Abstract

In landscapes where erosion rates exceed weathering, isolated bedrock outcrops may emerge as soil is removed. These outcrops, called tors, form when a heavily weathered area is subject to increased erosion rates, possibly due to dramatic climate shift. Mapping and measurements of tors in Gordon Gulch, a forested upland catchment in the northern Front Range of Colorado, shows that tor development is a function of erosion and weathering rates, bedrock characteristics, slope, and aspect. Satellite-based (GPS) measurements and geographic information system (GIS) analysis show that tor density and size are most closely related to the relative orientation of foliation with respect to local slope. Tors are much larger and more common on south-facing slopes because they are perpendicular to foliation of the gneissic units of the area, preventing downslope movement from taking advantage of weak layers. Rock type also influences tor characteristics, and the largest outcrops in the gulch were derived from the granitic units, despite the occurrence of sheeting fractures that should limit tor size. Most tors are developed in gneissic rock, which dominates the gulch. Chemical analysis of the gneissic units shows depletion of rare-earth elements typical of a protolith derived from intermediate volcanic rocks formed above a subduction zone. GIS-based analysis demonstrates that tors are most common on steeper slopes, and on the basin margin. Tors are not common near channels, where deposition rates are relatively high. However, tors are clustered on a steep, south-facing slope near a basin knickzone, where higher rates of erosion would be expected. Tors are relatively common on the ridge crests, where deposition is least probable. Understanding the controls on tor evolution is an important step in studying how the Critical Zone (the area between fresh bedrock and the top of vegetation) evolves over time. Studying tors can reveal how rock properties and erosion processes control the response of slopes to events such as channel downcutting and climate change. Investigations of tors also help to reveal specific critical- zone processes, such as how regolith thickness affects tor and outcrop formation rates. In areas where local erosion rates exceed weathering rates, as in Gordon Gulch, tors can be an excellent indicator of geologically recent hillslope evolution.

Citation

Trotta, James Richard. (2010): The Distribution of Tors in Gordon Gulch, Front Range, Colorado . Thesis. Degree of Bachelor of Arts with Honors in Geosciences. Williams College 2010.