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• <br />Some of the gravels on the terraces or on the present valley floors may have been deposited in <br />Pleistocene time by melt waters from valley glaciers, which are known to have existed in the headwaters of <br />Clear Creek and some of its tributaries. There is no evidence of glaciation in the Idaho Springs district other <br />than the gravel deposits. <br />STRUCTURE <br />The structural framework of the Idaho Springs district is outlined by the major units of conformable <br />gneisses, which are folded along northeast-trending axes (pl. 2 ). During Precambrian time these rocks, which <br />now strike generally northeast, were deformed at least twice, and may have been faulted. The first deformation <br />was pervasive plastic folding that took place at considerable depth at high temperatures and pressures and was <br />accompanied by intense recrystallization and the emplacement of many small bodies of granitic rocks. The <br />second deformation was characterized by intense granulation as well as by folding in a relatively narrow zone. <br />I t apparently took place at a somewhat shallower depth throughout a 2-mile-wide zone, termed the Idaho <br />Springs-Ralston shear zone, which extends at least 20 miles northeastward to the margin of the Front Range <br />(Tweto and Sims, 1%3). During the Laramide orogeny the rocks werejointed, intruded by a sequence of <br />porphyritic igneous rocks, and cut by an anastomosing network of faults. <br />The Precambrian structure of the district was described in detail by Moench (1964), and the joint <br />patterns and Precambrian structure of a larger area were described by Moench, Harrison, and Sims (1962) and <br />by Harrison and Moench (1961). These facets of the geology of the district are summarized here. <br />FOLIATION AND LINEATION <br />All the gneissic Precambrian rocks and some of the granitic rocks are characterized by well-developed <br />• foliation and lineation. Foliation in the gneissic rocks is expressed by compositional layering in the rock and by <br />preferred planar orientation of platy and tabular minerals. With few exceptions both features are parallel, and <br />they are parallel to the major lithologic layers shown on plate 2. This type of foliation probably represents <br />bedding in the original sediments. <br />Where the rocks were granulated by the younger Precambrian deformation, a cataclastic type of <br />foliation formed. This foliation is characterized by a subparallel mesh of close-spaced fractures that are healed <br />mainly by quartz. The cataclastic foliation typically is parallel to the older foliation described above, but locally <br />it breaks across it. <br />The granitic rocks commonly have a foliation that is termed a primary flow structure. In the <br />granodiorite, primary flow structure is expressed as an alignment of elongate inclusions parallel to discordant <br />contacts (Harrison and Wells, 1959, p. 12). In the biotite-muscovite granite, tabular feldspar crystals commonly <br />show a similar parallel alignment that has resulted from the flowage of partly crystallized magma. <br />Granodiorite and quartz diorite also show a secondary metamorphic foliation that is similar in <br />character to the foliation in the gneissic rocks. In the Chicago Creek area the secondary foliation in granodiorite <br />is locally superposed on the primary foliation and is continuous with the foliation in the gneissic rocks <br />(Harrison and Wells, 1959, p. 12). <br />The gneissic rocks of the district are characterized by many kinds of lineation, but five categories are <br />recognized: (1) the axes of small folds and crinkles; (2) elongate minerals, such as sillimanite and hornblende, <br />and elongate mineral aggregates, such as small pods of granitic material; (3) the axes of boudinage, or "pinch" <br />structures that formed by stretching; (4) rodding (rod-shaped features that result from the rolling or shearing <br />between layers); and (5) slickenside striae. <br />The lineations have systematic orientations that can be related to each of the two Precambrian fold <br />systems. Small folds, crinkles, and mineral alignments parallel the major folds of the older fold system <br />. throughout the district. Also, small warps, crinkles, boudinage, and sparse mineral alignments are oriented <br />approximately at right angles to the major folds and probably formed at a late stage in the older deformation. <br />13