The Mogollon Railroad

[nachoman]

while waiting for glue to dry on TrainNut's caboose, I have been putting rock castings on a portion of my layout. Having a geology degree, it should be fitting that my rocks be more than just crinkly plaster. But, I am also impatient and on a tight budget. I want the geology of my layout to depict two different types of rock units, and I decided since my layout is kinda one big ridge, I will have the different rock units occupying opposite sides of the layout.

Geologists joke that they are either "hard rockers" or "soft rockers". Hard rocks would be crystalline basement rocks such as granite, schist, and well cemented sedimentary rocks. Soft rocks are weak sedimentary units, dirt, gravel, and evaporates. My layout represents a copper mining town on the edge of a mountain range in southern Arizona. Hard rocks make up the range and the ore body, whereas soft rocks make up the adjacent basin and much of my railroad's right of way. This is typical of the Basin and Range geologic area that encompasses much of the western USA between the Sierra Nevada and Wasatch mountains, and from the northern part of Nevada sown to the southern border of Arizona and into Mexico.



Anyone who has visited the area recognizes the high, steep mountains rising up from low, flat valleys. A few tens of millions of years ago, Western North America was stretched. Normal faults developed and basins dropped down between rising mountain ranges. The basins filled with sediment derived from the ranges. As drainage networks developed, the basin sediments were further eroded, leaving behind ridges of this loosely cemented sediment. This sediment is often exposed as conglomerate is common in western basins. Local Phoenix area examples examples include the head of Camelback mountain and Papago buttes in phoenix.



I want one half of my layout to represent the hard ore-bearing rocks, and the other half to represent a basin conglomerate. I haven't decided how to make the conglomerate, but I went ahead and started making the hard rocks using standard plaster-in-molds techniques. They won't me truly representative of the type of rocks I am trying to model, but they will be quick and look good enough.