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The Origin and Characteristics of Soil Mounds

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The Origin and Characteristics of Soil Mounds THE ORIGIN AND CHARACTERISTICS OF SOIL MOUNDS AND PATTERNED GROUND IN NORTH CENTRAL OREGON by CLARK A. NELSON A Research Paper submitted to The Department of Geography in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE June 1977 ACKNOWLEDGEMENTS The completion of this paper would not have been possible without the support and interest of many people. To my major professor, Dr. Charles Rosenfeld, I wish to express a deep gratitude for initiating my interests in this topic and for supplying the inspiration to move when I was standing still. His knowledge of physical geography and his ability to combine youthful insights with the wisdom of experience never ceases to amaze me. He also served a valuable role as liaison between myself and the National Guard of Oregon from whom much research material was obtained. I would like to dedicate this paper to my parents and family who deserve a thanks which is beyond words. They have always believed in and encouraged me to pursue my own interests, and have given me unending support in various endeavors throughout my life. A special tribute must be made to all my friends, who, often unbeknownst to themselves, supplied a continuing source of encouragement, and helped to clarify the realities which often become obscured through deep involve- ment. Last but not least, special mention goes to the small but active band of coyotes residing near my research area. Their curiosity and nocturnal antics, though initially resulting in insomnia, soon were anticipated with relish upon subsequent returns to this area. TABLE OF CONTENTS ACKNOWLEDGEMENTS TABLE OF CONTENTS LIST OF FIGURES INTRODUCTION ............................. 1 PHYSICAL SETTING........................... 2 Geology.............................. 4 Soils............................... 6 Climate.............................. 8 Vegetation............................ 9 LandUse............................. 11 METHODOLOGY ............................. 11 PHYSICAL CHARACTERISTICS OF STONE RINGS AND SOIL MOUNDS IN NORTH CENTRAL OREGON...................... 13 Definitions............................ 13 Problems Encountered in Studying Relict Forms........... 14 Morphology............................ 15 Composition............................. 21 ORIGIN OF PATTERNED GROUND AND SOIL MOUNDS IN NORTH CENTRAL OREGON .......................... 25 EarlyAccounts.......................... 25 Pleistocene Environmental Conditions ............... 27 Processes Involved in Formation.................. 29 CONTEMPORARY GEOMORPHIC ACTIVITY................... 32 CONCLUSIONS .36 SUGGESTIONS FOR FURTHER RESEARCH................... 39 BIBLIOGRAPHY............................. 43 LIST OF FIGURES 1. Location of the Deschutes-Umatilla Plateau............ 3 2. Geology of the Deschutes-Umatilla Plateau ............ 5 3. Soils of the Deschutes-Umatilla Plateau ............. 7 4. Relationship between station elevations and monthly frequency of diurnal freeze-thaw cycles 1962-1971 ............ 10 5. Soil mound excavation ...................... 12 6. Schematic diagram of patterned ground development and the effectsof slope on shape ................... 17 7. Schematic diagram of the successive stages of the development ofsorted stone nets...................... 18 8. Aerial photograph of various soil mound configurations...... 19 9. Aerial photograph of soil mounds and stone stripes........ 20 10. Soil mounds and intermound drainage swale ............ 22 11. Cross-section of soil mound and stone ring............ 23 12. Stone stripe cross-section (two photos) ............. 24 13. Extent of glaciation and pluvial lake development in Oregon duringQuaternary ........................ 28 14. Sequence of the upfreezing of a stone within a. soil mass...... 31 15. Small stones heaved by contemporary frost action ..........14 16. Frost boil formed by contemporary frost action.......... 35 17. Contraction cracking in frost susceptible soil under current climatic conditions ...................... 36 18. Accumulation of frost shattering rubble ............. 38 THE ORIGINAND CHARACTERISTICSOF SOIL MOUNDS AND PATTERNED GROUND INNORTHCENTRAL OREGON ABSTRACT: Sections of the landscape in north central Oregon are dominated by numerous soil mounds surrounded by stone rings. Mounds vary in size and shape depending on slope and mound density. Inter- nal soil mass is free of stones to the bedrock level and is covered by successive layers of volcanic ash. Stone ring profiles exhibited strong vertical sorting of stones by size and a decrease in ring width with depth. Stone areas were generally void of soil except for a shallow layer on the bottom. Noticeable depressions in the bedrock layer were found beneath the rings. Inter-mound areas appear as large surface drainage swales with a very shallow soil layer contain- ing numerous stones. Formation of stone rings is attributed to the ejection of stones from the soil mass by intense frost action under a former periglacial climate. Increased subsurface drainage, piping, and erosion led to a lowering of the soil surface above the stone rings, thus segregating the high centered mounds. Small scale frost features continue to be formed though the climate is no longer of sufficient intensity to produce stone rings and soil mounds. INTRODUCTION There are a number of areas throughout the Pacific Northwest where relict patterned ground features exist. Though each area is unique in topography, soil conditions, and Pleistocene history, the patterned ground has a similar appearance with stone stripes and steps, stone nets, sorted stone rings,and their associated soil mounds. Landscapes composed of these features are often referred to as pimpled plains or biscuit scablands. These phenomena occur in specific regions on the Columbia Intermontarie Plateau as it extends into Washington, Idaho, and Oregon. My interests are specifically with the sorted stone rings and soil mounds as they exist on the Deschutes-Umatilla Plateau in Oregon. Upon initial in- vestigation, it became apparent that there is a dearth of information 2 concerning most aspects of these features. The patterned ground in this specific region has received only cursory treatment in relation to the Pacific Northwest as a whole. Theories as to the origin of these micro- relief features are wide-ranging and quite controversial. Few of the theories consider the association between the mounds and stone rings, though each appears as an integral part of the other. As these features are generally cold climate phenomena, I felt further study would be of importance in understanding the environmental! climatic realm of this region as it existed during the Pleistocene. Hopefully it will also give insight into processes currently active, form- ing similar features in arctic and alpine areas. The purpose of this research was to investigate and document the morphology, composition, distribution of, and relationship between the soil mounds and stone rings. I have used my research in conjunction with the previous work of other authors concerning similar features else- where, to develop a theory as to the origins of this biscuit-scabland topography. Finally, contemporary geomorphic processes active in my research area have been evaluated. Though these processes are comparable in their nature, the intensity with which they occur has greatly diminished since the Pleistocene, at which time the patterned ground is believed to have formed. PHYSICAL SETTING My research area is located on the Deschutes-Umatilla Plateau in north central Oregon (Figure 1). Though this broad region has a number of unifying, homogeneous characteristics, it contains a number of sub- regions, delineated on the basis of soil types, climate, and geology. DZCHUT5LJMAT1LLAOREGON PLATEAU 0..cI.,.tJst;II.Cent1M.I.t &..nd.,I.. P1.,... hssd.,1 Figure 1. Location of the Deschutes-LJmatilla0 10 20iI.t..Istt30 40 Plateau20 in north central Oregon. CA) 4 The Deschutes-Umatilla Plateau's northernborder extends along the Columbia River for approximately 250 km. The westernmargin adjoins the base of the Cascade Range and extends 11Ekm to the south. The southern boundary is marked by the northern slopesof the Blue Mountair,while the eastern margin extends south from the Coli.jmbia River approximately 15 km, in the vicinity of Pendleton. Geology The Deschutes-Umatilla Plateau is composed ofa vast volcanic sequence known as the Columbia River Basalts (Figure 2). At one time, numerous lava flows emanated from large fissures, obscuring the original topography with successive layers of basalt. Individual basalt flows were from lm to over 60m thick and covered approximately 500,000sq km in Washington, Idaho, and Oregon. Though the basalt flows decrease in thickness toward the southern plateau edge, theyaverage over 6On in depth throughout (Newcomb, 1969). Numerous ridges and exposed bedrock outcrops, representing lava flows of varying resistances,are apparent throughout the region. The Plateau is on an asymmetrical syncline, dippingnorthwards from to 3° from the Blue Mountains to the Columbia River (Newcomb, 1969). Elevations range from lOOm near the Columbia toover ilOOm in the vicinity of Shaniko. A short distance south of Shaniko, the Plateau terminates with a large escarpment. My research was conducted near Shaniko and northward on a broad, relatively undissectedplain, charac- terized by rolling hills with numerous bedrockoutcrops. -. Bar and ball onFault downthrown side Syncline I icr Dashed whereAnticiezQTs approximate Dotted wherenocline
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