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A unique circular feature of uncertain origin lies above the tree line on the eastern flanks of the Coast Range of British Columbia, Canada. It is composed of white, fractured, angular cobbles to boulders predominantly under 1 m across, arrayed in a slightly flattened circle nearly 50 m in diameter with the ring mostly about 4 m in width. The felsic granitoid lithology of the circle is unlike any in the immediate region, and no clasts of this composition occur within the circle. The debris rests on soliflucted soil containing rounded pebbles to cobbles of granodiorite that forms the regional lithology. The age of the circle is deglacial with post-glacial modification. Given the absence of similar lithologic units in the region and no obvious symbolic purpose for such a feature, an anthropogenic origin for the circle is improbable. Mechanisms for transporting the foreign rock unit to its final location, distributing fragments into a circle, and preserving it include flow of alpine and fringing continental ice masses, deglaciation, freeze-thaw cycles, and post-glacial solifluction and erosion. One large slab was either entrained within or fell onto a flowing glacier from some unknown outcrop at least 2 km from the site of the circle. During deglaciation, the insulating cap preserved the ice beneath it forming an isolated stagnant mass of ice. Freeze-thaw cycles likely affected the slab when it was on the surface of the ice, but it remained coherent, otherwise glacial flow or meltwater streams might have scattered loose clasts. Once the ice became stagnant, continued freeze-thaw cycles eventually created a rubble pile, which slid off the presumably symmetrical mass of ice to become arranged in a roughly circular ring. Having lost its protective cover, the ice melted and subsequent solifluction slightly modified the ring. [ABSTRACT FROM AUTHOR]

Functional structures from across the engineered and biological world combine rigid elements such as bones and columns with flexible ones such as cables, fibers, and membranes. These structures are known loosely as tensegrities, since these cable-like elements have the highly nonlinear property of supporting only extensile tension. Marginally rigid systems are of particular interest because the number of structural constraints permits both flexible deformation and the support of external loads. We present a model system in which tensegrity elements are added at random to a regular backbone. This system can be solved analytically via a directed graph theory, revealing a mechanical critical point generalizing that of Maxwell. We show that even the addition of a few cable-like elements fundamentally modifies the nature of this transition point, as well as the later transition to a fully rigid structure. Moreover, the tensegrity network displays a collective avalanche behavior, in which the addition of a single cable leads to the elimination of multiple floppy modes, a phenomenon that becomes dominant at the transition point. These phenomena have implications for systems with nonlinear mechanical constraints, from biopolymer networks to soft robots to jammed packings to origami sheets. [ABSTRACT FROM AUTHOR]

Collective cell migration is a highly regulated process involved in wound healing, cancer metastasis, and morphogenesis. Mechanical interactions among cells provide an important regulatory mechanism to coordinate such collective motion. Using a selfpropelled Voronoi (SPV) model that links cell mechanics to cell shape and cell motility, we formulate a generalized mechanical inference method to obtain the spatiotemporal distribution of cellular stresses from measured traction forces in motile tissues and show that such traction-based stresses match those calculated from instantaneous cell shapes. We additionally use stress information to characterize the rheological properties of the tissue. We identify a motility-induced swim stress that adds to the interaction stress to determine the global contractility or extensibility of epithelia. We further show that the temporal correlation of the interaction shear stress determines an effective viscosity of the tissue that diverges at the liquid-solid transition, suggesting the possibility of extracting rheological information directly from traction data. [ABSTRACT FROM AUTHOR]

The Madeira archipelago is situated in the Atlantic Ocean about 900 km south-west of Portugal and 700 km west of the Moroccan coast. The islands are volcanic and were produced by a large hotspot beneath the eastern Atlantic. The two main islands, the subject of this tour, are Madeira, the largest, and, 37 km to the north-east, Porto Santo. There are also the three Deserta islands 16 km to the south-east of Madeira which are uninhabited and are now a nature reserve. [ABSTRACT FROM AUTHOR]