226 University of California Publications in Geology [Vol. 9 



dred and fifty-foot contour line, or where the topography changes 

 abruptly from the gentle slopes of the Merced Valley to the steeper 

 slopes of the adjoining hills. The difference in the character of the 

 sediments, the difference in attitude of the strata, together with the 

 fact that they are in places separated by an erosion surface, makes 

 it clear that the Terrace Formations and the later Quarternary 

 deposits are lying unconformably upon the Merced Series. 



The upper horizon of the Merced Series has not been recognized 

 in the sections south and southeast of Colma. Its absence from 

 these sections may be explained by considering that a portion of 

 the Merced Series (that portion south and southeast of Colma) was 

 being subjected to erosion while the upper horizon, north of Thorn- 

 ton Station, was being deposited. 



The Terrace Formations consist principally of light-yellowish 

 sands and clays. There are numerous seams of gravel and coarse 

 sand interstratified with the finer sediments. Lignitic material and 

 well-preserved pieces of wood are not uncommon in the basal por- 

 tion of the series. One of the most noteworthy occurrences of 

 wood may be observed in the banks of one of the larger creeks 

 one mile nearly due south of Colma. Here the trunks of large 

 trees project out of the creek banks, in some cases extending across 

 the creek into the opposite bank. These trees are buried beneath 

 several hundred feet of sediments. In nearby streams the strata 

 of the Merced Series outcrops in the creek bottom, indicating that 

 the trees occur near the base of the Terrace Formations or later 

 Quaternary deposits. It seems probable that they existed on the 

 land surface of the Merced Series prior to the deposition of the 

 later sediments. The origin of these formations is to be accounted 

 for in a number of ways. It is the opinion of the writer that 

 they are not to any extent marine deposits, but that they are 

 mainly sediments that have been deposited in swamps, lagoons, 

 and in fresh-water lakes ; and to a less extent, are alluvial wash 

 and sand-dune accumulation. They are the equivalent of at least 

 a portion of the Santa Clara Formation and the Quaternary 

 gravels overlying the Santa Clara Formation. 



Fauna. — The fauna of the Merced Series, of the type section, 

 has not generally been considered to be distinctly determinative. 

 This has been due to the absence of certain characteristic forms, 

 such as the pectens, which are frequently relied upon to a great 

 extent as horizon determiners. The collections of Cooper, Lawson, 



