The Early and Middle Proterozoic rocks in the Needle Mountains include three distinct rock sequences (1) multiply deformed bimodal metavolcanic rocks, related sedimentary rocks, and plutonio units, all metamorphosed to medium grade; (2) multiply deformed clastic sedimentary rocks metamorphosed to low grade; and (3) weakly foliated to unfoliated plutonic rocks. Although many important questions remain unresolved, work by a number of individuals suggests the following history. Mafic and silicic volcanics, sediments, and intrusives of the Irving Formation and Twilight Gneiss accumulated at least 1,760 Ma and were multiply deformed and metamorphosed shortly after accumulation. Small post-tectonic plutons of the Tenmile and Bakers Bridge Granites invaded the metavolcanic sequence between 1,680 and 1,700 Ma. The timing of deposition of the conglomerates, clean quartzites, and pelites of the Vallecito Conglomerate and the Uncompahgre Formation with respect to events in the metavolcanic sequence is poorly constrained, largely because shear zones lie along crucial contacts. Recent work suggests that much if not all of this clastic sedimentary sequence may be parautochthonous and was probably deposited sometime after 1,680 to 1,700 Ma. Despite uncertainties in timing of deposition, it is clear that a second event of deformation and metamorphism affected the region, producing polyphase features in both the metavolcanic sequence and the clastic sedimentary sequence, as well as a weak foliation in the Tenmile Granite. The Eolus Granite and related intrusions, ranging in age from 1,320 to 1,450 Ma, crosscut all structural features in the region and provide a minimum age limit for the latest deformation in the region. This younger event is recorded nowhere else in Colorado but may correlate with events in northern New Mexico.

Abstract 0 Road log begins at the Keystone Lodge in Keystone, Colorado. North of the hotel, Proterozoic granite and felsic gneiss are exposed on the upper elevations of the un-named mountain. The lower slopes of the mountain consist of Pleistocene till that overlies the Precambrian rocks. Drive west on U. S. Highway 6 to the intersection with Interstate 70 (exit 205). 0.6 North of the highway, the Williams Range thrust fault that placed Proterozoic rocks over Cretaceous shale and sandstone is exposed just north of the borrow pit (situated above the shale cliff). The trace of the fault south is covered by Quaternary rocks in the Snake River area, is then exposed along the lower slopes of Keystone Mountain ( Widmann et al., 2003 ). Shale and sandstone of Cretaceous age crop out adjacent the road to Dillon and form the northern slope of Swan Mountain (south of the Snake River Arm of the reservoir) and the peninsula to the west ( Kellogg et al., 2008 ). 1.4 Highway crosses the Snake River. 2.4 Snake River crossing by highway. Pierre Shale (Cretaceous) forms the dark gray outcrops just north of the highway. 4.0 Dillon Reservoir. The west portal of the 21-mile long Harold Roberts Tunnel is located on the end of the peninsula west of here. The tunnel transfers water beneath the continental divide into the South Platte River which flows through Denver. Eastward, the Williams Range thrust is concealed beneath Quaternary units at about the elevation of the power transmission line along the bottom slopes of Tenderfoot Mountain (elevation of 11,441 feet). 6.2 Proceed west along Interstate 70. The Gore Range is directly west of here. The Gore Range is a north-trending basement uplift bounded on the east by the Blue River normal fault and on the west by the Gore fault zone. The Blue River Valley is to the 6.4 Dam for Dillion Reservoir. The exposed unit to the northeast of the dam is the Cretaceous Dakota Sandstone. The ridge to the southwest of the damconsists of the Jurassic-aged Entrada, and Morrison Formations and the Cretaceous Dakota Sandstone (Kellogg, 2002). 8.4 Scenic view turnoff for an overview of the Tenmile Range and west flank of the Front Range. The parking area for the overview is built on til

Abstract 0 Road log begins at the Keystone Lodge in Keystone, Colorado. North of the hotel, Proterozoic granite and felsic gneiss are exposed on the upper elevations of the un-named mountain. The lower slopes of the mountain consist of Pleistocene till that overlies the Precambrian rocks. Drive west on U. S. Highway 6 to the intersection with Interstate 70 (exit 205). 0.6 North of the highway, the Williams Range thrust fault that placed Proterozoic rocks over Cretaceous shale and sandstone is exposed just north of the borrow pit (situated above the shale cliff). The trace of the fault south is covered by Quaternary rocks in the Snake River area, is then exposed along the lower slopes of Keystone Mountain ( Widmann et al., 2003 ). Shale and sandstone of Cretaceous age crop out adjacent the road to Dillon and form the northern slope of Swan Mountain (south of the Snake River Arm of the reservoir) and the peninsula to the west ( Kellogg et al., 2008 ). 1.4 Highway crosses the Snake River. 2.4 Snake River crossing by highway. Pierre Shale (Cretaceous) forms the dark gray outcrops just north of the highway. 4.0 Dillon Reservoir. The west portal of the 21-mile long Harold Roberts Tunnel is located on the end of the peninsula west of here. The tunnel transfers water beneath the continental divide into the South Platte River which flows through Denver. Eastward, the Williams Range thrust is concealed beneath Quaternary units at about the elevation of the power transmission line along the bottom slopes of Tenderfoot Mountain (elevation of 11,441 feet).

Abstract As the late Pleistocene Cordilleran Ice Sheet (CIS) retreated from the southern Puget Lowland and thinned rapidly, marine waters invaded the central and northern lowland, floating the residual ice and causing wholesale collapse of the CIS from southern Whidbey Island to southern British Columbia. Massive, poorly sorted Everson glaciomarine drift was deposited contemporaneously over the entire central and northern lowland. More than 160 14 C dates show that the Everson interval began 12,500 14 C yr B.P. and ended 11,700 14 C yr B.P. Numerous marine strandlines record the drop in relative sea level in the Fraser Lowland from ~180 m (600 ft) at the end of the Everson interval to near present sea level. Following emergence of the Fraser Lowland, a lobe of the CIS advanced from the Fraser Canyon near Sumas to Bellingham during the Sumas Stade. As the ice retreated, at least eight end moraines were built successively across the lowland, each marking a position of ice advance or stillstand that records late Pleistocene climatic fluctuations. About 40 new 14 C dates indicate that the ages of these moraines span the Inter-Allerød–Younger Dryas intervals between 11,700 and 10,000 14 C yr B.P. The 14 C chronology allows correlation of the Sumas moraines with moraines in the Cascade Range, Rocky Mountains, Canada, Scandinavia, the European Alps, New Zealand, South America, and elsewhere. Late in the retreat of the ice, large outburst floods from an ice-dammed lake in British Columbia swept across the Sumas outwash plain, resulting in fluted topography and giant ripples on dune forms.