LANDSCAPE EVOLUTION IN CASTLE ROCKS STATE PARK, ALBION RANGE, SOUTHERN IDAHO

Castle Rocks State Park contains a cluster of spires composed of granitic Oligocene Almo pluton that rise up to 200 m above the floor of Big Cove in the Albion Range of southern Idaho. Intermediate and small-scale landforms provide evidence of a complex erosional history that includes multiple episodes of the classic two-stage process commonly invoked for the formation of tor and bornhardt landscapes. Perched quartzite cobbles and multiple levels of flared slopes on some spires result from exhumation and reburial produced by long-term variations in local climate. Many spires near the perimeter of the cluster have flared slopes at ground level indicating relatively recent exhumation. Although subsurface weathering along joints controls the overall distribution and shape of spires, the formation and growth of panholes and tafoni is the most important factor in shaping the spires once they are exhumed. Panholes are absent on freshly exposed rock and enlarge with time on sub-horizontal surfaces. The enlargement and coalescence of adjacent panholes is an important process in removing rock from the spires. The rate of rock removal is related to the residence time of water within the panholes and is thus tied to climatic variation. On the summits of many spires where the residence time of water is minimized, this process has produced spikes that we refer to as "pickelhauben" (singular pickelhaube) based on their resemblance to spiked helmets formerly worn by the German military. Tafoni are well developed throughout Castle Rocks. They initiate on joint surfaces of all orientations and enlarge through the creation of a sheltered microclimate that promotes the accumulation of salts. Granite spires 65 km south of Castle Rocks at Devils Playground near the Great Salt Lake Desert display a much greater development of tafoni that is likely related to their proximity to a salt source. The Castle Rocks are surrounded by alluvial fans composed primarily of Proterozoic quartzite cobbles and boulders. The upper slopes of the fans contain relict solifluction lobes and remnants of large-scale landslides formed during the most recent alpine glaciation of the Albion Range. The alluvial fan surface down-slope of Castle Rocks has aggraded 45 cm in the last 2450 years based on a radiocarbon date obtained from an archeological site.