Surface uplift and climate change - the geomorphic evolution of the western escarpment of the Andes of northern Chile between the Miocene and present

The Western Escarpment of the Andes at 18.30°S (Arica area, northern Chile) is a classical example for a transient state in landscape evolution. This part of the Andes is characterized by the presence of >10,000 km2 plains that formed between the Miocene and the present, and >1500 m deeply incised valleys. Although processes in these valleys scale the rates of landscape evolution, determinations of ages of incision, and more importantly, interpretations of possible controls on valley formation have been controversial. This paper uses morphometric data and observations, stratigraphic information, and estimates of sediment yields for the time interval between ca. 7.5 Ma and present to illustrate that the formation of these valleys was driven by two probably unrelated components. The first component is a phase of base-level lowering with magnitudes of∼300–500 m in the Coastal Cordillera. This period of base-level change in the Arica area, that started at ca. 7.5 Ma according to stratigraphic data, caused the trunk streams to dissect headward into the plains. The headward erosion interpretation is based on the presence of well-defined knickzones in stream profiles and the decrease in valley widths from the coast toward these knickzones. The second component is a change in paleoclimate. This interpretation is based on (1) the increase in the size of the largest alluvial boulders (from dm to m scale) with distal sources during the last 7.5 m.y., and (2) the calculated increase in minimum fluvial incision rates of ∼0.2 mm/yr between ca. 7.5 Ma and 3 Ma to ∼0.3 mm/yr subsequently. These trends suggest an increase in effective water discharge for systems sourced in the Western Cordillera (distal source). During the same time, however, valleys with headwaters in the coastal region (local source) lack any evidence of fluvial incision. This implies that the Coastal Cordillera became hyperarid sometime after 7.5 Ma. Furthermore, between 7.5 Ma and present, the sediment yields have been consistently higher in the catchments with distal sources (∼15 m/m.y.) than in the headwaters of rivers with local sources (<7 m/m.y.). The positive correlation between sediment yields and the altitude of the headwaters (distal versus local sources) seems to reflect the effect of orographic precipitation on surface erosion. It appears that base-level change in the coastal region, in combination with an increase in the orographic effect of precipitation, has controlled the topographic evolution of the northern Chilean Andes.

A Study of Montana Bentonite

Research has been undertaken both under Government and private auspices in an endeavor to develop uses for bentonite. Perhaps, the work done to date has had only in consideration the possible industrial importance of bentonite. No simple, quick methods for the determination of the properties or qualities of any particular bentonite have been developed. In an attempt to establish whether or not there is a means of making rapid simple determinations of the quality of Montana bentonitic clays, and in particular, with regard to the uses to which the clays may be suited. The problem also involves a study of Montana bentonite, and a comparison of it with the standard accepted bentonites and fullers earth on the market today.

Geology and Ore Deposits of the Butte-Highland Gold Mine.

The Butte-Highland mine is situated at the head of Basin Creek, in the Highland mining district, Silver Bow County, about 14 miles south of Butte. The tunnel portal and present surface plant are at an elevation of about 7350 feet above sea level, facing westward across the head of Basin Creek valley. The "ghost" mining town of Highland lies a mile to the east, near the forks of Fish Creek. Access to the mine is obtained at present from Beaudine's siding, 12 miles west. The property may also be reached, with difficulty, over poor roads from Limekiln hill, or from Moose Creek.

The General Geology of the Cardwell Mining District

The Cardwell Mining District is part of the greater Whitehall Mining District. The district is situated about four miles to the east and northeast of Whitehall in the southern end of the Bull Mountains which are near the Continental Divide. The first reported production was in 1896 after the dis­covery of the Mayflower Mine. Mining has been carried on in­termittently and on a small scale since that time.

The Geology of Lost Creek Canyon with Special Reference to Igneous Activity

This investigation was undertaken primarily as a problem in geologic mapping, coupled with a study of stratigraphy, glaciation, igneous phenomena, and structure. The area is admirably suited to a study of geology and geologic events. Because it is small in extent, the area was studied in some detail during the time which was devoted to field work. The record of igneous activity of past geological ages is re­markably well exposed, since Lost Creek Canyon was carved through the roof of a stock or batholith by the glaciers of the Pleistocene epoch.

Geology of some Gold Deposits of Montana

Gold is one of the rarer metals in nature, and chemically it is one of the most inactive. Gold forms stable, natural compounds with few other elements, and only with metals.

Geology and Ore Deposits of the Golden Era and Goldfinch Mines, Argenta Mining District, Montana.

This report includes the results of geological investigation of a small area in the northern part of the Argenta mining district. Approximately two square miles were mapped. The underground working of the three mines only were accessible: the Goldfinch. Golden Era, and Mayday mines.

Geology and Ore Deposits of the Golden Messenger MIne, Lewis and Clark County, Helena, Montana.

The Golden Messenger Mine which is approximately twenty-three miles northeast of Helena, Montana, near York, on Trout Creek, has long presented several problems of both theoretical and practical interest.

The Geology of the Southern Central Portion of the Morrison Cave Area.

Morrison Cave is located about 50 miles southeast of Butte, Montana. It was named after the man who discovered it. Later it was taken over by the State and renamed Morrison Cave State Park. Recently the government with the aid of the Civilian Conservation Corps has built a new road to the cave and has made the interior more accessible. The name of the cave is now Lewis and Clark Cavern National Monument.

The Geology and Ore Deposits of Jardine, Montana

The subject to be covered by this paper is based upon field study made during a six week stay at Jardine. The work began on June 19, 1937 and ended on July 31 of the same year.

Geology of a Portion of the Bull Mountain Range Jefferson County, Montana

Work was first done on a known section, the south Boulder Section, in order to familiarize the student with the formations. Most of the area was mapped by plane table and telescopic alidade, general features being surveyed by automobile traverse and a pacing traverse.

Geology of the Golden Sunlight Mine and Vicinity

The Golden Sunlight Mine is in the northern part of the Cardwell mining district on the eastern slope of a small range that rises by a series of benches to an elevation of 7,200 at a point five miles east of the town of Whitehall in Jefferson County.

The Geology and Mineralogy of the Independence Mining District

From 1888 until 1893 the district was a booming mining camp. Seven stamp mills were running and ore was be­ing mined from the Independence, King Solomon, Poorman, Golden, Hidden Treasure and Crown claims. The town of Independence, which was on the Boulder River at the fork of Basin Creek, boasted a population of 400 people.