DESCRIPTION OF THE RICO QUADRANGLE By Whitman Cross land F. Lu Baiisome. GEOGRAPHY AND GENERAL GEOLOGY OF THE QUADRANGLE. By Whitman Cross. INTRODUCTION. Features, of the Rico Mountains. The small the quadrangle the mesa floor is affected in pre­ F. M. Endlich examined the district to the east, group of mountains in the northeastern section of cisely similar manner by the uplift of those moun­ the one hundred and eighth meridian, passing The Rico quadrangle is situated in southwestern the quadrangle is in large degree a local center of tains, the steeper slopes of which begin a mile or through Telescope Mountain, being apparently the Colorado, about 50 miles west of the Continental uplift which is apparently independent of igneous two south of the quadrangle line. Between the general western boundary of his field of work. In Divide, in the zone bordering the San Juan Moun­ intrusion; but it is also to an important extent Rico and La Plata mountains the mesa is cut ,off 1876 W. H. Holmes made a rapid reconnaissance tains, almost at the head of the Dolores River. It characterized by many injected laccolithic masses. by the Dolores Valley and does not reappear on over the plateau country to the west. The compli­ is bounded by meridians 108° and 108° 15' west The intrusive rocks are of kinds common in the the eastern side because of the upturning of all cated geology of the Rico uplift, coming on the longitude and parallels 37° 30' and 37° 45' north so-called .laccolithic mountain groups of the pla­ formations on this general line, under the influence border zone between the fields of different men latitude, embracing about 236 square miles. teau country, embracing the La Plata, El Late, of the broad San Juan structure. working in different seasons, did not receive ade­ Carriso, Abajo, La Sal, and Henry mountains, Alnxost ,the entire surface of these mesa or pla­ quate attention, and the Hayden map of this area GENERAL, RELATIONS OF THE QUADRANGLE. most of which are plainly visible from the Rico teau remnants is covered by a forest growth in is, therefore, quite unsatisfactory. Relations io the plateau country. The Rico summits. This character of the Rico group was which white pine and aspen are the chief elements. J. B. Farish and T. A. Richard.- The only quadrangle lies in the north-south zone that not recognized during the Hayden Survey. The mesa border southwest of Bear Creek is espe­ geological explorations of the quadrangle since the marks the eastern border of a very notable pla­ The sedimentary section. In- general the sec­ cially characterized by a magnificent growth of time of the Hayden Survey have been connected teau surface which covers the greater part of the tion of sedimentary formations exposed in the stately aspens. At lower levels pin on, white pine, with mining developments in the Rico Mountains. area between the Colorado River in Utah and the valley of the Dolores River is that normal to cedar,, and scrub oak become more and more In the course of descriptions of some of the min­ San Juan Mountains of Colorado. Below the the zone about the San Juan Mountains. It is, prominent. ing properties near Rico there have been brief .dis­ gently undulating surface of this plateau many for example, like that shown in the adjoining The Rico Mountains. The summits of this com­ cussions of the geology of the mountain group. canyons have been carved by streams, one of the Telluride quadrangle by the erosion of the San pact and rather isolated group lie within an,oval These discussions were for the most part founded principal gorges being that of the Dolores River. Miguel River, and extends from the Mancos area about 7 miles in diameter from east to west on observations near and in the mines of New- Entering its canyon valley within the Rico quad­ (Cretaceous) shales down into the Carboniferous and 5 miles from north to south. The peaks are man Hill. In 1892 John B. Farish read a pap.er rangle this stream flows with irregular course for red beds. But in consequence of the Rico uplift nearly all included, within the northeast section of before the Colorado Scientific Society entitled "On about 18 miles in a southwesterly direction and and its bisection by the Dolores the lower Paleo­ the Rico quadrangle, but a few lie east of the one the Ore Deposits of Newman Hill, near Rico, Col­ then swings to a general north-northwest trend, zoic formations are shown locally, and even cer­ hundred and eighth meridian, in the Engineer orado" (Proc. Colorado Sci. Soc., vol. 4, pp. 1 SI- which it maintains for over 100 miles to the tain quartzites of the Algonkian. The formations Mountain quadrangle. ,,. 164). The description of the ore deposits was Grand River. thus revealed in the Dolores Valley have the gen­ The topographic map of the quadrangle shows preceded by some general remarks on the geology. The larger part of the plateau surface, lying eral character of the complete section more per­ the general character of the mountains as compared The structure of the mountains was recognized by between the Dolores and Colorado rivers is called fectly exposed in the Animas Valley, about 12 with the plateau area and the long lateral ridges of Earish as a domal uplift. the Great Sage Plain, while its direct continuation miles to the east. The Mesozoic formations are the Dolores Valley. The special sheet exhibits A detailed description of the Enterprise mine eastward and toward the head of the Dolores is the same that characterize the canyons of the the finer details of form and includes the peaks was published in 1896 by T. A. Rickard, then named the Dplores Plateau. plateau country to the west, :but it is known situated east of the quadrangle line. superintendent of the mine (Trans. Am. List. This broad plain surface is due chiefly to a that most of those formations exhibit .progres­ From these maps it may be seen that the Rico Min. Eng., vol. 26, pp. 906-980). In this paper heavy sandstone, the Dakota (Cretaceous), and its sive changes as distance from the Colorado moun­ Mountains consist of a circle of high and rugged there are but few statements concerning the gen­ undulations are in part structural, in harmony tain area increases. These changes have not yet peaks, divided into two crescent-shaped halves by eral geology. The strata about Rico are said to with the slightly varying dips of the sandstone, been examined in detail. the Dolores Valley. There are twelve peaks, each be fossiliferous and to belong to the lower Car­ and in part owing to remnants of the soft, thick exceeding 12,000 feet in elevation above sea level, boniferous, and the common igneous rock is called GEOGRAPHY AND TOPOGRAPHY OF THE QUAD­ shale formation normally overlying the sandstone. and the narrow crest connecting them rarely sinks porphyrite, and is concisely described by R. C. RANGLE. The Great Sage Plain of Utah has a general eleva­ below 11,500 feet on either side of the river. In Hills. Rickard refers to "a large dike of porphy­ tion of 6000 to 7000 feet above the sea. Eastward The Rico quadrangle presents three especially passing through the group the Dolores receives rite" crossing the valley north of Rico, "making the Dolores Plateau gradually rises with the dip of prominent types of topographic forms, each dom­ several important tributaries on each side, which a fault which breaks the continuity of the coun­ the sandstone until, on the western border of the inating a considerable part of the area. These not­ expose the internal structure of the group in many try on either side." It would appear that this Jlico quadrangle, it has an altitude of over 9000 able features are (1) the Dolores Plateau, (2) the important respects. These lateral gulches are all reference must be to the mass of schists with small feet. , Beyond that line it rises more rapidly as the Rico Mountains,, and (3) the Dolores Valley, with deep, with steep sides, and their streams are still dikes of hornblendic porphyry; but the position and Dakota sandstone and other formations take part its many lateral branches. actively engaged in the work of erosion. importance of the fault are not further indicated. in the local structures of the Rico and La Plata The Dolores Plateau. -The western .half of the The characteristic forms of peaks and gulches U. S. Geological Survey. In the course of the Mountains, to be described in detail. Rico quadrangle belongs to the Dolores Plateau. are illustrated in the photographs reproduced in present resurvey of. the Rico quadrangle the geo­ Relations to the. San Juan Mountains. The A glance at ,the topographic map shows that this folio. Fig. 1 in particular shows the details logic, complications in the Rico Mountains were southwestern front of the volcanic San Juan between the Dolores River and Stoner Creek, there of form .commonly present in the higher summits found to be,;so great that a detailed topographic Mountains lies 6 to 8 miles northeast of the Rico is a gently inclined mesa crossed by the western on the eastern side of the river. map and a.,. special report on its geology and quadrangle. The intervening space is character­ meridian of the quadrangle at an elevation of Timber line in the Rico Mountains lies between mineral resources were found necessary. This ized by irregular foothill topography, with features about 9400 feet. The flat crest of the narrow 11,500 and 12,000 feet, and its course may be report appeared in the Twenty-first Annual due in part to the upturning and erosion of various ridge between Stoner Creek and the West Dolores traced in several of the illustrations of the folio.