Sidney E. White

Rock glaciers and block fields, review and new data

Abstract Tongue-shaped and lobate rock glaciers are recognized in most alpine regions today. For the tongue-shaped, two situations emerge: those with buried glacier ice (debris-covered glaciers) called ice-cored rock glaciers, and those with interstitial ice known as ice-cemented rock glaciers. Those with ice cores are revealed by depressions between rock glacier and headwall cliff (where a former glacier melted), longitudinal marginal and central meandering furrows, and collapse pits. Ice-cemented rock glaciers ordinarily do not possess these features. As applied to 18 rock glaciers in the Colorado Front Range, 11 of 12 east of the Continental Divide are ice-cored, 6 west of the Divide are ice-cemented. The majority of lobate rock glaciers in the Colorado Front Range are on the south sides of valleys, and, except for talus, are the most voluminous form of mass wasting. All those active and above treeline have characteristics common to all rock glaciers. In addition, they originate from talus, contain interstitial ice, move outward from valley walls at 1–6 cm/yr, and transport more debris as a process of erosion than heretofore realized. Block fields and block slopes, in polar and alpine regions, are thin accumulations of angular to subrounded blocks, on bedrock, weathered rock, or transported debris. They extend along slopes parallel to the contour. Block streams are similar but extend downslope normal to the contour and into valleys. They are made of interlocked blocks without interstitial detritus, but many have finer material deeper inside. The fabric of surface blocks indicates that motion most likely occurred during a periglacial time when interstitial debris, now washed or piped out, permitted movement of the whole deposit.

Alpine mass movement forms (noncatastrophic) : classification, description, and significance.

A variety of noncatastrophic and distinctly slope, with the rare (in the alpine) block field, alpine mass movement forms may be classi- and block stream as subtypes. These mass fled under three basic terms: (1) talus, with movement forms reveal past and present local rockfall talus, alluvial talus, avalanche talus, and microclimates, rates of erosion, postglaavalanche boulder tongue, and protalus ram- cial valley wall, cliff, and ridge crest changes, part as subtypes; (2) rock glacier, subdivided and periglacial events during Neoglaciation. into tongue-shaped and lobate; and (3) block

Pleistocene glaciation of volcano Ajusco, central Mexico, and comparison with the standard Mexican glacial sequence

Abstract Three Pleistocene glaciations and two Holocene Neoglacial advances occurred on volcano Ajusco in central Mexico. Lateral moraines of the oldest glaciation, the Marques, above 3250 m are made of light-gray indurated till and are extensively modified by erosion. Below 3200 m the till is dark red, decomposed, and buried beneath volcanic colluvium and tephra. Very strongly to strongly developed soil profiles (Inceptisols) have formed in the Marques till and in overlying colluvia and tephra. Large sharp-crested moraines of the second glaciation, the Santo Tomas, above 3300 m are composed of pale-brown firm till and are somewhat eroded by gullies. Below 3250 m the till is light reddish brown, cemented, and weathered. Less-strongly developed soil profiles (Inceptisols) have formed in the Santo Tomas till and in overlying colluvia and tephra. Narrow-crested moraines of yellowish-brown loose till of the third glaciation, the Albergue, are uneroded. Weakly developed soil profiles (Inceptisols) in the Albergue till have black ash in the upper horizon. Two small Neoglacial moraines of yellowish-brown bouldery till on the cirque floor of the largest valley support weakly developed soil profiles with only A and Cox horizons and no ash in the upper soil horizons. Radiocarbon dating of organic matter of the B horizons developed in tills, volcanic ash, and colluvial volcanic sand includes ages for both the soil-organic residue and the humic-acid fraction, with differences from 140 to 660 yr. The dating provides minimum ages of about 27,000 yr for the Marques glaciation and about 25,000 yr for the Santo Tomas glaciation. Dates for the overlying tephra indicate a complex volcanic history for at least another 15,000 yr. Comparison of the Ajusco glacial sequence with that on Iztaccihuatl to the east suggests that the Marques and Santo Tomas glaciations may be equivalent to the Diamantes glaciation First and Second advances, the Albergue to the Alcalican glaciations, and the Neoglacial to the Ayolotepito advances.

Quaternary glacial stratigraphy and chronology of Mexico

The volcano Iztaccihuatl in central Mexico was glaciated twice during the middle Pleistocene, once probably in pre-Illinoian (or pre-Bull Lake) time, and once in late Illinoian (or Bull Lake) time. Glaciation during the late Pleistocene was restricted to the late Wisconsin (or Pinedale). A maximum advance and one readvance are recorded in the early part, and one readvance in the latter part. Three or four small neoglacial advances occurred during the Holocene. Two other volcanoes nearby, Ajusco and Malinche, have a partial record of late Pleistocene and Holocene glaciations. Three others, Popocatepetl, Pico de Orizaba, and Nevado de Toluca, have a full Holocene record of three to five glacial advances during Neoglaciation.

Late Pleistocene Glacial Sequence for the West Side of Iztaccihuatl, Mexico

A sequence of late Pleistocene glacial deposits on the west side of the volcano Iztaccihuatl, Mexico, represents (oldest to youngest) the Tomicoxco, Diamantes, Alcalican, and Ayolotepito substages, determined by study of stratigraphy of lava and ash alternating with glacial deposits, comparisons of morainal topography and degree of weathering and of soil development, and similarity of sedimentary characteristics. Till-like sediments in older alluvial deposits along the west base of the mountain suggest an earlier glaciation; these extend down to an altitude of 2450 m. Pyroclastic sediments overlie these, and all are severely eroded, weathered, and support a thick yellow Podzolic soil. During the Tomicoxco Substage, of two distinct ice advances, moraines of Nexcualango Till were deposited across lower mountain slopes; outwash reached the Mexico basin and is the only outwash correlated with a former lake level (the highest) in the basin. Moraines on the mountain are eroded and support a strongly developed gray-brown Podzolic soil. Volcanic activity occurred intermittently high on the north end. A belt of striking moraines of Hueyatlaco Till representing the Diamantes Substage, showing evidence of an interval of erosion and of soil formation, was built higher across the mountain, covering in places a new flow. Lava inundated the upper ends of several moraines of this till, and all the region was mantled with ash and lapilli. The moraines have been eroded and bear a moderately developed soil. Glaciation during the Alcalican Substage left small moraines of Milpulco Till in the southwest valleys only. Black ash then fell on the mountain; a weak soil is developed on these latest deposits. During the Ayolotepito Substage, a final but minor episode of refrigeration, huge moraines of Ayoloco Till were built in the mouths of the valley heads. Since then the little glaciers in the valley heads have retreated, leaving three or four recessional moraines.

Alpine Subnival Boulder Pavements in Colorado Front Range

Boulder pavements on valley floors in or near stream channels, downstream from breaks in slope, beneath persistent snowbanks, and above treeline occur in four glaciated valleys east of the Continental Divide, Boulder District, Colorado Front Range. They are at altitudes of 3,230 to 3,620 m, triangular to rectangular in ground plan, slope a few degrees downvalley with nearly all blocks or boulders tightly packed with a flat face upward, and made of local bedrock or till. These alpine subnival boulder pavements may be due to the joint effort of frost action, meltwater saturation and removal of fine sediment, and rotation and flattening of blocks by rock weight, snow pressure, and possible snow creep.