Magnetic polarization of sedimentary rocks and the Earth's magnetic history

Magnetic polarization measurements were made on 99 samples of flat-lying, undisturbed sedimentary rocks collected at eight sites in the States of Colorado, Utah, Idaho, Wyoming, Washington, and South Dakota. The majority of samples ranged in age from Pliocene to Eocene, being thus from 5 or 10 million to 50 or 60 million years old. A few samples were obtained from Jurassic formations 125 to 150 million years old. The direction of polarization in the plane of bedding, or declination, was found in two-thirds of the samples to fall in a narrow range centering on geographic rather than present geomagnetic north. Present geomagnetic north at the eight sites ranges from 13° to 22° east of geographic north. The dip or inclination of polarization in two-thirds of the samples falls in a small range centering on 63°, which compares closely with the value of 69° for the average inclination of the earth's present field at the-sites sampled. The results, though few, are consistent with the idea that for 50 million years or so the polarity of the earth's magnetic field has been the same as now and the magnetic axis has had an average orientation that coincides with the earth's geographic axis.