Changes in the percentages of clay minerals and coarse fraction minerals with depth in Deep Sea Drilling Project cores from Delgada submarine fan suggest that the rate of motion between California and the Pacific plate has increased with time during the past 15 m.y. The major changes in mineralogy indicate changes in provenance. The times of these changes found in fan sediment and palcogeographic lithologic reconstructions of California demon- strate an increase in rate of motion from about 3.0 cm/yr 15 m.y. ago to 6.5 cmfyr at preent. During the time interval of 4-9 m.y. ago the data suggest a rate of motion of 4.5 cm/yr. Rates of motion between lithospheric plates are of major concern in determining the quan- titative nature of the dynamic earth system. Spreading centers as plate boundaries have been emphasized in the study of rates of motion. Rates of movement in these areas are usually calculated from the apparent age of basement rock, as deduced from the geomagnetic time scale of Heirtzler et al. (1968), and the dis- tance between rocks of different deduced ages. Studies of the SaN Andreas fault, a transform fault plate boundary, have considered move- ment between the Pacific and the American plates primarly in terms of a constant rate of motion with time (Dickinson and Grantz, 1968; Atwater, 1970; Hayes and Pitman, 1970). Such an extrapolation of a constant rate of motion with time was necessary because of poorly understood dynamics of a transform fault plate boundary and magnetic anomalies representing only the last 4 m.y. of history between these two plates (Anderson, 1971; Larson et al., 1968). Possibilities of variable rates of motion with time are discussed by Atwater (1970), Dickinson et al. (1972), and Hein (1973). Hu.f)- man (1972), working within the San Andreas fault zone and therefore not necessarily mea- suring total relative motion between the Pacific and the American plates, suggests a change in the rate of motion of the San Andreas fault sometime in the last 22 m.y. Larson (1972), from a study of the Gulf of California and from the results of previous studies, suggests a signifi- cant change in motion of these two plates 10 m.y. ago. Frerichs and Shive (1971) speculate about episodic movement on spreading centers with an increase in spreading rate 10 m.y. ago, although they are not concerned specifically with the American and Pacific plates. Talwani et al. (1971) provide concrete evidence, from distribution of sediment, of an increase in spreading rate at 10 m.y.B.P. in the North Atlantic. This paper concerns a method by which the history of relative movement can be determined between California and the Pacific