1916 1 Martin: Pliocene of Middle and Northern California 



225 



vium and surface soils they entirely cover the floor of the Merced 

 Valley and extend far back on the hills to considerable elevations, 

 manteling a large portion of the Merced Series. 



In general, the relations of the Terrace Formations to the 

 underlying series are obscure. Between the ocean beach and Lake 

 Merced, north of Thornton Station, the strata of the upper horizon 

 of the Merced Series, the Gasteropod Zone, are only very gently 

 inclined toward the north and east. The overlying Terrace Forma- 

 tions are poorly stratified and it is a difficult matter to locate a 

 difference in attitude of the two formations. In the sea-cliff 

 about one mile south of the outlet to Lake Merced Professor Law- 

 son recognized an erosion surface. The occurrence of dark brown, 

 somewhat peaty, beds immediately above the erosion surface sug- 

 gested to Professor Lawson the possibility that this might mark 

 the base of the Terrace Formations. 



In laying its track from San Francisco to Halfmoon Bay the 

 Ocean Shore Railroad Company has cut a bench in the sea-cliffs 

 from Thornton Station to Mussel Rock. A few hundred yards 

 south of Thornton Station the railroad cuts have exposed a good 

 section in which a distinct unconformity may be observed. This 

 unconformity is marked by an erosion surface and a difference in 

 attitude of the strata. The beds above the erosion surface consist of 

 rather coarse sands, gravels, and clays. The underlying strata are a 

 part of the Merced Series and are probably the upper part of the 

 lower horizon instead of the upper horizon found north of Thornton 

 Station. This unconformity is considered to mark the base of the 

 Terrace Formations. 



In following up any of the larger creeks which cut across the 

 strike of the strata of the Merced Series, south and southeast of 

 Colma, one usually finds the soft, unconsolidated, light-yellowish 

 sands and clays in the lower portions of the streams. The erosiorj 

 of these sediments is quite characteristic and resembles the Bad 

 Lands topography. The stratification is not well marked, but occa- 

 sionally bedding planes can be observed and they are usually hori- 

 zontal. In continuing up stream the gray sandy shales of the 

 Merced Series are encountered. The stratification of the latter is 

 inclined northward at considerable angles. At no place in these 

 sections is it possible to find the exact contact between the two 

 formations, but the zone of separation can be quite easily located. 

 It crosses the larger creeks at about the two hundred or two hun- 



