Geologic maps are made by finding out the type, composition, age, and distribution of
surficial deposits. In 1998-2000, Marith
Reheis and Darrell Kaufman surveyed the surface
and near surface sediments around Bear Lake using aerial photographs, soil probes and shovel
trenches, seeking to locate and map sediments laid down by higher lake levels or by the Bear River.In order to map surficial deposits across a large area like the Bear River drainage, we examine aerial photographs (like the one at right) to make initial interpretations. Deposit types, such as landslides, river terraces of different ages, and channels incised into lake plains, are tentatively identified on air photos. Promising locations are then examined in-person to verify or ėground-truthî the interpretation. Ideally, we look for exposures, like river banks, road cuts, or gulleys, where the sedimentary layers are visible. When no existing exposures are available, we use soil probes and shovels to create an exposure. The field survey also lets us examine the associations between deposits. For example, lake deposits located on top of river deposits would indicate that at some time in the past high lake levels flooded at least part of the river basin. While in the field we collect shells and charcoal from specific sedimentary layers, for analysis in a radiocarbon or amino acid dating lab. The resulting dates to help us determine the age of each deposit. |
Geologic map information is used in several ways to help us understand changes in stream flow
and lake level. First, the distribution and age of deposits like stream terraces, landslides,
and glacial features in the Bear River drainage tells us when large-scale erosion was moving
sediment from hillslopes, transported by the river, and deposited in Bear Lake. Some rocks
are more susceptible to erosion when the climate is cold and wet, while other rocks erode
faster in dry climates. Thus, the type of the sediment carried by Bear River may change when
the climate changes, and we can see this in the river terraces and lake deposits.
Second, detailed mapping of marsh, lake, and river deposits lying between the Bear Lake and
Bear River today will help us understand the lake/river dynamics over the last few thousands
of years. Check out the preliminary results from mapping.At left, University of Northern Arizona graduate student Ben Laabs points to a Bear lake beach gravel with his shovel. The deposit is at 1844 m (6050 ft), about 39 m (128 ft) above maximum Bear Lake levels today, and is located on the east side of Dingle Swamp. Click here to read more about sediment analysis.
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Other related links:
U.S. Department of the Interior | U.S. Geological Survey
URL: http://esp.cr.usgs.gov/info/lacs/mapping.htm
Page Contact Information: ESP Web Team
Page Last Modified: Wed 2-May-2001 10:08:19 MDT