Interpreting What is Rural and Urban for Western U.S. Counties

The paper examines how rural and urban are interpreted and applied to counties in eleven states in the western United States. After reviewing various conceptual approaches, we turn to a three‐part analysis of county commissioners' perceptions and census data to identify characteristics associated with urban‐ness and rurality. The analysis involves comparing qualitative and quantitative survey and interview data, using a multiple regression analysis to correlate census variables with commissioners' perceptions of their home counties, and using cluster analysis techniques on census variables to identify patterns and unevenness in rurality and urbanness. Three characteristics mentioned by the widest range of commissioners and found to be statistically significant in the regression analysis were population concentration, total population, and the agricultural land base. Population concentration, in particular, was identified most frequently by interviewees as the single most important variable in characterizing an urban county and had the most meaningful contribution to predicting commissioners' perceptions of their home counties in the regression analysis. The cluster analysis identified five county types: largest urban centers, growing regional hubs, high growth rural, dispersed rural, and stable rural agriculture. These county types were widely distributed, reflecting the spatial unevenness of macroscale processes operating across eleven western states.     

Geomorphology, Stratigraphy, and Radiocarbon Chronology of LlanquihueDrift in the Area of the Southern Lake District, Seno Reloncaví, and Isla Grande de Chiloé, Chile

Glacial geomorphologic features composed of (or cut into) Llanquihue drift delineate former Andean piedmont glaciers in the region of the southern Chilean Lake District, Seno Reloncav', Golfo de Ancud, and northern Golfo Corcovado during the last glaciation. These landforms include extensive moraine belts, main and subsidiary outwash plains, kame terraces, and meltwater spillways. Numerous radiocarbon dates document Andean ice advances into the moraine belts during the last glacial maximum (LGM) at 29,363–29,385 ¹⁴C yr BP , 26,797 ¹⁴C yr BP , 22,295–22,570 ¹⁴C yr BP , and 14,805–14,869 ¹⁴C yr BP . Advances may also have culminated at close to 21,000 ¹⁴C yr BP , shortly before 17,800 ¹⁴C yr BP , and shortly before 15,730 ¹⁴C yr BP . The maximum at 22,295–22,567 ¹⁴C yr BP was probably the most extensive of the LGM in the northern part of the field area, whereas that at 14,805–14,869 ¹⁴C yr BP was the most extensive in the southern part. Snowline depression during these maxima was about 1000 m. Andean piedmont glaciers did not advance into the outer Llanquihue moraine belts during the portion of middle Llanquihue time between 29,385 ¹⁴C yr BP and more than 39,660 ¹⁴C yr BP . In the southern part of the field area, the Golfo de Ancud lobe, as well as the Golfo Corcovado lobe, achieved a maximum at the outermost Llanquihue moraine prior to 49,892 ¹⁴C yr BP . Pollen analysis of the Taiquemmire, which is located on this moraine, suggests that the old Llanquihue advance probably corresponds to the time of marine isotope stage 4. The implication is that the Andean snowline was then depressed as much as during the LGM. A Llanquihue-age glacier expansion into the outer moraine belts also occurred more than about 40,000 ¹⁴C yr BP for the Lago Llanquihue piedmont glacier.     

A comparison of archaeointensity results from Chinese ceramics using microwave and conventional Thellier's and Shaw's methods

Palaeo- and archaeointensity determinations of the geomagnetic field are limited by mineralogical alteration that may occur when samples are heated in the laboratory to produce a TRM ( Thellier & Thellier 1959 ; Shaw 1974 ). By using microwave excitation of the magnetic grains we have been able to create a TRM without significantly heating the bulk samples, thereby avoiding thermal alteration ( Walton et al . 1993 ; Shaw et al . 1996 ). When applied to Peruvian ceramics in a manner analogous to the standard Thellier method, the microwave technique was found to reduce the scatter dramatically in the archaeointensity results ( Shaw et al . 1996 ). In the present study we apply this microwave technique to a collection of Chinese ceramics covering the time interval 2700–7500  yr BP. These ceramics have already been investigated using adaptations of both Thellier's ( Coe 1967 ) and Shaw's ( Rolph & Shaw 1985 ) palaeointensity methods ( Yang et al . 1993a ). Although an acceptable agreement was found between those two methods, the equivalent virtual axial dipole moments (VADMs) were significantly lower than for the global model of McElhinny & Senanayake (1982 ). The present study, using the microwave technique with cooling-rate correction, has provided more consistent VADMs, and is much closer to the global model.     

Seasonal variation of the soil seed bank of grasses in central Argentina as related to grazing and shrub cover

The objectives of this research were to (a) study seasonal variation of the seed bank of grasses in two areas with different grazing history, (b) analyse the effect of the presence of shrubs on the seed bank and (c) study seed viability and dormancy in buried seeds of Piptochaetium napostaense and Stipa tenuis. The seed density of grasses was in general low and showed seasonal variation. Most of the grasses showed maximum seed density in December when seed dispersal occurs. Shrub cover did not show a marked influence in seed accumulation. The relatively high number of damaged seeds suggests that predation may be an important factor in seed bank dynamics in the Caldenal. Seeds of dominant perennial forage grasses showed dormancy that could explain, at least in part, persistence of these species in spite of periodical disturbance.     

The effects of post-seismic motions on the moment of inertia of a stratified viscoelastic earth with an asthenosphere

Summary. We give the analytical formulation for calculating the transient displacement of fields produced by earthquakes in a stratified, selfgravitating, incompressible, viscoelastic earth. We have evaluated the potential of viscous creep in the asthenosphere in exciting the Chandler wobble by a four-layer model consisting of an elastic lithosphere, a two-layer Maxwell viscoelastic mantle, and an inviscid core. The seismic source is modelled as an inhomogeneous boundary condition, which involves a jump condition of the displacement fields across the fault in the lithosphere. The response fields are derived from the solution of a two-point boundary value problem, using analytical propagator matrices in the Laplace-transformed domain. Transient flows produced by post-seismic rebound are found to be confined within the asthenosphere for local viscosity values less than 10²⁰P. The viscosity of the mantle below the low-viscosity channel is kept at 10²²P. For low-viscosity zones with widths greater than about 100 km and asthenospheric viscosities less than 10¹⁸P, we find that viscoelasticity can amplify the perturbations in the moment of inertia by a factor of 4–5 above the elastic contribution within the time span of the wobble period. We have carried out a comparative study on the changes of the inertia tensor from forcings due to surface loading and to faulting. In general the global responses from faulting are found to be much more sensitive to the viscosity structure of the asthenosphere than those produced from surface loading.     

Palaeomagnetic study of the (Lower Cambrian) Longmyndian sediments and tuffs, Welsh Borderlands

Summary. This study covers detailed thermal, chemical and alternating field demagnetization from 50 sites distributed through all units of the Longmyndian sedimentary succession (with the exception of the Stretton Shales), and eight sites in the tuff bands comprising the Batch Volcanics. The resultant effect of treatment is to remove a low blocking temperature steep component and isolate a coherent remanence direction D = 116°, I = 76° the total NRM is composite and not an adequate indicator of the palaeofield. Chemical demagnetization indicates that both hematite and magnetite grains contribute to this component and these two phases are magnetized in the same direction. A negative fold test shows that this remanence was acquired during or after the Longmyndian folding. Formation of the major magnetic constituent, authigenic magnetite, is linked to dewatering during rapid uplift following the folding which is dated by both the Rb–Sr and fission track methods at c . 525 Ma. The study defines a palaeopole of this age remote from the later APW path for Britain and links the Late Precambrian–Lower Cambrian path defmed from basement rocks of England and Wales with the Ordovician and younger results. Palaeomagnetic results from tuff bands within the sediments and Lower Silurian age intrusions cutting the outcrop are also reported.