Establishing a sequence of events responsible for creating a particular feature or landscape is a primary goal of geology. This involves establishing which event or feature came first, whether it be the sequence of rocks in a mountain range or the pattern of glacial episodes in New England. Geologists often use the principle of “cross cutting relations” as a means of establishing such a chronology. This principle states that a feature is older than any feature that bisects (or cuts) it. For example, a fault (crack in the Earth that moves) must be younger than any rock unit displaced by the fault. This is intuitive, because one cannot cut a cake unless the cake is first baked.
A geomorphologist is a particular type of geologist that deals with the shape of the Earth’s surface, whether it be the action of rivers or the evidence of glaciers. The same cross cutting principles hold true for surface processes, and I noticed a perfect example while browsing Google Earth.
Queen Creek is a mostly ephemeral stream running east to west southeast of Phoenix, Arizona. Its headwaters are somewhere in the mountains between the towns of Globe and Superior, Arizona, and flows towards the suburbs of southeast Phoenix. Near the community of Queen, the creek leaves the mountains and empties into the relatively flat Gila River valley.
As Queen Creek leaves the mountains, an interesting drainage pattern develops. The creek occupies a relatively wide and deep channel that cuts across another drainage network. As seen in Google Earth, the character of Queen Creek is completely different than the network of smaller channels in the valley. One would normally expect the smaller channels to feed the larger Queen Creek as tributaries, but in many cases the smaller channels flow away from Queen Creek. The smaller channels divide and terminate near the mountain front, whereas Queen Creek crosses into the mountains and into a different basin. In the valley, the Queen Creek channel appears to cut into, and is topographically lower than the surface containing the smaller channel network. While I can’t say for certain because I have not investigated this area on the ground, I can reasonably conclude that Queen Creek is younger than the network of other channels by the principle of cross-cutting relations.
Obviously, then, a network of channels developed that terminated near the mountain front. At some later time, something changed, and Queen Creek now dominates the modern drainage pattern. Without investigating this area in the field, it’s hard to say what initiated the change. It could something drastic like tectonic activity, a gradual climate shift, one stream capturing another via mundane erosional processes, or even activities of mankind. Similar drainage network oddities are common in Southwest North America, and each oddity tells a story of some change to the status quo.
I spend a good portion of my work week looking at aerial imagery. While I can’t explore even a quarter of the interesting things I see, I can still contemplate their origins from the imagery.
P.S. – Please excuse the rather lousy power point annotations to the Google imagery, but that’s the best software I have available on my very old home computer. It’s difficult to see what I am talking about in this format. But, if you are really interested, the coordinates for the area are on the bottom of each photo, and you can explore the area on your own in Google Earth.