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Antelope Island Field Trip


        The Central Wasatch Mountains lie directly astride the boundary between two major physiographic provinces: the Basin and Range, and the Middle Rocky Mountains.  These provinces are further divided into five smaller physiographic units:  (1) Great Salt Lake Valley, (2) Wasatch Mountains, (3) Back Valleys, (4) Eastern Highlands, and (5) Lake Islands.  For political purposes the area is defined as Weber, Davis and Morgan Counties, Utah.

Great Salt Lake Valley
        The Great Salt Lake Valley extends westward from the base of the Wasatch Mountains.  Its western portion is a broad, almost flat plain composed largely of lake bottom deposits from ancient Lake Bonneville, partly modified by young erosion and deposition principally by streams.  The eastern portion, that part lying adjacent to the Wasatch Mountains, is steeper and characterized by local faulting, Bonneville-aged beaches, deltas, alluvial fans, and recent cutting by streams.
        This plain is bordered by the north-south trending Wasatch Mountains.  The valley represents a large down dropped graben, or a tilted half graben, and sediments up to 6000-8000 feet thick are present.

Wasatch Mountains
        The Wasatch Mountains are the most conspicuous physiographic feature of north-central Utah.  The main ridge of the range lies along a north-south axis through the center of the study area.  It contains peaks that rise 5000 feet above the valley floor.  The highest peak is Ben Lomond Peak, elevation 9712 feet.
        Generally the western side of the range is steeper than the eastern side.  However, both sides exhibit areas of gentle slopes and steep inclines, the latter partly due to the presence of faults along which the mountain block were uplifted.  This uplift is evidenced on western slopes by numerous triangular facets and fault scarps.  Two large east-west canyons, Weber Canyon occupied by the Weber River, and Ogden Canyon, occupied by the Ogden River, are found within the study area.  In addition, numerous small canyons have been cut into the slopes on both sides of the range.  Some of these contain perennial streams, but most are ephemeral.
        The Wasatch Mountains were formed by two separate events with combinations of thrust and normal faults.  During the late Mesozoic, compression forces began to build the ancestral Wasatch Mountains by thrust faulting.  These compression forces caused mountains to form that contained older Pre-Cambrian metamorphic rocks and Paleozoic sedimentary rocks.  These rocks were completly folded and faulted to the degree that old layers are often found on top of much younger ones, as can be readily seen on the sides of Ogden Canyon.  These early mountains probably largely eroded.  Regional normal faulting then occured during Tertiary to recent times, lifting the Wasatch Mountains and the Bear River Range to their present positions and down dropping the Great Salt Lake and back valleys.  Water and glacial action then shaped the mountains into their present physiography.

Lake Islands
        Two islands of the Great Salt Lake are located within the three counties.  Antelope Island, the largest of the two, is located in Davis County.  It is a north-south trending island 15 miles long, and three to four-miles wide.  Its maximum elevation is 6400 feet rising some 2200 feet above the surface of the lake.  Several small, ephemeral streams and some small springs are located in the bottoms of its shallow canyons.
        Fremont Island, named in honor of Captain John C. Fremont, is located in Weber County.  It trends northwest-southeast and is three-fourths-mile long and one-fourth-mile wide.  Its maximum elevation is 4995 feet.  The island has no streams and only one small spring.  
        Both islands probably originated as uplifted blacks (horsts) during the same time as the present Wasatch Range and the Eastern Highlands were formed.  
        Antelope Island consists of older Precambrian metamorphic and igneous rocks and late Precambrian to Paleozoic sedimentary rocks mantled locally by a thin layer of Quarternary lake deposits, colluvium and alluvium.  Fremont Island consists only of late Precambrian sedimentary rocks with a thin cover of Quaternary lake deposits.


        The area is broadly defined as semi-arid.  However, due to great differences in elevation and exposure, considerable variation in mean annual precipitation is found ranging from a semi-arid condition of six inches or less on the islands of the Great Salt Lake to a sub-humid climate of 45 inches along the tops of the Wasatch Mountains.
        In general, seasons are characterized by a wet spring (March to May), warm-to-hot, dry summer (June to August), cool, dry autumn (September to November), and cold, snowy winter (December to February).
        Summer storms in the low valleys are infrequent while storms of short duration and high intensity are common in the mountains.  The greatest recorded storm intensity in Farmington Canyon was 2.04 inches per hour.  These summer storms are usually a product of convective thunderstorms although some originate when weak cold fronts moving eastward from the Pacific lift the warm, moist air that has come into our area from the Gulf of Mexico.  Prevailing winds are from the south and southwest.
        Winter storms generally originate with strong frontal systems moving in from the Pacific Northwest and account for 80 percent of the precipitation with most of this falling as snow at elevations above 7000 feet.  Snow records from Farmington Canyon show an average depth of snow pack at 7600 feet elevation of 53.3 inches with a water content of 18.6 inches.  Snow depths of 18 feet or more in drifts along the mountain tops are common.
        Smaller climatic differences such as those found on north-or south-facing slopes of ridges or canyons, particularly if they are steep, are also important.  South-facing slopes have greater insulation, higher ground temperature and drier soils.  Air temperatures are generally cooler in the canyon bottoms and on the north-facing slopes, while relative humidity is quite variable.
        Moisture content of the snow pack is also quite variable.  In general south-facing slopes and areas covered by heavy stands of conifers have a greater percentage of moisture per unit of snow depth than those open areas on north-facing slopes.  These small, local climatic differences greatly influence the distribution of plants and communities within the study area.


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