1 68 J. HARLEN BRETZ 



extra-morainic in position and lying back of the ice limit. These 

 areas are all alike in being natural prairies because of the coarseness 

 of the soil and in bearing a surface deposit of black silt of variable 

 thickness. Many of them exhibit a very interesting surficial 

 development into mounds of fairly uniform size and distribution 

 composed of mingled gravel and silt without stratification. Where 

 typically developed, they resemble a field of closely spaced hay- 

 cocks. Their origin is not clear. Grand Mound Prairie bears 

 these tumuli over a considerable portion of its extent. 



Some distance back from the frontal edge of the terminal moraine 

 between Tenino and Little Rock a new railroad grade affords 

 frequent exposures of the Vashon till overlying drift of much 

 greater age and with bedrock often appearing beneath the drift. 

 Hills of the moraine occur on the east side of Black River a mile 

 south of Little Rock, while across the river on the west, a morainic 

 tract of low relief occurs about a mile wide. In this tract is a 

 splendid exposure of Vashon till highly charged with rounded 

 gravel which is doubtless overridden and incorporated outwash 

 material. 



Mima Prairie, southwest of Little Rock, is another part of the 

 outwash gravel plain and forms a sharp re-entrant angle in the 

 surface till exposures, though hardly recording such an ice margin 

 form, the till being probably buried beneath this northward angle 

 of the outwash. Between Mima Prairie and the Black Hills, 

 unweathered Vashon till was observed in a gravel pit with a thick- 

 ness of three feet overlying a very red and decayed till of undeter- 

 mined depth. Small pebbles of the latter were often easily cut 

 in two with a knife, while those of the overlying Vashon were firm 

 and unweathered. 



No drift is found back in the Black Hills except a sprinkling of 

 pebbles in re-arranged residual material on the slopes which face 

 the broad drift plain eastward. The region is exceedingly difficult 

 to examine, the forest being almost impassable. Entrance into 

 the hill region is gained on a logging railroad and on various trails. 

 One road crosses near the northern part of the hills, passing west 

 from Olympia close to Summit Lake. Drift has been found near 

 this lake on the north slope of the hills up to an altitude of 1,460 



