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The Geology of the San Juan Islands




Principal Features. The coal-bearing sandstones and conglomerates occurring on the northern part of Lummi Island and in the vicinity of Bellingham Bay to the eastward are here referred to as the Chuckanut formation. These rocks form the lower part of White's Puget group.28

28White, C. A., On the Puget Group of Washington: Amer. Jour. Sci., 3rd ser., pp. 443-450, 1888.

The sandstones of the Chuckanut formation are generally cross-bedded and somewhat arkosic. They are usually cemented less firmly than the sandstones belonging to the Nanaimo series and they contain more interbedded lignitic material. As a general rule the Chuckanut sandstones are lighter in color than those of the Nanaimo series. The conglomerate strata occurring in the Eocene and Upper Cretaceous formations appear to be identical with regard to the nature of the materials composing their boulders and pebbles.

The Chuckanut formation was evidently laid down in brackish or even fresh water, and fossil leaves and plants are very abundant in many of the strata.

Lithology and Structure. The rocks of the Chuckanut formation exposed on Lummi Island consist of cross-bedded and poorly consolidated arkosic sandstones and conglomerates. The conglomerates are indistinguishable from those occurring in the Nanaimo series. The sandstones are composed of undecomposed fragments of granitoid and volcanic rocks mixed with fragments of chert and argillite. Quartz is frequently a subordinate constituent only, although in some horizons it is very abundant. Lignitic material derived from fossil palm trees is found interbedded with and scattered through the sandstone strata.

On Lummi Island the rocks belonging to the Chuckanut formation trend northwesterly. They occupy the bottom of a distorted syncline, the greater part of which has been eroded away. The Chuckanut sediments have been laid down upon the eroded surfaces of the Eagle Cliff porphyrites.

PLATE XVIII. Above: The sandspit and lagoon at Argyle, San Juan Island. Below: The Upper Cretaceous rocks at Point Thompson, Orcas Island.

Age and Correlation. The sedimentary strata occurring on the northern part of Lummi Island are identical in lithology and plant remains with those outcropping on the mainland to the eastward. The rocks of the Chuckanut formation are well exposed along the Chuckanut Drive on the Pacific Highway.

It is interesting to note that the rocks of this district contain several plant species which are also found in the upper Cretaceous rocks to the south of Point Doughty on Orcas Island. Newberry29 compared the fossil plants from both localities and decided that they were identical and of upper Cretaceous age.

29Newberry, J. S., Description of the Fossil Plants Collected by George Gibbs, Geologist to the United States Northwest Boundary Commission under A. Campbell: Jour. Boston Soc. Nat. Hist., vol. 7, pp. 506-525, 1863.

Recent work on these fossil plants by Dr. Knowlton has placed the rock formations in the vicinity of Bellingham Bay in the lower Eocene. According to Dr. Knowlton the beds do not represent the lowermost portion of the Eocene and their flora is much different from that occurring on Orcas Island. The writer found that the fossil leaves occurring on Orcas Island are interbedded with strata which contain a marine upper Cretaceous fauna.

The Chuckanut flora is very different from the middle or upper Eocene flora occurring in the vicinity of Seattle.

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Last Updated: 28-Mar-2006