GRAVEL-COVERED TERRACES 269 



In the vicinity of Fruitland, a few miles east of the Great Hogback, 

 the two lower gravel horizons are well exhibited. The intermediate one, 

 at least 150 or 200 feet above the lowest, may be seen capping the ledge 

 of white Laramie ( ?) sandstone, known as the "Pictured rocks." Here 

 the gravels were found by Cross to consist of large and small boulders and 

 pebbles of many volcanic rocks, of granite and schist, and notably of the 

 extremely hard Algonkian quartzites of the Needle mountains. Boulders 

 as much as 3 feet in diameter were noted and many of 1 and 2 feet diam- 

 eter. Isolated gravel-capped hills, often of level top, and rising above 

 this gravel plain of the Pictured rocks, occur on both sides of the San 

 Juan. 



The beautiful gravel terraces on the south side of the San Juan, oppo- 

 site the Great Hogback, form the subject of a sketch by Holmes in the 

 Hayden Eeport for 1875 (plate xxxix), and a photograph taken by Cross 

 of the same view has been used by Gilbert and Brigham in their "Intro- 

 duction to Physical Geography" (figure 36), but erroneously entitled as 

 representing "Terraces of the Uncompahgre valley, Colorado." Inas- 

 much as the terraces of the San Juan extend with visible continuity far 

 below the Great Hogback, it is almost certain that they are gravel-covered 

 for many miles beyond the limit of present observation, near the Great 

 Hogback. The coarseness of the gravels at this locality renders such 

 an assumption natural. 



Northward from the San Juan river, terraces and. plains, seemingly 

 extensions of those which are gravel-covered, reach far up the Mancos 

 valley and across to the slopes of the El Late mountains. It appears 

 probable that, whether gravel-coated or not, these topographic features 

 are contemporaneous in origin with those under discussion here. 



On the western side of the San Juan mountains, in the valleys of the 

 Dolores and San Miguel rivers, gravel benches are known several hun- 

 dred feet above the present stream beds, but only near the mountains, as 

 far as our observations go. It is probable, however, that these elevated 

 gravels will in time be correlated with some of the more extensive de- 

 posits of the northern and southern slopes of the San Juan mountains. 



During a reconnaissance made in 1905, Cross noted the absence of 

 gravel deposits over the Dolores plateau between Mancos and the Abajo 

 mountains, but about this group, as also around the La Sal moun- 

 tains, there are evidences of ancient Local gravel terraces or plains 

 extending for some miles farther from these local uplifts than do the 

 moraines of the recent glaciers. The latter ice-streams in both of these 

 mountain groups of the plateau country scarcely extended beyond the 

 sharply defined zone between mountain and plain. 



