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.     

Frozen Ground Phenomena in the Vicinity of Terra Nova Bay, Northern Victoria land, Antarctica: A Preliminary Report

Frozen ground phenomena in the Northern Foothills, Northern Victoria Land, Antarctica, include large–scale polygons, 15–20 m in diameter, and small frost mounds, 1–5 m high. The polygons are most widespread on terrain formed upon Younger Drift and are usually surrounded by interpolygon furrows or troughs, 10–30 cm deep and 10–100 cm wide. The troughs contain shallow wedges of sandy gravel (sand wedges) near the surface but excavations into underlying permafrost indicate that small ice wedges or ice veins are locally present. Field and anecdotal evidence suggest that thermal contraction cracking is active under today's climate. Frost mounds occur in association with a number of perennially frozen lakes in the region. In most cases they appear related to frost and icing blister activity caused by the episodic injection of free water from below. The debris–covered nature of the centre of Enigma Lake is best explained in terms of basal ice accretion beneath the lake–ice cover.     

上海淤泥质潮滩潮锋作用及其形成机制初步探讨*

本文具体讨论了上海淤泥质潮滩的潮锋作用过程,定性给出了潮锋的形成机制,认为潮锋作用及其水体高含沙量现象的本质在于“潮锋带水体水流加速效应”,即,由于潮滩坡面变化平缓的特殊性,涨潮水体前锋沿程存在着一持续时间可达数十分钟的水流加速过程.最后,本文初步探讨了潮锋作用在滩面塑造过程中的地位,并重新定义了潮锋作用的内容.     

Radiation from an impulsive line source in an unbounded homogeneous anisotropic medium

Summary. The space-time elastic wave motion generated by an impulsive line source in a homogeneous anisotropic medium is calculated with the aid of the Cagniard-de Hoop method. Two types of sources are considered in detail, viz. a line source of expansion (model for an explosive source) and a line force (model for a mechanical vibrator). Numerical results are presented for the radiated particle velocity in the medium including those regions of space where shear-wave triplication occurs. There is a marked difference in the time response observed for the two types of sources and for the different positions of the receiver with respect to the source position. These waveform differences are important when the radiated wave is used to determine experimentally the elastic properties of the medium. As compared with the traditional Fourier-integral transform method to handle this problem, the computation time with the present method is considerably less.     

Human and Environmental Factors Affect Patterns of Crop Diversity in an Ethiopian Highland Agroecosystem

The Ethiopian highlands are a center of diversity for numerous globally important crops. Heterogeneous landscapes, traditional agricultural practices, and inaccessibility have created and maintained diverse subsistence agroecosystems. We surveyed sixty-three farms in twelve communities of the Gamo highlands and found high levels of on-farm diversity of crop species and varieties. The extent and nature of this diversity are related to both ecological factors, namely elevation, and anthropogenic factors such as land-use history, accessibility to markets and extension, and strength and scale of farmer exchange networks. These human landscape features have the potential to both enhance and mediate effects of elevation and other environmental factors on crop diversity. The interaction of environmental and anthropogenic forces on the diversity of crops grown by farmers affects the strategies farmers employ to adapt their farming to changing conditions.     

ENVIRONMENTAL CONSEQUENCES OF HUMAN ACTIVITY ON THE YELLOW RIVER AND ITS DELTA,CHINA

The Yellow River is unique among the major rivers of the world because of its extremely heavy sediment load in relation to its small discharge. The Yellow River has changed courses numerous times during the past 5000 to 6000 years. With its large sediment load, most of which comes from the Loess Plateau of central China, the river has created a number of deltas along the coast of northern China. The abandoned deltas are eroding, whereas the present delta being formed in Bohai Bay is growing rapidly. Human activity, including extensive agriculture, heavy water consumption, the use of river-control structures, and war, has been responsible for many of the changes occurring in the Yellow River and its delta. [Key words: hydrology, loess, floods, Yellow River, Bohai Bay, China.]