AREAL GEOLOGY OF LOWER CLEAR CREEK 



(COLORADO)^ 



By James Underbill 



Introduction 



The area under consideration, shown on the key map, Fig. i, embraces 

 the northern portion of what I have called the Evergreen Quadrangle, 

 about i8o square miles, and the southern portion of the Black Hawk 

 Quadrangle, about 60 square miles. I have also covered in great detail, 

 as far as the dikes are concerned, the region embraced in the so-called 

 Idaho Springs and Central City special maps, an area of about 18 square 

 miles. 



For the Black Hawk I had the excellent topographic map of the 

 United States Geological Survey on a scale of 1-45,000, or about i^ inches 

 to the mile . For the special area I had the Idaho Springs and Central 

 City specials on a scale of 1-12,000, or about 5 inches to the mile. 



There being no topographical map of the Evergreen Quadrangle, one 

 had to be made before geological mapping could be done, and this has 

 been in part constructed as follows. Where the Georgetown and Black 

 Hawk Quadrangles overlapped on the Evergreen Quadrangle, the topog- 

 raphy of these maps has been used. On the south and east the topog- 

 raphy of the Platte Canon and Denver sheets respectively has been 

 used. Surrounded as the Evergreen Quadrangle has thus been, it has 

 been admirably checked on all sides. Several triangulation points of the 

 U. S. Geological Survey on the Evergreen Quadrangle, with some other 

 triangulation points on other quadrangles that may be seen from it, have 

 been platted. Other points have been more or less accurately determined 

 with the pocket sextant. Then the township lines were laid on with 

 checks by the topographic sheets above mentioned, and finally the 

 sections and their topographic "guesses" — for they are little bet er — 

 were sketched in. These topographic features have been found reason- 

 ably accurate at the intersections with the township lines, but as a rule 



' Presented to the Graduate Faculty of the University of Colorado in a partial fulfilment of the require' 

 ments for the degree, doctor of philosophy. 



263 



