UW logo University of Washington Publications in Geology
The Geology of the San Juan Islands


The geological record as revealed in the rocks exposed on the San Juan Islands is fragmental in many respects, but when taken with the records shown in the surrounding areas, a fair conception of the outstanding geological events may be had.



The geological record, as revealed on the San Juan Islands, opens with the middle or upper Devonian period. Periodically alternating conditions of sedimentation occurred in a wide, shallow, and subsiding epicontinental sea, producing thin alternating layers of fine-grained and semi-colloidal silica sand and layers of mud or silt. The deposition of these alternating layers of sediment continued until a total of many thousand feet of strata were deposited. Colonies of corals, brachiopods, and other sedentary animals were able to establish themselves in the more favored localities during the process of the deposition of the sediments. These gave rise to the limestone lenses which occur, intermittently throughout this rock group.


The conditions of sedimentation existing in the late Devonian period were continued on into the Mississippian. The dividing-line between the two periods was probably marked by the temporary addition of pyroclastics to the normal sediments. The closing stages of the Mississippian period were probably marked by a second period of volcanic activity in the surrounding regions which contributed pyroclastic material to the sediments. During this time the floor of the sea was being gradually uplifted and the character of the sediments was changing to the coarser-grained varieties. During the stage the basal tuffaceous graywackes of the Leech River group were probably in the process of deposition. The Mississippian period was probably terminated by a general uplift of the region.


The Pennsylvanian period presumably opened with a submergence of the region until conglomerates and breccias were deposited upon the surface of the graywackes. A further submergence caused the character of the sediments to become finer and finer-grained until nothing but silt was deposited. Periodically alternating conditions in the sedimentation caused thin alternating strata of light and dark colored carbonaceous mud to be deposited. These alternating conditions of sedimentation continued for a long lapse of time during which an occasional limestone bed was formed. The Pennsylvanian period was probably closed by another general uplift of the San Juan Island region.


The dividing line between the Pennsylvanian and the Permian periods has not been established in the San Juan Island area. If the conglomerates, graywackes, and slates occurring in the upper part of the Leech River group really belong to the Paleozoic, they probably belong to the Permian period. In that case, the whole area was uplifted above sea-level at the close of the Paleozoic era.



It is possible that the movement which resulted in the uplift of the Paleozoic sediments was connected with the intrusion of the dunites of the Fidalgo formation. During the latter part of the Triassic period the area was down-warped, and the sea waters came in and deposited the sediments of the Haro formation. At the same time, tremendous and repeated eruptions of andesitic material occurred on Vancouver Island.


The outpouring of volcanic materials continued on into the Jurassic period. In the San Juan Island region the post-Triassic erosion surface was not far from the present erosion surface. At any rate the intrusions of the Eagle Cliff porphyrite assumed ellipsoidal structures indicating that the intruded rocks were both near to the surface and very moist.

Near the close of the Jurassic the area was uplifted and the Paleozoic and Mesozoic sediments were subjected to a period of intense folding and faulting. The area was intruded by large batholithic masses of granodiorite and the overlying sediments were in many places destroyed by the numerous off-shoot dikes thrown out by the batholiths.


The intrusion of the late Jurassic batholiths was followed by a prolonged period of erosion and deposition. So great was the lapse of time represented by the lower Cretaceous period, that the batholith was partly de-roofed in some of the neighboring areas by the close of the Knoxville stage. The conglomerates do not contain abundant boulders of granodiorite however, until upper Cretaceous time.


Erosion and deposition of the sediments derived from the uplifted land areas continued during the upper Cretaceous period. By this time the batholiths were exposed over considerable areas in the neighboring regions. The conglomerates and arkosic sandstones of the Nanaimo series were laid down up on surfaces that were usually near sea-level. Clapp in his study of the Nanaimo region considers the Nanaimo series to have been laid down upon erosion surfaces that were not far from the present level.

Patos Islands show an excellent though exceedingly fragmental record of the fact that a large river delta existed at that locality during upper Cretaceous time.



The Mesozoic era was closed by a general withdrawal of the sea and an accompanying uplift of the whole region. The Cenozoic era began with a gradual submergence of the region, the waters entering both from the northward and from the area now occupied by the Strait of Juan de Fuca. As the submergence continued, the waters encroached farther and farther eastward upon the land areas. The sediments of the Chuckanut formation on Lummi Island were laid down upon the eroded surfaces of the Eagle Cliff porphyrites and the Leech River sediments. During this period the waters occurring in the eastern and southeastern portion of the map-area were brackish. Towards the close of the Eocene period the whole region was uplifted above sea-level and gently folded.


During the early part of the Oligocene period the rocks occurring in the San Juan Island region were still further uplifted and folding and faulting took place. This was apparently followed by a prolonged period of erosion, for no rocks of middle or late Tertiary age are found in the region.


During the early and middle Miocene the region was exposed to erosion, and the more elevated portions were no doubt reduced to areas of low relief. Towards the latter part of the Miocene period, when the newer Cascade Mountains were in the process of formation, the rocks of the San Juan Island region were uplifted and folded into a mountain range trending in a northwest and southeast direction. At this time the river systems which drained southern British Columbia were probably prevented from reaching the ocean by way of the Strait of Juan de Fuca.


In the San Juan Island region the Pliocene period is represented by a long erosional interval during which the uplifted mountain range was deeply dissected by ravines and valleys.

PLATE XXI. Above: Roche Harbor, San Juan Island. Below: One of the quarries of Roche Harbor Lime and Cement Company, Roche Harbor.



The uplifted mountain range which crossed the San Juan Island map-area was subjected to the erosion of repeated glacial invasions during the Pleistocene period. The region was subjected to at least two distinct glacial advances and future investigations will probably reveal the proof of additional and still earlier glacial invasions. The glacial episodes were separated by intervals of time during which the climate was relatively warm.

While the outline of the present topography of the San Juan Island region is no doubt of pre-glacial origin, the earlier topography has been greatly modified as a result of glacial erosion.


The retreat and close of the Vashon glacial invasion marks the beginning of the Recent period. The loose and unconsolidated sediments deposited during the different glacial invasions and interglacial intervals were left in such a position that in some cases they were exposed to the action of the waves. These sediments were rapidly eroded away and often carried considerable distances by the strong tidal currents. High sea-cliffs were developed as the waves cut their way into the deeper and more elevated deposits of glacial material. Sand bars, sand spits, and sandy hooks were formed by the action of the waves and tidal currents. Had it not been for the great strength of the tidal currents, the erosional action of the waves would have been felt to a far greater extent. In many of the channels the tide-rips are more or less active at all times excepting at high tide, and it is therefore at high tide that the storm waves reach the shore without being broken up into interfering components which destroy or limit their eroding power. The abundant wave-cut benches at high tide-level show how efficient the waves are as agents of erosion when they are not broken up by the tide-rips.

The retreat of the last glacier was evidently followed by a general uplift, for excellent examples of recently uplifted wave-cut benches may be seen in all parts of the region at elevations of 15 to 25 feet above high tide-level.

<<< Previous <<< Contents >>> Next >>>

Last Updated: 28-Mar-2006