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The Geology of the San Juan Islands






In a restricted sense the San Juan Islands consist only of the area included within San Juan County, Washington. To the eastward occur islands belonging to Skagit and Whatcom Counties, and these are commonly regarded as a part of the San Juan Island group. The area considered in this report includes the American islands located within Washington Sound. The center of the map-area can be located by the point of intersection of the 123rd meridian of west longitude with the north latitude line of 48° 30'.

The San Juan Islands are bounded on the west by the waters of Haro Strait and Boundary Pass which serve as the International boundary line. Still farther westward lies Vancouver Island and the smaller Canadian islands which fringe its eastern margin.

The San Juan Islands were first surveyed and divided into townships in 1874 and 1875. During the years 1888 to 1897 the islands were accurately surveyed by the United States Coast and Geodetic Survey. The results of this survey are shown in reduced scale in the United States Coast and Geodetic Survey chart No. 6380. Corrections as well as new data have been added to this chart from year to year and its accuracy has been thereby increased and kept up to date.

PLATE I. The Cone Islands. (Photography by A. O. McCormick).

The region included in this report embraces an area of land and water 33 miles long and 27 miles wide, or an approximate surface of 890 square miles. The waters of the map-area are dotted with islands, some large and mountainous while others are small and covered by water at high tide. A careful count of all the isolated land areas exposed at lower low tide, revealed no less than 786 islands and reefs within the map-area, 743 of which occur in San Juan County. At high tide the number of island masses is only 457, with 428 within San Juan County. These are grouped into approximately 175 islands and reefs of sufficient importance to warrant individual names.

A list showing in detail the areas of the islands of the San Juan group is found on page 178.

The total amount of land included in the map-area is 206 square miles. It includes 175 square miles or all of the land area of San Juan County, together with about 10 square miles belonging to Whatcom County and 21 square miles belonging to Skagit County. Although Samish Island has an area of 931 acres, but 234 acres are included in the map-area.


During the summer of 1922 the writer spent two months in the field doing reconnaissance work and making a systematic collection of rock samples and fossils. The following winter was occupied in preparing and examining thin sections of the samples.

During the summer of 1923 he spent four months in the field examining the formations in more detail and making more extensive collections of fossils, several new localities having been discovered. Late in the autumn of that year the writer made a brief examination of the formations on Vancouver Island from Nanaimo to Victoria. Several short trips to the San Juan Islands were made during the following winter to re-examine certain areas in more detail.

The entire area was again covered during the summer of 1924 when three months were spent in the field. Additional collections of rocks and fossils were made and certain areas were further studied. Short trips to the San Juan Islands were made during the following winter, and also during the spring and summer of 1925 in order to study again certain problems in the structural geology of the region.

The writer wishes to thank the members of the Puget Sound Biological Station for the many courtesies extended to him.

For helpful suggestions in connection with the igneous geology of the region, he wishes to express his indebtedness to Professor George E. Goodspeed, Jr. He wishes to acknowledge the assistance received from the late Professor Edwin J. Saunders on problems dealing with the glacial geology of the map area. For helpful suggestions regarding the correlation and nomenclature of certain sedimentary rock formations, he wishes to express his indebtedness to Dr. Charles E. Weaver and Professor M. A. Hanna.

For helpful suggestions in the compilation of this report, especial thanks are due to Dr. Charles Schuchert of Yale University, and to Dr. T. W. Stanton of the United States National Museum. The writer wishes to express his appreciation of the many helpful suggestions received from Dean Henry Landes, under whose direction the work was carried out.


Allan, J. A., Saltspring Island and East Coast of Vancouver Island: Geol. Survey Canada, Sum. Rept. 1909, pp. 98-102, 1910.

Contains a description of the so-called Sansum formation, which for a time was erroneously considered as a metamorphic phase of the Nanaimo series.

Bretz, J. Harlen, Glaciation of the Puget Sound Region; Wash. Geol. Survey Bull. 8, 1913.

The most complete work dealing with the Pleistocene glaciation of the Puget Sound country. It contains a bibliography of earlier writers on that subject.

Clapp, C. H., Southern Vancouver Island; Geol. Survey Canada, Mem. 13, 1912.

Gives a general description of the geology of Southern Vancouver Island. Many of the formations described in this report are also to be found on the San Juan Islands. It contains a complete bibliography of the literature dealing with the geology of Southern Vancouver Island.

Geology of the Victoria and Saanich Map-Areas; Geol. Survey Canada, Mem. 36, 1913.

Gives a description of the Late Jurassic intrusive that occur across the channel to the west of the San Juan Islands.

Geology of the Nanaimo Map-Area; Geol. Survey Canada, Mem. 51, 1914.

Gives a detailed description of the coal measures and rocks belonging to the Nanaimo series.

Sooke and Duncan Map-Areas, Vancouver Island; Geol. Survey Canada, Mem. 96, 1917.

Contains a description of the Sicker series. This work was done in collaboration with H. C. Cooke.

Clapp, C. H., and Shimer, H. W., The Sutton Jurassic of the Vancouver Group, Vancouver Island; Proc. Boston Soc. Nat. Hist., vol. 34, pp. 425-438, 1911.

Describes and figures several Jurassic species occurring in the Sutton limestones.

Daly, R. A., North American Cordillera at the Forty-Ninth Parallel; Geol. Survey Canada, Mem. 38, 1912.

Contains a description of the Chilliwack series, the Hozomeen series, and the Pasayten series, all of which are probably represented in the rocks occurring on the San Juan Islands.

Darton, N. H., Structural Materials in parts of Oregon and Washington; U. S. Geol. Survey Bull. 387, 1909.

Contains a brief description of the structural materials and cement materials of San Juan County.

Dawson, G. M., Report on a Reconnaissance of Leech River and Vicinity; Geol. Survey Canada, Report of Progress 1876-77, pp. 95-102, 1878.

The Leech River "formation" or "series" was first described in this report.

Report on a Geological Examination of the Northern Part of Vancouver Island and Adjacent Coasts; Geol. Survey Canada, Ann. Report 1886, pp. 1B-107B, 1887.

Includes a description of the Vancouver group.

Notes on the Cretaceous of the British Columbia Region. The Nanaimo Group; Amer. Jour. Sci., 3rd ser., vol. 39, pp. 180-183, 1890.

The Nanaimo "group" was defined. The age of both the Nanaimo and the Puget groups were regarded as Laramian.

Diller, J. S., Mineral Resources of South-Western Oregon; U. S. Geol. Survey Bull. 546, pp. 14-17, 1914.

Describes radiolarian cherts of Late Devonian age that are similar to those of the Orcas group.

U. S. Geol. Survey, Geol. Atlas, Redding Folio (No. 138), 1906.

Contains a description of radiolarian cherts of Late Devonian age.

Eckel, E. C., Cement Materials and Industry of the United States; U. S. Geol. Survey Bull. 243, pp. 323-325, 1905.

San Juan Island limestones are described briefly and an analysis of the Roche Harbor rock is included.

Gabb, Win. M., Geol. Survey Calif. Paleontology, vol. I, 1864; vol. II, 1869.

Describes and figures many of the Cretaceous species occurring in the Nanaimo series.

Gibbs, George. This author published an account of a geological reconnaissance of the northwestern boundary of the United States in the Journal of the American Geographical Society for 1873. He mentioned the elevated marine terraces on San Juan Island.

Heer, Oswald. Fossil Plants of Vancouver and Bellingham Bay; Amer. Jour. Sci. 2nd ser., vol. 28, pp. 85-89, 1859.

Contains a description of fossil plants collected by Dr. John Evans on Vancouver Island and at Bellingham Bay. The species are compared and correlated with the Tertiary flora of Europe.

Landes, Henry, Cement Resources of Washington; U. S. Geol. Survey Bull. 285, pp. 377-383, 1906.

Describes the limestone and clay of San Juan and Orcas islands.

Non-Metalliferous Resources of Washington, Except Coal; Wash. Geol. Survey, vol. 1, Ann. Report for 1901, Part 3, pp. 7, 24-27, 1902.

Describes the sandstone quarry at the Sucia Islands, and the limestone quarry and lime-burning plant at Roche Harbor.

Lesquereux, Leo, Species of Fossil Plants from Bellingham Bay, etc.; Amer. Jour. Sci., 2nd ser., vol. 27, pp. 360-363, 1859.

A description of six fossil plant species from Nanaimo, and eight species from Bellingham Bay.

Mangum, A. W. and party, Reconnaissance Soil Survey of the Western Part of Puget Sound Basin; U. S. Department of Agriculture, Bureau of Soils, 1912.

Classifies and maps the soils occurring in the western part of the Puget Sound Basin, including the San Juan Island map-area.

McLellan, R. D., The Devonian Orcas Group of Washington; Amer. Jour. Sci., 5th ser., vol. 8, pp. 217-222, 1924.

Describes the San Juan series whose lower member, the Orcas group, was determined by means of fossils to be of Late Devonian and Mississippian age.

Meany, E. S., Origin of Washington Geographic Names; Univ. of Wash. Press, Seattle, 1923.

A dictionary of the origins of geographic names in the State of Washington.

Meek, F. B. On Cretaceous Fossils from Vancouver and Sucia Islands; Trans. Albany Institute, vol. 4, pp. 37-39, 1857.

Descriptions of New Cretaceous Fossils from Vancouver and Sucia Islands; Proc. Acad. Nat. Sci. Philadelphia, vol. 13, pp. 314-318, 1861.

Descriptions and Illustrations of Fossils from Vancouver and Sucia Islands, and Other Northwestern Localities; Bull. U. S. Geol. Survey Territories, vol. 2, pp. 351-374, 1876.

These three reports contain descriptions of Cretaceous fossils occurring in the Nanaimo series. The last of the three reports gives figures of the fossils.

Milton, W. W. F., A History of the San Juan Water Boundary Question; pp. 14-32, London, 1869.

A general survey of the natural resources of the San Juan Islands which was made for the British in connection with the San Juan boundary dispute. He said, "A circumstance of great importance with this island (San Juan Island) is the existence upon it of extensive deposits of limestone. * * * On the western shore, near the base of Mount San Juan (Mount Dallas), immense masses raised up into perpendicular walls, are seen at several localities, covering an area of many acres. The northeastern corner of the island is composed of an extensive ledge of the same material. A very small island (O'Neil Island) lying close to the northeast end of San Juan Island, containing only a few acres, is composed almost entirely of limestone. * * * A deposit of coal is found near Point Doughty at the northwest end of the island (Orcas Island) similar to that at Nanaimo on Vancouver's Island and at Bellingham Bay on the mainland."

Newberry, J.S., Description of Fossil Plants from Orcas Island, Bellingham Bay, etc.; Boston Jour. Nat. Hist., vol. 7, pp. 506-524, 1863.

Gives a description and comparison of the fossil plants occurring near Point Doughty on Orcas Island, and at Bellingham Bay on the mainland.

Newcombe, C. F., Pleistocene Raised Beaches at Victoria, B. C.; Ottawa Naturalist, vol. 28, pp. 107-110, 1914.

Discusses the upraised wave-cut benches that occur at Victoria, B.C.

dPardee, J. T., Chromite Ores in Washington; U. S. Geol. Survey Bull. 725, pp. 61-65, 1922.

Gives a description of the chromite deposits that occur on Cypress Island.

Richardson, James, Coal Fields of the East Coast of Vancouver Island; Geol. Survey Canada, Rept. of Progress, 1871-72, pp. 73-100, 1873.

Report on the Coal Fields of Nanaimo, Comox, Cowichan, Burrard Inlet, and Sooke, B. C.; Geol. Survey Canada, Rept. of Progress, 1876-77, pp. 160-192, 1878.

These reports contain a preliminary description of the coal resources of Vancouver Island and vicinity.

Saunders, E. J., Reconnaissance Soil Survey of the Western Part of the Puget Sound Basin, Washington; U.S. Department of Agriculture, Bureau of Soils, pp. 16-29, 1912.

Describes and explains by means of charts and diagrams, the distribution of monthly and yearly precipitation, the occurrence of the latest and earliest frost, and the general temperature conditions in the western part of the Puget Sound Basin.

Schofield, S. J., The Geological Record of the Cordillera in Canada; Trans. of the Royal Soc. of Canada, vol. 17, 3rd ser., pp. 79-103, 1923.

Discusses the Pre-Cambrian and Paleozoic paleogeography of Western Canada and favors the idea that the eastern margin of the continent of Cascadia was located immediately to the west of the present coastline of Canada, and that no landmass of great size existed between it and the Canadian Shield.

Shedd, Solon, Building and Ornamental Stones of Washington; Wash. Geol. Survey, vol. 2, Ann. Rept. for 1902, pp. 71-74, 1903.

Describes the sandstone quarries on Stuart Island and the Sucia Islands.

Smith, G. O., U. S. Geol. Survey, Geol. Atlas, Mount Stuart Folio (No. 106), 1904.

Gives a description of the Easton schist, Peshastin formation, and Hawkins formation, which are probably to be correlated with some of the rocks occurring on the San Juan Islands.

Smith, G. O. and Calkins, F. C., A Geological Reconnaissance across the Cascade Range near the Forty-ninth Parallel; U. S. Geol. Survey Bull. 235, 1904.

Describes rock formations that are probably to he correlated with the Paleozoic and Mesozoic rocks of the San Juan Islands.

Weaver, C. E., Preliminary Report on the Tertiary Paleontology of Western Washington; Wash. Geol. Survey Bull. 15, 1912.

Contains a brief account of the outstanding geological events that have taken place in the vicinity of Puget Sound.

The Tertiary Formations of Western Washington; Wash. Geol. Survey Bull. 13, 1916.

The most complete work dealing with the stratigraphy and paleontology of the Tertiary formations of Western Washington. Contains a large bibliography of works that have either a direct or indirect bearing on the geology of the San Juan Islands.

White, C. A., On the Puget Group of Washington Territory; Amer. Jour. Sci., 3rd ser., vol. 36, pp. 443-450, 1888.

Names and describes the Puget group of Washington.

On Invertebrate Fossils from the Pacific Coast; U. S. Geol. Survey Sull. 51, 1889.

Describes and figures a number of Cretaceous fossils occurring on Waldron Island, Skipjack Island, the Sucia Islands, and Vancouver Island. Contains a description of the Puget group.

Correlation Papers, The Cretaceous; U. S. Geol. Survey Bull. 82, 1891.

Whiteaves, J. F., On the Fossils of the Cretaceous Rocks of Vancouver and Adjacent Islands in the Straits of Georgia; Geol. Survey Canada, Mesozoic Fossils, vol. I, parts 1-5, 1876-1903.

The most complete work dealing with the Cretaceous fossils occurring on Vancouver and the Sucia Islands.

Wilkes, Charles, Narrative of the United States Exploring Expedition of 1838-1842, 1845.

Names and describes some of the islands of the San Juan group.

Willis, Bailey, Stratigraphy and Stucture of .the Puget Group; Geol. Soc. America, Bull. vol 9, pp. 2-6, 1897.

Excellent article on the physiography, stratigraphy, and structure of the Puget Sound region, with a description of the appearance and character of the country bounding the Sound.

Woodward, Henry, On Some Decapod Crustacea from the Cretaceous Formation of Vancouver Island; Report of the British Assoc. for Adv. of Sci., pp. 696-697, 1895.


The San Juan Islands consist of an archipelago situated in that body of water known as Washington Sound, which separates southern Vancouver Island from the mainland to the east. The islands represent the highest points of a submerged mountain range that connects Vancouver Island with the mainland, and forms the southern boundary of the Gulf of Georgia.

The complex systems of channels and harbors composing Puget Sound and Washington Sound are supplied with ocean water by the Strait of Juan de Fuca. The major portion of the enormous volume of water that flows in and out of the Gulf of Georgia at every tide is derived from the southward, and therefore it must pass through the channels surrounding the San Juan Islands. Very heavy tide-rips are consequently produced in many of the channels.

For the most part the individual islands are rugged and mountainous. The map-area includes 15 mountain peaks with elevations exceeding 1000 feet, and these are typically located near the shore-lines and rise precipitously from the water's edge. Where sufficient soil permits, the more elevated portions of the islands are heavily wooded, especially on their northern slopes.

The islands usually present irregular shore-lines deeply indented by narrow fjord-like harbors. The intervening channels, which are commonly narrow and U-shaped due to glacial erosion, often attain depths of 600 feet, and occasionally exceed 1000 feet in depth.

The San Juan Islands range in size from that of Orcas Island with approximately 57 square miles, to that of the smallest reefs which are scarcely awash at low tide. The majority of the islands have an area of less than one-quarter of an acre.

The nucleus of the San Juan Island group is formed by Orcas, San Juan, and Lopez islands. These large islands are bounded on the west by Haro Strait and Boundary Pass, which contain the smaller islands belonging to the Stuart Island and Waldron Island groups. On the south the whole San Juan Island group is bounded by the open waters of Washington Sound and the Strait of Juan de Fuca. The nucleus of the San Juan Island group is separated from the mainland to the east by the waters of Rosario Strait, which contain Cypress, Guemes, Lummi, Sinclair, and other lesser islands and reefs. The San Juan Island group is bounded on the north by the Gulf of Georgia, which contains the small islands, Patos, Sucia, Matia, Barnes, and Clark, together with several smaller islands and reefs that fringe the northern shores of Orcas Island.

A large number of islands and reefs occur in the channels separating Orcas, San Juan, and Lopez islands. The largest of these, Shaw Island, has an area of nearly 5000 acres.

San Juan Island is separated from Lopez, Shaw, and Orcas islands, by the waters of San Juan Channel. The narrows occurring at the southern entrance of San Juan Channel were originally given the name of Little Belt Passage. This name has fallen into disuse and it is now known as Cattle Point Narrows.

A narrow channel trending east and west, separates Shaw and Orcas islands. This is called Harney Channel. Shaw Island is separated from Lopez Island by Upright Channel which trends northeasterly and blends with Harney Channel. Still farther eastward these combined channels merge into Lopez Sound and East Sound at their common meeting point.

Orcas Island is nearly cut into two islands by the narrow fjord-like harbor known as East Sound, which opens towards the south and merges with Lopez Sound, a somewhat similar depression with a general north-south trend that penetrates the northern margin of the Lopez Island landmass. The land occurring on the east side of Lopez Sound has been penetrated by several narrow channels connecting directly with Rosario Strait, and consequently the eastern shore of Lopez Sound is formed by Lopez Island only near the southern end, while farther north its place is taken by Decatur, Blakeley, and Obstruction Islands.

PLATE II. Above: The eastern lobe of Orcas Island as seen from Cypress Island. At the extreme left is Mount Entrance, in the center is Mount Constitution, and to the right is Mount Pickett. Below: Eagle Cliff, Cypress Island.

The San Juan Islands are composed chiefly of Paleozoic and Mesozoic sedimentary rocks, the former having been intruded and metamorphosed by igneous rocks of Mesozoic age. The Paleozoic strata, which include the bulk of the sedimentary rocks of the region, have been folded into a broad syncline that plunges towards the eastward or southeastward at a moderate angle. The structure of these rocks has been greatly complicated by later folding and faulting.

In the eastern part of the map-area, large irregular masses of peridotite have been intruded into the Paleozoic sedimentary rocks. The Paleozoic sedimentary strata in all parts of the map-area, have been almost everywhere intruded by a series of dikes and sills of basic igneous rocks that apparently served as feeders to flow rocks that have since been removed by erosion.

Some parts of the map-area have suffered further from the intrusion of a series of igneous off-shoots derived from the late Jurassic batholith that is now exposed along the shore of Vancouver Island to the west of the San Juan Island map-area. The intrusion of these off-shoots into the strata that were already shattered by folding and previous intrusions of igneous materials, took the form of an injection breccia. This injection breccia was composed of igneous rocks, both acid and basic, and these now occur intermingled with the older igneous materials, and occasionally remnants of the sedimentary host rocks are to be seen.

The islands forming the northern and northwestern margins of the map area are composed of closely folded though unmetamorphosed sedimentary rocks of late Mesozoic and early Tertiary age. Within the map-area these sedimentary rocks have not been intruded by any igneous materials.

The islands have been completely overridden by glaciers and even the top of Mount Constitution is deeply striated and polished by glacial action. Considerable areas on many of the islands are covered with glacial drift and several excellent examples of recessional moraines occur within the map-area.

The latest geological movement appears to have been a general uplift, and excellent examples of recently upraised wave-cut benches are to be seen in many places at elevations of about 20 feet or higher.

During the winter season the great storm waves that otherwise would be very effective agents of erosion, are broken up by the interference of the tide rips, and it is only at slack tide (particularly near the time of high tide, when the tide-rips are not active) that the storm waves are free from such interference. (See Plate IX). As a result, a remarkably well developed wave-cut bench near high tide-level is encountered on all exposed shores throughout the region.

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Last Updated: 28-Mar-2006