarge it.)..
Soza Mesa is named after the descendants of a Spanish soldier whose family moved from Tubac to Tucson in the late 18th Century, some of whom proceded into the San Pedro River Valley in the 19th Century. There they established an extensive domain and a quite prominent dynasty. For more information, see Edward Soza.