EPOCHS SEPARATING THE GROUPS. 63 



to continue farther eastward beyond the belt examined; and it may be 

 found as examination is carried farther in that direction to be conformable 

 with the next lower. 



BRIDGER GROUP. 



The plane of demarkation between this group and the next in order is 

 not always well defined. The change from the green sands or bad-land 

 rocks to the indurated sandstones and limestones of the Upper Green River 

 is transitional; yet the epoch of change is important, for the Upper Green 

 River attenuates both to the east and west, and in the latter direction it 

 entirely disappears, so that the Bridger beds lie with an apparent conformity, 

 but actual unconformity, on the beds of the Lower Green River. This is 

 seen in the vicinity of Carter Station. 



UPPER GREEN RIVER GROUP, 



These beds are interpolated between the Bridger and Lower Green 

 River, as described above, only in a portion of the country where the latter 

 two occur. The Tower Sandstone which forms the base of the Upper 

 Green River Group is laid down unconformably on the Lower Green 

 River, the unconformity being represented by gentle valleys of erosion; 

 and there seems to have been a period of erosion or dry land conditions 

 separating the Tower Sandstone from the Plant Beds of the Upper Green 

 River also, and during this dry land period the sands of the Tower Sand- 

 stone were eroded by rains and drifted by winds. After the deposition of 

 the sands, the bottom of this great Green River lake was left bare for a 

 time and the sands drifted in dunes. 



LOWER GREEN RIVER GROUP. 



The Lower Green River beds represent a period when minutely lami- 

 nated bituminous shales and more massive limestones were deposited, the 

 limestones prevailing as you descend in the series. These beds are all 

 fresh water and are separated from those below by an abrupt plane of 

 stratification which marks a change in the character of the sediments. 

 The lower beds are soft, friable, and highly colored bad-land sandstones. 

 But this plane of separation means something more. The conditions which 



