United States Department of Agriculture Pyramid Creek Geologic Special Interest Area Eldorado National Forest

The Geology That You See

Taking a short hike from how the road winds around the Pyramid Creek Trailhead, the nose of the two moraines. you will find easily accessible The larger of the two moraines exposures of petrographic is the older one. It forms the features visible within the arcuate shaped, linear granodiorite bedrock and ridgeline that extends out View form Horsetail Falls looking to the south at Lovers Leap and Highway 50. excellent views of late from the mountain front and Pleistocene glaciaon. extends to the flat just west of Camp Sacramento. This A great view of the broad U‐ moraine was created by glacial shaped glaciated valley of during the Tahoe glaciaon. Pyramid Creek can be seen The age of the Tahoe from Highway 50, looking to glaciaon is generally the north from just east of the considered to be from 118,000 Pyramid Creek Trailhead. to 56,000 years ago. Horsetail Falls, a series of picturesque cascades and falls, Nested inside of the larger marks one of the steps formed moraine, and forming a lower, as the glacier scoured the parallel ridgeline west of the ancient valley. As you walk main ridge, is the Tioga age along the trail that leaves lateral moraine. This moraine Pyramid Creek Trailhead and marks the maximum extent of Glaciers sculpted the valley of Pyramid Creek flowing follow Pyramid Creek, areas of the ice that accumulated down the valley from Horsetail Falls and filling it almost glacial polish, striaons, during the Tioga glacial stage, to the top of Lover’s Leap. errac boulders carried by which occurred about 20,000 glaciers, and glacially years ago. produced gouge and chaer One of the other prominent marks can be seen. features you see west of A large nested pair of Pyramid Creek Trailhead is lateral moraines can be found Lovers Leap. This high cliff was to the east of Pyramid Creek. in part, shaped by the glaciers Highway 50 is built across that poured out from Pyramid these moraines. As you Creek. Glaciers from a large Striations indicate the direction proceed east on Highway 50, ice cap that covered much of the glacier flowed. watch for areas with large the Crystal Range and rounded boulders and noce Desolaon Valley flowed down USDA is an equal opportunity provider and employer.

Eldorado Naonal Forest website: www.fs.usda.gov/eldorado 2019 What Was That You Said? Glossary The ancient Many of the technical terms commonly used granodiorite bedrock by geologists are shown in italics. Here are exposes features that some definions or explanaons for these terms: record how this rock was formed. Bedrock is the term used to refer to the hard rock that underlies soil or unconsolidated sediments. Country rock is another term used for bedrock. Errac boulders are blocks of rock (oen rounded) that were carried by past glaciers and dropped as the glaciers melted and Above: Chatter marks created receded. by rocks frozen into the ice ripping at the surface. Glacial polish, striaons, gouge, and chaer marks are features formed by glacial ice and chunks of rock frozen into the base of the Intrusions or xenoliths are the remnants of the parent rock. glacier move over the bedrock surface. (note: lens cap gives a sense Glacial polish is the smooth, polished surface of size.) of the bedrock, striaons are linear scratches seen in the glacial polish, gouges are deeper, Pyramid Creek and extended westward, covering the lower, more pronounced scratches in the bedrock, rounded knob you can see just below Lovers Leap, but did not reach and chaer marks are small, closely spaced the top of Lovers Leap. The glacier covered the area near crescenc grooves formed as frozen rocks Strawberry and extended to the western edge of the large meadow held in the ice chip into the bedrock below. at Strawberry. Granodiorite is an igneous rock that formed As you follow the trail leaving Pyramid Creek Trailhead and get into deep in the earth and is generally light in open areas with large expanses where the granodiorite bedrock is color with scaered black or dark minerals exposed, you can see clusters of dark grey “blobs” of rock within the Inclusions are fragments of older rock within lighter colored granodiorite. These inclusions or xenoliths are the younger, igneous rock. The term xenolith commonly a few inches to several feet in diameter. These inclusions is from the Greek, meaning “foreign rock” are thought to be remnants of the parent rock into which the and is the formal term used by geologists for granodiorite was intruded. If you can envision ancient sea floor inclusions in igneous rocks sediments and volcanic rocks buried up to several miles deep. Then, some 130 million years ago, molten rock created by intense heang Moraines are accumulaons of loose soil and deep in the earth rose up into the older rocks, following fractures rock that were formed by past glaciers. and other zones of weakness. In places, the molten rock ripped up There are many types of moraines. Moraines or engulfed the original country rock which can now be seen as found along the edges of the glaciers are these darker colored xenoliths. known as lateral moraines. Some of the other features Petrographic—descripons of the minerals you can see within the and textures of rocks such as grain size, granodiorite include veins of layering, etc. quartz and feldspar, and Pleistocene is the geologic me period from swirling paerns of black about 1.8 million years ago to as recent as minerals which preserve 10,000 years ago and includes the period of evidence of the moons of the world’s recent repeated glaciaons. the molten rock at the me when it was cooling and Schlieren are irregular dark or light streaks hardening. These unique in igneous rocks which differ in composion paerns in the rock are from the main mass. known as schlieren and can Erratic boulders left behind as the ice Tahoe age and Tioga age are two me receded. be found in many outcrops of periods used to describe the ages of glaciers granic rocks across the Sierra in the Sierra Nevada; Tioga glaciers were Nevada. more recent than Tahoe age glaciers.