502 / HARLEN BRETZ 



highest terrace of the localities known is lower with distance down- 

 stream. Apparently they form a simple system of river terraces. 

 So far as known their surfaces do not bear the drifted erratics. 

 They are beheved to have been formed during and after the sub- 

 siding stages of the submergence. One set of these terraces, here 

 named the Portland Delta, is described as follows: 



The eastern portion of the city of Portland is built on a series 

 of terraces of river gravel, the highest of which is about 300 feet A.T. 

 The 300-foot portions lie east of Mount Tabor and Rocky Butte and 

 extend eastward up the river to the mouth of the Gorge. Consider- 

 able tracts at 300 feet A.T. alio lie on the Washington side of the 

 Columbia, east of Vancouver. This 300-foot plain has been dis- 

 sected by subsequent meanderings and braidings of the Columbia, 

 and a large portion of it has been brought down to about 

 200 feet A.T. One of these abandoned Columbia channels is 

 traceable from Rocky Butte in the northeast part of Portland 

 westward to the Willamette in the heart of the city. Another is 

 conspicuous immediately east of Vancouver, Washington, across 

 to the 300-foot plain. 



The gravel composing the 300-foot plain and 200-foot terraces 

 is fresh material, discolored only slightly by weathering at the 

 surface, and all of its constituent pebbles are hard. This criterion 

 distinguishes it from terraces of the much older Satsop formation 

 on the Washington side, the altitudes of the lower of which are by 

 coincidence almost the same as the highest of this series. 



The gravel is almost everywhere disposed in fore-set beds which 

 dip westward and northwestward. Any gravel pit in Portland 

 shows this, and the long cut of the Oregon- Washington Railroad 

 and Navigation Company in the eastern part of Portland shows it 

 most strikingly. 



This deposit of fresh, delta-bedded gravel, with summit level 

 of 275-300 feet A.T., does not extend down the Columbia farther 

 than 10 miles north of Vancouver, Washington. Its outline is that 

 of a broad fan or Greek A in the Willamette structural valley, with 

 its apex at the mouth of the gorge. It appears clearly to record 

 static water at this level in the Columbia Valley west of the Cascade 

 Mountains. The absence, so far as known, of the drifted bowlders 

 on its surface, the absence of infiltrated clay in it, and the evidence 



