OF-16-05 Geologic Map of the Longmont Quadrangle, Boulder and Weld Counties, Colorado
2017-04-12 | Dr. John Hopkins
We just uploaded the most recent of our STATEMAP mapping products to our online store: the Geologic Map of the Longmont Quadrangle, Boulder and Weld Counties, Colorado. The STATEMAP series in general provides a detailed description of the geology, mineral and ground-water resource potential, and the geologic hazards of an area. This particular 7.5-minute, 1:24,000 quadrangle is located immediately east of the Front Range uplift of Colorado and includes most of the town of Longmont within its borders. The geologic map plates were created via traditional field mapping, structural measurements, photographs, and field notes acquired by the investigators. Richard F. Madole, PhD Scientist Emeritus at the USGS was the lead geologist for the project. This free release from the CGS includes two plates (with a geologic map, cross-section with correlation, oblique 3D view, legend, and description) along with the corresponding GIS data package that allows for digital viewing, all in a single zip file.
From the map history
The Longmont quadrangle is in the northern part of the Colorado Piedmont, which is a section of the Great Plains that is bounded on the west by the Front Range and on the east by the High Plains section of the Great Plains. It is distinguished primarily by the fact that it has been stripped of the Miocene fluvial rocks (Arikaree and Ogallala Formations) that cover most of the High Plains. Headward erosion of the South Platte and Arkansas Rivers and their tributaries caused most of the stripping. Like much of the Colorado Piedmont, the Longmont quadrangle is an area of low hills and plains underlain by Upper Cretaceous (100–66 Ma) sedimentary rocks. Most of these rocks consist of fine-grained sediment (clay, silt, and fine sand) that accumulated in a broad seaway (Western Interior Seaway). This seaway connected the areas of the present-day Arctic Ocean and the Gulf of Mexico and extended from Minnesota and western Iowa on the east to central Utah on the west
Even before urbanization, Upper Cretaceous bedrock was exposed in only a few places in the Longmont quadrangle because loess of late Pleistocene age (126 ka to 11.7 ka) blankets about 85 percent of the area. Deposition of most loess is attributed to northwesterly winds, which during the last glaciation (between about 40 ka and 12 ka) were stronger than they are today, blowing across extensive areas upwind from the Longmont quadrangle that are underlain by siltstone, mudstone, and shale. Thus, eolian sediment covers almost all bedrock and surficial deposits (loose, uncemented sediment as opposed to rock) that were at the surface prior to the end of the last glaciation. The floors of the major streams in the Longmont quadrangle also bear the imprint of Pleistocene glaciations. The gravel deposits that are mined in several places along St. Vrain and Boulder Creeks consist mostly of granitic and gneissic rocks that were derived from the Front Range and transported to the piedmont during glaciation. The headwaters of the St. Vrain, Lefthand, and Boulder Creeks were glaciated repeatedly during Pleistocene time. The principal glaciers in these areas were 10–12 miles (16-20 km) long and as much as 600–1150 ft (2-350 m) thick.
This mapping project was funded jointly by the U.S. Geological Survey through the STATEMAP component of the National Cooperative Geologic Mapping Program, which is authorized by the National Geologic Mapping Act of 1997, and also by the CGS using the Colorado Department of Natural Resources Severance Tax Operational Funds. The CGS matching funds come from the severance paid on the production of natural gas, oil, coal, and metals. Geologic maps produced through the STATEMAP program are intended as multi-purpose maps useful for land-use planning, geotechnical engineering, geologic-hazard assessment, mineral-resource development, and ground-water exploration.