As you may recall, a few months ago I took advantage of some exceptionally good weather to hike up Camelback Mountain near my house. I posted a photos of a bee hive and a chuckwalla; here are four more observations.
Two rock types comprise Cambelback Mountain. Precambrian-age Granite makes up the back of the mountain. Overlying the granite, a much younger Tertiary-age conglomerate and sandstone (appropriately named the “Camel’s Head Formation”) forms the head. Between the two rock units is what geologists call an unconformity. An unconformity is a period of “missing” geologic time. At an unconformity, no rocks record the passage of time.
Despite the missing rock record, unconformities themselves reveal clues to the geologic history of an area. An unconformity usually indicates tectonic activity such as uplift or tilting that altered the geologic setting. In this particular case, much younger sedimentary rock overlies much older intrusive igneous rock. Intrusive igneous rocks form as magma cools deep within the crust. We don’t know all of what happened during the time of this unconformity, but we do know the granite was uplifted to the surface, as all of the overlying rock has since been removed. Over the next billion years or so, all kinds of things could have happened – uplift, erosion, subsidence, and deposition. We know the last event prior to the deposition of the Camel’s Head sediment was a period of erosion, because the granite did not form at the Earth’s surface and nothing lies between the granite and the Camel’s Head Formation. Eventually, the geologic environment changed from erosion to deposition, allowing deposition of the sand and gravel that makes up the Camel’s Head formation. This particular unconformity represents over a billion years of “missing” geologic time.
2. Large conglomerate boulder.
Large conglomerate boulders rest near the base of the Camel’s Head, and several are adjacent to the Echo Canyon trail. This particular block is about the size of a small house. The Camel’s Head Formation must be well-indurated for a boulder of this size to survive a tumble down the hillside.
A closer look at the boulder reveals mostly angular gravel and cobbles comprise the coarse fraction of the conglomerate. In many beds, the gravel shows a strong fining-upwards character (the gravel pieces get smaller higher in the sedimentary bed).
3. Igneous dike.
This dark-colored igneous dike cuts through the Camel’s Head formation. I didn’t hike over for closer inspection, but it appears to be about a meter wide and made of basalt. This dike was formed as magma intruded into a crack in the already-deposited Camels’ Head Formation. The dike cuts the Camel’s Head Formation, so the dike is younger.
Included within the granite are these areas of darker, finer-grained minerals. They range in size from a US quarter to about the size of a baseball. The darker minerals indicate a different chemical composition than the granite, and together with the finer texture indicate these areas are a different rock type included within the granite. Granite is an intrusive igneous rock formed when magma cools within the Earth’s crust. These darker included rocks are likely fragments of the magma chamber wall that broke off as the magma cooled.