J. Hoover Mackin

CONCEPT OF THE GRADED RIVER

Grade is a condition of equilibrium in streams as agents of transportation. The validity of the concept has been questioned, but it is indispensable in any genetic study of fluvial erosional features and deposits. This paper modifies and extends the theory of grade originally set forth by Gilbert and Davis. A graded stream is one in which, over a period of years, slope is delicately adjusted to provide, with available discharge and the prevailing channel characteristics, just the velocity required for transportation of all of the load supplied from above. Slope usually decreases in a downvalley direction, but because discharge, channel characteristics, and load do not vary systematically along the stream, the graded profile is not a simple mathematical curve. Corrasive power and bed rock resistance to corrasion determine the slope of the ungraded profile, but have no direct influence on the graded profile. Chiefly because of a difference in rate of downvalley decrease in caliber of load, the aggrading profile differs in form from the graded profile; the aggrading profile is, and the graded profile is not, asymptotic with respect to a horizontal line passing through base level. It is critical in any analysis of stream profiles to recognize the difference in slope-controlling factors in parts of the overall profile that are (1) graded, (2) ungraded, and (3) aggrading. A graded stream responds to a change in conditions in accordance with Le Chatelier9s general law:—“if a stress is brought to bear on a system in equilibrium, a reaction occurs, displacing the equilibrium in a direction that tends to absorb the effect of the stress.” Readjustment is effected primarily by appropriate modification of slope by upbuilding or downcutting, and only to a minor extent or not at all by concomitant changes in channel characteristics. Paired examples illustrate (1) the almost telegraphic rapidity with which the first phases of the reaction of a graded stream to a number of artificial changes are propagated upvalley and downvalley and, (2) the more or less complete readjustment that is effected over a period of thousands of years to analogous natural changes. The engineer is necessarily concerned chiefly with short-term and quantitative aspects of the reaction of a graded stream to changes in control, while the attention of the geologist is usually focused on the long-term and genetic aspects of the stream9s response to changes. But the basic problems are the same, and a pooling of ideas and data may enable the engineer to improve his long range planning of river control measures and permit the geologist to interpret, in quantitative terms, the deposits of ancient streams.

Erosional history of the Big Horn Basin, Wyoming

GENERAL SETTING The Big Horn Basin is an elliptical lowland, limited on the west by the Absaroka and Beartooth units of the Rocky Mountain system, and on the south, east, and northeast by the crescentic arch of the Owl Creek, Big Horn, and Pryor Mountains (Fig. 1). The geomorphic history of the basin is divided into two, more or less distinct periods, the first characterized by relative uplift of the surrounding mountain ranges and by widespread fluvial aggradation of the resulting structural depression, and the second by production of the present topographic lowland through partial re-excavation of the Basin fill. The field work on which this paper is based was confined almost entirely to the broad, gravel-capped terraces and interstream benches formed during the second period by degrading streams, and the greater part of the paper is devoted to an analysis of the manner of development and the geological . . .

Origin of Lunar Maria

Lunar maria are not simply large lunar craters. Most craters have raised rims; most maria are bordered partly or wholly by broad zones of downwarping. A satisfactory theory for the origin of maria must provide a genetic link between the origin of the depression and the origin of the material which floors it. This paper advances the concept that the maria are the result of giant meteorite impacts with resultant magma formation, circumferential slumping, nuee-ardente eruption, and pooling in topographic low areas. Slumped rims of craters occur on craters with a diameter of 20 ± 5 km or larger. Terrestrial slumps can be shown to be caused by removal of lateral support and movement of the mass initiated by yielding near the base of the free face. Accepting the theory that most lunar craters were formed by impact, then craters over 20 ± 5 km in diameter, but not smaller, penetrated to depths where temperatures were such as to permit the rock to deform rapidly in the lower part of the crater wall. Slumps thus may provide a clue to the thermal gradient in the outer part of the Moon at the time of slumping. Craters 100 km or more in diameter appear to have slumped rims of such magnitude that magma formation and flow is indicated. Frothing of the magma (formation of nuees ardentes) in the low gravity and hard vacuum on the Moon seems likely, as has been demonstrated by laboratory experiments. Mare-scale eruptions indicate ignimbrites of thousands of cubic kilometers (same order of size as the largest terrestrial ignimbrites) and slumping of the St. Lawrence type. This concept implies that the typical mare is floored by a stratigraphic sequence consisting of: (a) the fall-back breccia and partly molten material of the first eruptive phase, (b) an ignimbrite representing the normal nuee-ardente phase of the same eruption, (c) ignimbrites representing younger eruptions elsewhere on the Moon, and (d) minor lava flows. Lunar ignimbrites explain differences in elevation of adjacent flat regions separated by mountains or crater rims; eliminate the necessity of individual feeders to furnish molten material to floor each crater as well as the surrounding plains by the same age and type of material; and allow the preservation, with reduced relief, of pre-existing topography (“ghost craters” and other compaction features common in ignimbrite terrains).

STRUCTURE OF THE GLENARM SERIES IN CHESTER COUNTY, PENNSYLVANIA

The Woodville structure is a hook-shaped belt of Baltimore Gneiss surrounded by Glenarm rocks. The hypothesis that the structure is a nappe, rather than an upwarp involving two gneiss-cored anticlines, is based primarily on the orientation of B lineation in the Glenarm rocks, taken in conjunction with the map pattern; the southwestward plunge of the Glenarm rocks under the gneiss along the northeastern side of the hook indicates that this segment of the border of the gneiss is the inverted limb of a recumbent fold.

An Occurrence of "Cave Pearls" in a Mine in Idaho

Pisolites up to 13 mm. in diameter occur in depressions in a veneer of calcium carbonate which mantles rubble in an abandoned mine. They were formed by precipitation from mine water, and the high polish that characterizes some specimens is due to agitation caused by dripping water. The period of formation is 35-42 years.

The Problem of the Martic Overthrust and the Age of the Glenarm Series in Southeastern Pennsylvania

The Glenarm series of schists, marbles, and quartzites of the Atlantic Piedmont has been assigned to the pre-Cambrian in numerous recent papers. The apparent superposition of these rocks on strata of known Paleozoic age near the northwestern margin of the Piedmont province is explained by the Martic low-angle overthrust fault. It is the purpose of this paper to reopen the Glenarm-Martic problem by presenting evidence that suggests, first, that the Glenarm Series in southeastern Pennsylvania may be wholly or in part Paleozoic, and, second, that the Martic overthrust does not exist.