In the same way, the Earth tells a story through the pattern of wrinkles on the surface of the planet. Some rocks are softer, others are harder, and some are in horizontal layers while others are tilted. Some aren't in layers at all, such as plutonic intrusive rocks, so the type of rock exposed is fairly uniform, unless it has been broken up by jointing. Water and other agents of erosion attack the rocks in a differential manner, and reveal something of the underlying structure in the way that streams cross the landscape (the most commonly recognized drainage patterns are shown in the diagram below).
|The top left frame should be labeled "Dendritic", and the top right "Parallel". From NASA (http://daac.gsfc.nasa.gov/geomorphology/GEO_4/GEO_CHAPTER_4.shtml)|
The first photo above doesn't reveal a lot of rock, as most of the surfaces are covered by grass and brush. If you concentrate on any one drainage, you will notice that many small tributary channels gather into larger and larger channels in the way that a tree has multitudes of branches that end up at a single trunk. This is an example of dendritic ("tree-like") drainage pattern, which indicates that the underlying rock is fairly uniform. There is no evidence of sedimentary layering, and thus the rocks must be part of the Salinian Block, the Coast Range Ophiolite, or the Franciscan Complex (layers are found over short distances in the latter two, but not enough to effect the overall drainage pattern). Of course, this part of the Coast Ranges is right in my backyard, so I knew I was seeing mainly the ophiolite found in the upper part of Del Puerto Canyon above the town of Patterson.
I saw one other really distinctive drainage pattern on my trip, but far removed from these first two photos. If you look at the photo below, you might pick out that many of the smaller channels are dendritic, but that the larger channels are roughly parallel to each other. This is called a parallel drainage pattern, caused by streams eroding uniform, but slightly sloping rocks. I was lost at the moment because we had been flying over clouds for several hours, but the moment I saw these channels, I knew exactly where we were: on the flanks of the Valles Caldera (sometimes called the Jemez Caldera). There is no other place in New Mexico (and indeed most of the southwest) quite like it.
|Image courtesy of NASA (http://earthobservatory.nasa.gov/IOTD/view.php?id=50666)|