CRATER LAKE was discovered on June 12, 1853, by J. W. Hillman at the head of a party of prospectors from Jacksonville, Oregon. The discovery failed to arouse the attention it deserved, for in those days the interest of the western pioneers was concentrated on the search for gold and on the Indian wars. In 1862, and again in 1865, the lake was "rediscovered" independently by others.¹ Yet as late as 1872, when W. G. Steel arrived in Oregon, there were few who had ever heard of the lake, and it took him well-nigh ten years to find a single person who had actually been there. When, in 1885, Steel found an opportunity to visit the lake, he was so profoundly impressed by its beauty that he set himself steadfastly to the task of making it more widely known and having it. established as a national park. In 1886, largely because of his efforts, President Cleveland issued an order withdrawing from settlement and sale ten townships surrounding and including Crater Lake, and in 1893 these were included in the Cascade Range Forest Reserve. Finally, after seventeen years, Steel achieved his object. In 1902, by an act of Congress, the lake and its immediate surroundings, totaling some 250 square miles, were set aside for the public use as a national park.

It was largely because of Steel's insistence that the first scientific survey was made. In 1885, he accompanied Major C. E. Dutton and Professor Joseph Le Conte to the lake, and in the following year Dutton returned with a party from the United States Geological Survey. A special map of the lake and its environs was made by M. B. Kerr and Eugene Ricksecker, and 168 soundings were made, among them one of almost moo feet. It was an astonishing find, and aroused widespread interest.

Dutton had previously spent much time in the Hawaiian Islands and was familiar with the large calderas of Kilauea and Mauna Loa. His opinion concerning the origin of Crater Lake therefore carried great weight. His was indeed the first scientific attempt to solve the problem. In a letter addressed to Powell, then director of the Geological Survey, he wrote:

More than half a century has passed since Dutton put forward this hypothesis. Since then radically different hypotheses have been suggested. Yet the present survey has shown that Dutton was essentially correct, not only as to the origin, but also as to the age of Crater Lake.

The main task of studying the region in detail, Dutton left to his associate on the Geological Survey, J. S. Diller. Diller had visited Crater Lake as early as 1883, in company with Everett Hayden, and had spent several days exploring parts of the rim and Wizard Island. Between 1883 and 1886, he had been engaged in a study of the Lassen Peak district and had made reconnaissance trips to many other volcanoes in the southern Cascades. He was therefore especially well qualified to undertake the survey of Crater Lake. His first published accounts appeared in 1897, but his final report, supplemented by petrographic descriptions of the rocks by H. B. Parton, did not appear until 1902.³ It was a splendid piece of work, all the more to be praised when we recall how he was hampered by lack of time, by difficulties of access, and by the scarcity of suitable places to camp.

It would occupy too much space here to review Diller's findings in full; many of them are referred to below. His main conclusion concerning the origin of Crater Lake should, however, be emphasized. Like Dutton, he could not subscribe to the view that explosion by itself was the cause. He realized that in order to feed the powerful glaciers which once spread down the sides of the old volcano, the summit must have been far higher than the present rim of the caldera, and he supposed that it may even have exceeded 14,000 feet in height. Approximately 17 cubic miles of material must therefore have disappeared to form the caldera. But if this volume of rock had simply been removed by explosion, a tremendous layer of debris should be found close to the rim. No such layer was to be seen. Accordingly he concluded that the top of the old volcano must have been engulfed. He supposed that engulfment was brought about, as in the case of the calderas of Hawaii, by withdrawal of support from beneath the top of the volcano following copious eruptions of lava far down the flanks, and though he searched in vain for the products of such eruptions, he did not despair that some day they would be found.

More than thirty years elapsed before the next important contribution was made to an understanding of the geology of Crater Lake. In 1934, B. N. Moore⁴ published an account of the great pumice sheet that encircles the lake. He was the first to recognize the dual character of the pumice deposits. One type is heavily charged with large lumps, is typically unsorted, and contains many charred logs. This type he realized to be the product of glowing avalanches or nuées ardentes like those erupted from Mount Pelée during the devastating activity of 1902. The other type of pumice is spread over hill and valley as a continuous mantle. This deposit is far more extensive, covering an area of more than 5000 square miles. It is granular and well sorted, and with increasing distance from the lake it becomes thinner. Clearly this pumice was ejected high above the volcano and was drifted northeastward by the prevailing winds, to fall in showers along the path.

Another notable advance in the interpretation of Crater Lake was made by W. W. Atwood, Jr.⁵ who in 1935 published the results of glacial studies commenced four years earlier. Atwood was the first to realize that during the slow growth of the volcano glaciers repeatedly advanced and retreated on its flanks. He revealed the story of a long struggle between the erosional and constructional processes that shaped the ancestral Mount Mazama.

In 1936, J. E. Allen⁶ examined the internal structure of the dacite flows of Llao Rock, Grouse Hill, and Cleetwood Cove, and determined the manner of extrusion. At the same time he proved that the Cleetwood lava, which extends from the caldera rim to the water's edge, is not really a "backflow," as Diller had supposed, but is the filling of an inclined feeder laid bare by the collapse of Mount Mazama.

Finally, in the same year, W. D. Smith and C. R. Swartzlow⁷ cast doubt on the idea that Crater Lake was formed by engulfment. On the basis of evidence offered by new exposures, they maintained that Diller had been misled concerning the volume of coarse ejecta around the rim. In their opinion, there seemed ample fragmental material to account for the formation of the caldera by devastating explosions. They thought that much of the shattered top of the volcano was blown far beyond the confines of the park, that much lies concealed beneath the ubiquitous mantle of pumice, and that much has been removed by erosion; and it appeared to them that the shape of the caldera itself was indicative of explosion. The challenge of their conclusions fostered new interest in the problem of Crater Lake.

Such, in brief, had been the progress of research when the present survey was begun, in 1936. The main objects of this survey were to determine the age and origin of the caldera. To accomplish these ends, it seemed desirable to reconnoiter a wide area surrounding the park in order to appreciate its geological setting with respect to neighboring volcanoes and to discover the nature of the basement beneath Crater Lake. Work begun ten years earlier among the volcanoes of the Lassen Peak region and on Mount Shasta was therefore continued northward into Oregon to connect with the survey of Mounts Thielsen and Bailey made in 1932. In this way a general picture was obtained of the structure and volcanic sequence of the southern Cascades over a distance of more than 100 miles. It seemed desirable also to examine other calderas for the light they might throw on the mode of formation of Crater Lake. With this in mind, a study was made of the large volcanic depressions of New Zealand, of the Javanese calderas, including Krakatau, Tengger, and Idjen, and of several calderas in Japan, particularly those of Aso, Towada, and Toya. The question of how Crater Lake was formed was then approached with greater confidence.

The present report deals chiefly with the results of field work and only secondarily with the microscopic petrography of the rocks, a subject of which Patton has already presented an admirable account in his contribution to Diller's monograph.

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