Plutonic Relationships

We are somewhere near the 100th anniversary of the completion of one of the most remarkable geologic maps ever, and yet the map was never published during the author’s lifetime and indeed has largely been overlooked despite the tremendous strides in understanding and incredible effort the map represents.

When the US Geological Society decided in 1913 that a full and complete study of Yosemite Valley was needed, they tapped topographer François Matthes and geologist Frank Calkins.  Matthes, who moved from the topographic to the geologic branch, has since been in the limelight in Yosemite, interpretive materials for most of the twentieth century based on his input. It was Calkins, though, who mapped the bedrock of the valley in remarkable detail on the equally remarkable topographic baseman prepared by Matthes a decade earlier.

Calkins1

At the time, geologists were still struggling to understand how the large mass of granitic rock of the Sierra had come to be.  Henry Ward Turner, working with Waldemar Lindgren on the original folio maps of the northern Sierra, wrote in 1896 that “the intrusion of the large granodiorite masses…took place on so gigantic a scale that the mind strives with difficulty to comprehend the mechanics of the process.” Turner would be the one to really find the key to understanding Sierra magmatism when his mapping in Yosemite led him to identify individual bodies of plutonic rocks, each chemically or mineralogically distinct, and see that the relations at the boundaries between these bodies indicated that one side was solid while the other was molten. Turner had identified the base unit of geologic mapping in igneous terrains, the pluton–a term not to be used until 1928, when it finally supplemented the overused “batholith”. Sadly, Turner’s work only was mentioned almost as an aside in a 1900 paper on the Quaternary geology of the valley before he headed into the mining industry.

Although Turner had left, his initial mapping and insights were available to Calkins as he accompanied Matthes into the field in 1913.  Many geologists were struggling with the indications that a cooling body of magma might differentiate (Bowen’s famous series was published in 1915), and so considered the variations in the petrology of the granitic basement to possibly reflect one or another stage in the cooling of a massive molten body. What Turner started and Calkins so ably finished was to recognize that there were individual pulses of magma, and these were mappable bodies.  Although the needs of Matthes’s studies of glaciation in Yosemite only required fairly crude mapping of these bodies (for use in identifying glacial erratics), Calkins took exceptional care.  Even so, when he completed his fieldwork in 1916, he apparently could not bring himself to publish the detailed map of the complex geology of the valley, only using it as part of a far smaller scale map published in 1930 in the Yosemite Valley Professional Paper mainly written by Matthes.

Calkins’s work was arguably the first modern geologic map in the Sierra and could well be one of the first of its kind anywhere in the world. The level of detail in his mapping clearly conveys some of the igneous processes at work as in the piece of the map above, where the intrusion of the Sentinel Granodiorite (Ks) dikes and dismembers the edge of the older El Capitan Granite (Kec). It would take admirers of Calkins work, people who had themselves mapped in this terrain, to bring the map finally to publication in 1985 as USGS Miscellaneous Investigations Map 1639.

The impact of Calkins’s work is unclear; it would take a proper historian of geology to go through letters and meeting notes to see if Calkins’s mapping had broader influence. But it clearly was seminal in the Sierra (compare, for instance, Knopf’s mapping farther south at about the same time with what Calkins did); the rescue and publication of this map nearly 70 years after its completion speaks volumes about its perceived value.  Indeed, only recently has some work in the valley exceeded Calkins’s level of detail.

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  1. Mountains that Remade America | The Grumpy Geophysicist - April 20, 2017

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