MAKviNE.] GEOLOGY NORTHERN LIGNITIC AREA. 173 



along the eastern base of the Ligh, even-topped Willow Creek lignitic 

 ridge. The line of junction of the sedimentary rocks of this ridge and 

 of those beneath it, with the metamorphic rocks of the Medicine Bow 

 ridge, must lie along the valley between the two ridges. Unfortunately 

 this was not examined. 



The facts presented near the Frazier and Grand Elvers, the apparent 

 gentle westward dip of the Willow Creek ridge, and the smoothed ap- 

 pearance of the west slopes of the Medicine Bow ridge, would all seem to 

 point to the fact that all along between the two ridges the Cretaceous 

 underlying the lignitic, together with the breccia between the two, are 

 abruptly turned up against the Medicine Bow ridge, dipping steeply 

 westward away from it. From Park View appearances seemed to indi- 

 cate such upturned ridges of rock coming out from behind the north 

 end of the Willow Creek ridge, lying along the base of the Medicine 

 Bow ridge, and passing on north into the North Park. The narrow- 

 ness of the valley between the two ridges, as well as the apparently 

 slight western inclination of the lignitic of Willow Creek ridge, how- 

 ever, may indicate that a great fault lies between the two ridges, sepa- 

 rating the archtean on the east from the Cretaceous and lignitic forma- 

 tions in the west, the down-throw being upon the west side. Such a 

 structure is by no means improbable, as the tendency of all the folds 

 throughout this region is to an abrupt western down-throvv', which often 

 passes into a fault. 



Where Willow Creek breaks through the doleritic breccia-bed the 

 latter forms a well-defined ridge a few hundred feet high, striking a little 

 east of north, with irregular curving, and dipping west about 60^. It 

 is here only about 500 feet thick, and considering its greater thickness 

 near the Grand, it is probably thinning in going northward. Passing 

 up the Willow through the gap formed in the ridge by the stream, a 

 small, sharp valley is found on either hand behind the ridge, with some 

 exposures of soft brown sandstones, mostly shaly, dipping 00° west. 

 A thousand feet beyond, a ridge parallel with the breccia ridge is passed, 

 which is composed of coarser and harder brown sandstone, mostly of 

 granitic material, like the lignitic terraces of the Grand, dipping 50° 

 westward. Two or three hundred feet farther on a smaller ridge shows 

 a dip of about 40^, and but little over half a mile above the breccia 

 ridge, the dip is reduced to live or ten degrees, or even less. Here 

 the dip is more across the river to the north than up it, the valley being 

 a partial mouoclinal, though the dip is very gentle. The terraces near 

 the stream are somewhat marked, but best so on its northeast side. 

 The first slope is between four and five hundred feet high, quite abrupt, 

 in escarpments of coarse gray sandstones, some rather finely and com- 

 pactly banded, and conglomerates, all of metamophic debris, sometimes 

 30 feet abrupt, with slopes of 5 to 10 feet of softer shaly beds. Some 

 exceedingly fine, compact, dark-bluo argillaceous bands occur, carrying 

 small black spots of carbonaceous material. Dips of 5^ to 10^ or 12'^ 

 generally occur, mostly to the north, the observer constantly rising 

 through the formation in going up the stream. At a point about south- 

 east by east of Park View, however, a dip of about 10° to the south 

 occurs along the stream for a half mile, ending at the north in a dip of 

 about 30O to the south, till the up-throw of the beds on the northern 

 side is perhaps 150 feet, when the gentle northern dip is again resumed. 

 About in line with this small southward-bending fold, and perhaps a pro- 

 longation of it, there occurs upon the southern spurs of Park View Peak, 

 a short south dip of about 15°, but the usual northern dip is here, like- 

 wise, at once resumed. In a spur just norj:hwest of Park View, the re- 



