Northwestern Colorado Research Articles

ESPs Successfully Meet Challenges of CO2 Use in Tertiary Oil Recovery

Technology Update Numerous operators are using CO2 injection as a means of significantly extending the oil life of a reservoir. Most oil fields initially produce without artificial lift (AL). Then as the reservoir pressure depletes some form of AL is employed. In many cases, secondary-recovery techniques, such as drilling horizontal producers or initiating water injection, are employed. Tertiary-recovery methods, which include thermal recovery and gas injection, can further expand recoverable reserves and extend field life. Each of these new tactics has presented learning curves for the operator and electrical submersible pump (ESP) provider. The CO2 learning curves are shrinking as the knowledge base grows. Success stories are growing as well. A major producer in Saskatchewan, Canada, has applied and refined miscible CO2 injection within a program of water-alternating-gas (WAG) injection that has added 155 million bbl of oil reserves and 25 years of life to the field. This operator has also implemented a program of CO2 sequestration. At the Rangely field in northwestern Colorado, CO2 injection has been incorporated into the WAG process. The injection program has added 114 million bbl of oil to recoverable reserves. Because of the high produced-fluid volumes resulting from enhanced-recovery methods involving water and gas injection, AL by means of electrical submersible pumps (ESPs) is often applied. ESP-System Challenges Posed By CO2 Injection Several potential problems affecting pump run life and performance can occur when an ESP is installed in a producing well where CO2 is present (Fig. 1). Carbonic acid attacks Asphaltene buildup Aromatic attacks on elastomers Gas impedance of pump performance Carbonate scaling Carbonic acid. The injected CO2 when mixed with water can form carbonic acid. This acid will attack the iron present in carbon-steel materials in the pump, tubing, and casing. If the fluid speed is low enough, the newly formed iron carbonate (siderite) will remain, creating a protective layer. The mechanical integrity of the component will not be degraded. If the fluid speed is great enough, the iron carbonate layer will be removed. This uncovers a new layer containing iron for the carbonic acid to attack. This mechanism will degrade the mechanical integrity of a carbon-steel component. CO2 corrosion exhibits itself uniquely from H2S, dissolved oxygen, chloride, or galvanic corrosion. Aggressive CO2 corrosion will create deep pits. These pits generally have sharp edges and rounded bottoms. As these pits deepen they will expose internal components to the well fluid. The corrosion pits commonly occur on the outside diameter of the housing material, but occasionally occur on the inside diameter of the pump housings (Fig. 2).

A NEW LOOK AT FREMONT CORDAGE FROM MANTLE’S CAVE IN NORTHWEST COLORADO

The perishables from Mantle’s Cave (5MF1), a nonresidential, Fremont rockshelter located in northwestern Colorado, continue to inform us about the prehistoric people who occupied this area. Because cordage construction techniques, in particular, may be used to identify social groups, the large cordage assemblage from Mantle’s Cave was reanalyzed. This paper discusses the results of the reanalysis, which focused on the direction of the final twist (S vs. Z). The cordage assemblage from Mantle’s Cave viewed as a whole suggests use of the site by one social group. However, the analysis of the cordage within the context of the caches it was recovered from reveals much variability. In addition, new chronometric data indicate intermittent use of the site for over 2,400 years. Reasons for the variability observed in cordage construction techniques for the rich assemblage are proposed.

Antiquity and Evolution of Prosternal Horns in Baridine Weevils (Coleoptera: Curculionidae)

Among weevils of the subfamily Baridinae (548 extant genera and ca. 9,000 species), unique prosternal horns and associated sheaths have evolved independently multiple times. These structures are utilized in a unique form of male-male aggression in which males push, rather than overturn as in most other horned beetles, their competitor. Herein we report the first fossils of male Baridinae exhibiting such prosternal horns. Two males recovered from the middle Eocene Green River Formation in northwestern Colorado serve to provide a minimum age (at ca. 47 million years old) for both the origin of horns and its associated form of male-male combat in Baridinae.

Impacts of the introduced biocontrol agent, Rhinocyllus conicus (Coleoptera: Curculionidae), on the seed production and population dynamics of Cirsium ownbeyi (Asteraceae), a rare, native thistle

The release of non-native insects to control noxious weeds is commonly used to combat invasions without disturbing the environment through chemical or mechanical methods. However, introduced biological control agents can have unintended effects. This study was initiated to evaluate potential non-target effects of the flowerhead weevil, Rhinocyllus conicus Frölich, on Cirsium ownbeyi S.L. Welsh, a rare, native and short-lived perennial thistle in northwestern Colorado, northeastern Utah, and southwestern Wyoming. C. ownbeyi represents one of 22 known native hosts on which this introduced weevil has naturalized. The study population remained stable over the eight years of the study despite floral damage by the biocontrol beetle. The growth rate ( λ) from a count-based population viability analysis of the population was 1.03; however, large inter-year variation indicates this rare species is still vulnerable to local extirpation. The biocontrol weevil consistently damaged the developing seeds over the course of the study independent of changes in overall population size and variation in the number of flowering individuals. The target species, Carduus nutans L. (musk thistle) is generally absent near the study plots, which may limit the population levels of R. conicus that can be sustained in this area. Although R. conicus utilizes C. ownbeyi as a host plant, the late flowering period of this native thistle and the small size of the flower heads may limit the demographic impact of R. conicus on C. ownbeyi.

Notice of release of White River Germplasm Indian ricegrass

White River Germplasm Indian ricegrass ( Achnatherum hymenoides (Roem. & Schult.) Barkworth [Poaceae]) has been released for use in rangeland seedings. This plant material traces from a seed morph collected in northwestern Colorado in Rio Blanco County, near the White River. White River Germplasm displays good germinability in a species characterized by high levels of seed dormancy. We anticipate this material will be used in the Upper Colorado Plateau of eastern Utah and western Colorado and the Basin Province of southern Wyoming.

The role of the particle size distribution in assessing aerosol composition effects on simulated droplet activation

Abstract. Variations in the chemical composition of atmospheric aerosols alter their hygroscopicity and can lead to changes in the cloud-active fraction of the aerosols, or cloud condensation nuclei (CCN) number concentration. To investigate the importance of this effect under different atmospheric conditions, cloud droplet formation was simulated with a Lagrangian parcel model. Initial values of updraft speed and temperature were systematically varied along with aerosol number concentration, size and hygroscopicity (represented by the hygroscopicity parameter, κ). A previous study classifies the sensitivity of CCN activity to compositional changes based on the supersaturation reached in the parcel model. We found that these classifications could not be generalized to a range of aerosol size distribution median radii. Instead, variations in sensitivity with size depend on the location of the dry critical radius for droplet activation relative to the size distribution median radius. The parcel model output was used to construct droplet activation lookup tables based on κ that were implemented in the Regional Atmospheric Modeling System (RAMS) microphysical scheme. As a first application of this system, aerosol hygroscopicity and size were varied in a series of RAMS mesoscale simulations designed to investigate the sensitivity of a mixed-phase orographic cloud case to the parameter variations. Observations from a recent field campaign in northwestern Colorado provided the basis for the aerosol field initializations. Model results show moderate sensitivity in the distribution of total case precipitation to extreme changes in κ, and minimal sensitivity to observed changes in estimated κ. The impact of varying aerosol hygroscopicity diminished with increasing median radius, as expected from the parcel model results. The conclusions drawn from these simulations could simplify similar research in other cloud regimes by defining the need, or lack of need, for detailed knowledge of aerosol composition.

The pollen record of a 20th century spruce beetle ( Dendroctonus rufipennis) outbreak in a Colorado subalpine forest, USA

The frequency and intensity of ecosystem disturbance, including outbreaks of forest insects and forest fires, is expected to increase in the future as a result of higher temperatures and prolonged drought. While many studies have concentrated on the future climatic impacts on fire, little is known about the impact of future climate on insect infestation. Paleoecological techniques are important in this regard in identifying the potential relationships between climate and insect outbreaks in the past, as a predictive tool for the future. We examine a high-resolution 20th century record of spruce beetle ( Dendroctonus rufipennis) infestation from a small, subalpine lake, comparing the paleoecological record to the historical and tree-ring record of the event. An extensive spruce beetle outbreak occurred in northwestern Colorado during the 1940s and 1950s, causing widespread mortality of mature Picea engelmannii. Pollen analysis of this period documents the decline of Picea and its replacement locally by Abies lasiocarpa, paralleling age and composition studies of modern forest stands in the region. This study is a proof of concept that, when applied to longer sedimentary records, could produce a detailed record of infestation for the Late Holocene or older time periods. This information will be useful to forest managers in efforts to plan for the effects of D. rufipennis infestations, and subsequent succession within high elevation conifer forests.

Multicriteria GIS modeling of wind and solar farms in Colorado

The majority of electricity and heat in Colorado comes from coal and natural gas; however, renewable energy sources will play an integral role in the state’s energy future. Colorado is the 11th windiest state and has more than 250 sunny days per year. The objectives of this research are to: 1) determine which landcover classes are affiliated with high wind and solar potential; and 2) identify areas that are suitable for wind and solar farms using multicriteria GIS modelling techniques. Renewable potential (NREL wind speed measurements at 50 m above the ground and NREL annual insolation data), landcover, population density, federal lands, and distance to roads, transmission lines, and cities were reclassified according to their suitability. Each was assigned weights based on their relative importance to one another. Superb wind classes are located in high alpine areas. Unfortunately, these areas are not suitable for large-scale wind farm development due to their inaccessibility and location within a sensitive ecosystem. Federal lands have low wind potential. According to the GIS model, ideal areas for wind farm development are located in northeastern Colorado. About 41 850 km 2 of the state has model scores that are in the 90–100% range. Although annual solar radiation varies slightly, inter-mountain areas receive the most insolation. As far as federal lands, Indian reservations have the greatest solar input. The GIS model indicates that ideal areas for solar development are located in northwestern Colorado and east of Denver. Only 191 km 2 of the state had model scores that were in the 90–100% range. These results suggest that the variables used in this analysis have more of an effect at eliminating non-suitable areas for large-scale solar farms; a greater area exists for suitable wind farms. However, given the statewide high insolation values with minimal variance, solar projects may be better suited for small-scale residential or commercial projects.

Raptor Electrocution Rates for a Utility in the Intermountain Western United States

ABSTRACT We estimated electrocution rates for raptors and common ravens (Corvus corax) for the Moon Lake Electrical Association in northeastern Utah and northwestern Colorado, USA. From July 2001 to May 2003, we conducted mortality searches at randomly selected distribution line segments and poles within 3 regions, but rate estimates (0.0036–0.0112 deaths/pole/yr) may have been biased by the effects of scavengers and by long sampling intervals (≥ 3 months), which prevented us from determining the cause of death for most birds because of advanced decay. In 2002–2003, we conducted carcass removal experiments in the Rangely Oil Field (ROF) in northwestern Colorado to estimate scavenging effects, and in 2003–2004, we reduced sampling intervals to 1 month and searched for dead birds at all distribution poles in the ROF. The shorter sampling interval nearly tripled the number of birds suitable for necropsy, but we were still unable to establish cause of death for >40% of our sample. Instead of eliminating the unknowns from rate estimates, we estimated minimum annual electrocution rates using only confirmed electrocutions and maximum annual electrocution rates based on all available mortalities, including mortalities without known causes. Golden eagles (Aquila chrysaetos) accounted for 63% of dead birds found in 2003–2004, but they were removed by scavengers at rates well below hawks and owls (6.8% vs. 55.6%). We compared maximum rates for the ROF in 2003–2004 with the rates estimated from a survey conducted at the same poles in 1999 to assess the effects of retrofitting conducted by Moon Lake from 1999 to 2003. Electrocution rates in 2003–2004 were 47% lower than those in 1999. Raptor densities in the ROF did not change during our study, suggesting the reduction was not the result of changes in raptor populations. However, estimates of raptor densities in 1999 were not available, and we cannot be sure that numbers of birds using the oil field in 1999 were similar to those in 2003–2004. Our research emphasizes the difficulties of estimating electrocution rates precisely but suggests that utilities will have the greatest effect on mortality by monitoring power lines at large scales and focusing subsequent mitigation efforts in areas that pose the greatest risk to the greatest number of birds.

Structural and tectonic evolution of the Douglas Creek arch, the Douglas Creek fault zone, and environs, northwestern Colorado and northeastern Utah: Implications for petroleum accumulation in the Piceance and Uinta basins

The Douglas Creek arch is a north–south-trending faulted anticline that separates the Uinta basin of northeastern Utah from the Piceance basin of northwestern Colorado. Previous work indicates that the arch initially formed during the Laramide orogeny as part of a broad, north–south-trending uplift that extended from northwest Colorado into southern Wyoming and included the Rock Springs uplift. The axis of this structure was offset sinistrally and truncated by the late-Laramide uplift of the Uinta Mountains. This study examines available geologic, structure, Bouguer gravity, aeromagnetic, seismic, and paleomagnetic data to investigate this late-Laramide history, as well as indications of younger, post-Laramide tectonic events that have shaped the Douglas Creek arch and environs. This study also uses the existing data to evaluate the genetic relationships between Precambrian basement structures and shallower structures formed in the sedimentary cover. Results of this study suggest that a major east–west-oriented structure, the Douglas Creek fault, likely has a Precambrian ancestry and was reactivated during the Phanerozoic. Structures in the study area are consistent with periodic sinistral slip, dominantly along the Douglas Creek fault, most recently during late-Laramide tectonic events. Northwest-striking fractures flanking the Douglas Creek arch and extending into the surrounding basins are likely synthetic strike-slip faults related to a subsequent period of dextral slip on the Douglas Creek fault. This deformation could be the result of the northwest translation of the Colorado Plateau and opening of the Rio Grande rift during post-Laramide Tertiary extension. Wrench faulting has created enhanced permeability and numerous structural traps for petroleum accumulation across the Douglas Creek arch and in the surrounding basins. Wrench structures are identified by their distinct geometries, and the origins of their individual features can be discerned using supporting subsurface data. Knowledge of the genesis of the wrench system allows for better understanding of wrench structures and thus a better potential for success in the search for oil and gas.

Tropospheric Microbes Are Surprisingly Diverse Yet Stable

Airborne microbial diversity is much greater than expected, albeit spare compared to that in the ocean and in the soil, according graduate student Robert M. Bowers, his advisor Noah Fierer, and their collaborators at the University of Colorado, Boulder, and elsewhere, who collected their data at Storm Peak lab in northwestern Colorado at an elevation of 3,200 meters. Moreover, the species of bacteria that they observe in the troposphere remain relatively stable over time, despite changing conditions. Further, ice nucleation—a precursor to precipitation—does not correlate with the local abundance of known ice-nucleating microbes. Details appear in the August Applied and Environmental Microbiology (75:5121– 5130).

Characterization of Airborne Microbial Communities at a High-Elevation Site and Their Potential To Act as Atmospheric Ice Nuclei

Bacteria and fungi are ubiquitous in the atmosphere. The diversity and abundance of airborne microbes may be strongly influenced by atmospheric conditions or even influence atmospheric conditions themselves by acting as ice nucleators. However, few comprehensive studies have described the diversity and dynamics of airborne bacteria and fungi based on culture-independent techniques. We document atmospheric microbial abundance, community composition, and ice nucleation at a high-elevation site in northwestern Colorado. We used a standard small-subunit rRNA gene Sanger sequencing approach for total microbial community analysis and a bacteria-specific 16S rRNA bar-coded pyrosequencing approach (4,864 sequences total). During the 2-week collection period, total microbial abundances were relatively constant, ranging from 9.6 x 10(5) to 6.6 x 10(6) cells m(-3) of air, and the diversity and composition of the airborne microbial communities were also relatively static. Bacteria and fungi were nearly equivalent, and members of the proteobacterial groups Burkholderiales and Moraxellaceae (particularly the genus Psychrobacter) were dominant. These taxa were not always the most abundant in freshly fallen snow samples collected at this site. Although there was minimal variability in microbial abundances and composition within the atmosphere, the number of biological ice nuclei increased significantly during periods of high relative humidity. However, these changes in ice nuclei numbers were not associated with changes in the relative abundances of the most commonly studied ice-nucleating bacteria.

Notice of release of ‘Continental’ basin wildrye

‘Continental’ basin wildrye (<i>Leymus cinereus</i> (Scribn. &amp; Merr.) A. Löve [Poaceae]) has been released as a cultivar for use in rangeland seedings. It was developed from a hybrid between an induced octoploid (2<i>n</i> = 56), generated from the natural tetraploid ‘Trailhead’ (2<i>n</i> = 28), and the natural octoploid ‘Magnar’ (2<i>n</i> = 56). Continental has shown similar or superior stand establishment to Trailhead and Magnar in evaluation trials in west-central Utah, northeastern Utah, southwestern Wyoming, and northwestern Colorado.

Eocene tortoise beetles from the Green River Formation in Colorado, U.S.A. (Coleoptera: Chrysomelidae: Cassidinae)

Abstract The fossil history of leaf beetles (Chrysomelidae) is relatively poorly documented despite an abundance of available material. Of particular interest is the origin and radiation of the diverse tortoise beetles, a derived group within Cassidinae s.l. (=Cassidinae + Hispinae) defined by the exophagous life history and specialized morphology of the immature stages. Cassidinae is also a group with relatively few fossil records that can be assigned with any degree of certainty. Here we report two of the oldest definitive tortoise beetle fossils, Eosacantha delocranioidesgen.n. et sp.n. and Denaeaspis chelonopsisgen.n. et sp.n., from the Eocene Green River Formation (ca. 47 million years old) in northwestern Colorado, U.S.A. Owing to the fine level of preservation, many important features can be observed and are coded into the recent cladistic analysis for the subfamily. Phylogenetic analysis highlights that both genera have affinities with modern lineages, one restricted to the Old World and the other restricted to the Neotropics. Although Cassidinae as a whole extend into the Cretaceous, the available information suggests that the tortoise beetles perhaps originated and diversified during the Early Tertiary. As such, the morphological and biological transitions from the leaf‐mining hispiforms to the distinctive tortoise‐like cassidiforms, with their elaborate defensive larval shields and other unique behaviours, probably took place during the latest Paleocene or earliest Eocene. These Green River fossils are the oldest yet to document the specialized morphology associated with the transition in cassidine feeding and immature biology.

Cableless system meets challenge of acquiring seismic to define subtle fractures in complex shale

Don Chitwood, John Tinnin, Cara Hollis and Fernando Hernandez provide a detailed description of a cableless land seismic operation carried out by ION Geophysical over Durham Ranch, a remote and environmentally sensitive area of northwestern Colorado.

New Mentzelias (Loasaceae) from the Intermountain Region of Western United States

Abstract Three new species and two new varieties of Mentzelia belonging to the multistemmed subshrub group of section Bartonia are described. Typical of the group, they are narrow endemics occurring in relatively barren clay soils in the Intermountain Region of interior western U.S. Mentzelia tiehmii and Mentzelia argillicola are from Nevada, Mentzelia memorabalis from northern Arizona, Mentzelia multicaulis variety uintahensis from northeastern Utah and northwestern Colorado, and Mentzelia multicaulis variety flumensevera from central Utah. Communicating Editor: Aaron Liston

The effect of drainage reorganization on paleoaltimetry studies: An example from the Paleogene Laramide foreland

Using multiple isotope systems, we examine the complex effects of drainage reorganization in the Laramide Foreland in the context of stable isotope paleoaltimetry. Strontium, oxygen and carbon isotopic data from lacustrine carbonates formed in the southwestern Uinta Basin, Utah between the Late Cretaceous and late Middle Eocene reveal a two stage expansion in the drainage basin of Lake Uinta beginning at ~ 53 Ma culminating in the Mahogany highstand at 48.6 Ma. A marked increase in 87Sr/ 86Sr ratios of samples from the Main Body of the Green River Formation is interpreted as the result of water overflowing the Greater Green River Basin in Wyoming and entering Lake Uinta from the east via the Piceance Creek Basin of northwestern Colorado. This large new source of water caused a rapid expansion of Lake Uinta and was accompanied by a significant and rapid increase in the O isotope record of carbonate samples by ~ 6‰. The periodic overspilling of Lake Gosiute probably became continuous at ~ 49 Ma, when the lake captured low- δ 18O water from the Challis and Absaroka Volcanic Fields to the north. However, evaporation in the Greater Green River and Piceance Creek Basins meant that the waters entering Lake Uinta were still enriched in 18O. By ~ 46 Ma, inflows from the Greater Green River Basin ceased, resulting in a lowstand of Lake Uinta and the deposition of bedded evaporites in the Saline Facies of the Green River Formation. We thus show that basin development and lake hydrology in the Laramide foreland were characterized by large-scale changes in Cordilleran drainage patterns, capable of confounding paleoaltimetry studies premised on too few isotopic systems, samples or localities. In the case of the North American Cordillera of the Paleogene, we further demonstrate the likelihood that (1) topographic evolution of distal source areas strongly influenced the isotopic records of intraforeland basins and (2) a pattern of drainage integration between the hinterland and foreland may correlate in space and time with the southward sweep of hinterland magmatism.

Modeling of gas generation from the Cameo coal zone in the Piceance Basin, Colorado

The gas generative potential of the Cretaceous Cameo coal in the Piceance Basin, northwestern Colorado, was evaluated quantitatively by sealed gold tube pyrolysis. The H/C and O/C elemental ratios show that pyrolyzed Cameo coal samples follow the Van Krevelen humic coal evolution pathway, reasonably simulating natural coal maturation. Kinetic parameters (activation energy and frequency factor) for gas generation and vitrinite reflectance (Ro) changes were calculated from pyrolysis data. Experimental Ro results from this study are not adequately predicted by published Ro kinetics and indicate the necessity of deriving basin-specific kinetic parameters when building predictive basin models. Using derived kinetics for Ro evolution and gas generation, basin modeling was completed for 57 wells across the Piceance Basin, which enabled the mapping of coal-rank and coalbed gas potential. Quantities of methane generated at approximately 1.2% Ro are about 300 standard cubic feet per ton (scf/ton) and more than 2500 scf/ton (in-situ dry-ash-free coal) at Ro values reaching 1.9%. Gases generated in both low- and high-maturity coals are less wet, whereas the wetter gas is expected where Ro is approximately 1.4–1.5%. As controlled by regional coal rank and net coal thickness, the largest in-place coalbed gas resources are located in the central part of the basin, where predicted volumes exceed 150 bcf/mi^2, excluding gases in tight sands.

The Dynamics of Land Development in Resort Communities: A Multiagent Simulation of Growth Regimes and Housing Choice

Over the past thirty years, recreation communities in many parts of the globe have gone through cycles of diversification and integration into complex recreation regions. As resort communities mature, they face increasing pressures on scarce recreational resources, demands for economic diversification, and changing attitudes toward tourism on the part of local residents. A variety of land-use management practices and economic development initiatives has emerged in resort towns in response to resource congestion and other growth issues. In this paper we explore alternative growth strategies through a simulation of housing decisions by primary actors in resort land markets. We use a multiagent system to model the dynamics of growth regimes, assess the influence of recreational and town amenities, and evaluate the effect of alternative growth processes on long-term development patterns. Our case study area is Steamboat Springs and surrounding parts of Routt County, a four-season recreational region in northwestern Colorado.

Structural and tectonic evolution of the Cherokee Ridge arch, south-central Wyoming: Implications for recurring strike-slip along the Cheyenne Belt suture zone

The Cherokee Ridge arch is an east–west-trending structure that separates the Washakie Basin of south-central Wyoming from the Sand Wash Basin of northwestern Colorado. It is characterized by a distinct east–west-trending lineament on Landsat images and a zone of northwest–southeast-striking, en echelon, high-angle faults that cut Upper Cretaceous to Miocene sedimentary rocks. The lineament is interpreted to be the surface expression of the Cheyenne belt, a buried suture zone that separates Proterozoic crust of the Colorado province from Archean crust of the Wyoming province. Results of this study indicate that the surface and near-surface structures along the Cherokee Ridge arch are the result of periodic strike-slip motion along the Cheyenne belt since Late Cretaceous time and possibly before. Previous structural and paleomagnetic studies support the conclusion that left-lateral transpressional motion occurred during the Late Cretaceous and early Tertiary as the area was under the contractional tectonic regime of the Laramide orogeny. The left-lateral shear zone is interpreted to be a lateral ramp accommodating west-directed thrusting that culminated in the formation of the Rock Springs uplift as the Colorado Plateau rotated in a clockwise direction at the end of the Laramide orogeny. A significant amount of vertical reverse displacement would also have occurred at this time along the south-dipping shear zone, giving the arch its present-day structural relief. This interpretation is also consistent with uplift of the Uinta Mountains during the Eocene. Right-lateral transtensional motion is interpreted to have taken place after the Miocene as the Colorado Plateau rotated to the west relative to the more stable Wyoming province during Tertiary extensional events. Northwest translation of the Colorado Plateau during Cenozoic extension is also supported by structural and paleomagnetic studies. Most surface structures observed along the arch are related to this most recent period of wrenching.