Laserfiche WebLink
G30LOGY. <br />The most outstanding phvsiographic festure of the <br />Boulder 9rea is the abrupt wall-like mountain front <br />forming the boundary between the ?rant Range and the <br />Piedmont Brea, <br />The narrow foothills area along the western margins of <br />the Piedmont is characterized b•: a series of ?olded wind <br />faulted sedimentary strata, the more resistent beds of <br />which form prominent hogback ridges. <br />The foothills area at the base of the mountain front is <br />characterized by numerous broad, gentl^ sloping inter- <br />sream surfaces that stand at •:arious steolike le•rels <br />above modern stream aalle~rs. The hick-level ceomorph'._c <br />surfaces usually occur as fFn-shaped pediments msntled <br />by coarse alluvial deposits. These deposits lie on <br />erosion surfaces that truncate the titled strata of i;he <br />foothills belt. Below these pediment surfaces, the <br />modern streams generally occuo- relatively wide and :°lat- <br />floored valleys. <br />Geologic formations ran Ping from Precambran to Recen'; <br />in age occur within the surve•r area. The~• consist o° <br />Drecambrian metamorphic and igneous rocks; sedimentary <br />• rocks of Daleozoic and ~•~esozoic are; a few small bodies <br />of igneous intrusive rock of Tertiary- age; and uncon- <br />solidated surficial deposits of quaternary (~leiocene <br />and Recent) afire. The oeoloQic formations present also <br />include Boulder Creek and Silver Plume Granite of Pr~j- <br />camhrian age. <br />The most ex±_•naive sedimentar~r formations in the surve•• <br />area are the Dierre Shale, the ?ox Hills Sandstone, <br />and the ;aramie '~ormation. The Pierre Shale crops o~zt <br />,)ust to the east of the foothills area and throu{~hou~ <br />the northeastern part of the Boulder 4rea. The ?ox ;ills <br />Sandstone and the Laramie Formation cro_o out in the <br />southeastern part of the count?, <br />Sedimentar^ formations that are older than the Pierrs <br />Shale consist of a series of sandstone and shale beds, <br />the sandstone predominating. These formations crop ~~ut <br />mainly as a series of hogback rid€ea in the relative:L*• <br />narrow foothills area along the mountain front. The;.- <br />include the mountain rormation, Lyona sandstone, L~rk:Lns <br />?ormation, Ralston Creek Formation, P4orrison ='ormati~~n, <br />Dakota Group, Penton Shale, and Niobrara formation, <br />• <br />