A photo from Wright State Geological Sciences Circus

Thrust Fault

This a photo of a thrust fault in the Ordovician Selona Formation on the north side of US 322 north of State College, Pennsylvania. The fault occurs in the Nittany Mountain Syncline, which is a part of the Nittany Anticlinorium. In an interpreted image, I show the fault plane itself in yellow-green, and my interpretation of a correlated bed across the fault in magenta, which shows the throw on the fault. Sorry there is no rock hammer in the picture - I don't know what I was thinking.

Trace Fossils

These are photos of kinds of trace fossils, or ichnofossils. Ichnofossils are structures in sediment left behind by the burrowing activities of ancient organisms. They are given scientific names and are classified as if they are actually organisms, although two different organisms can make the same ichnofossil, and a single organism can be responsible for two different ichnofossils. The top photo is an ichnofossil called Ophiomorpha, which is in the sandstone of the Parkman Member of the Clagget Formation in Elk Basin on the Montana/Wyoming border, taken June 17, 1997. The next photo shows Diplocraterion, which is in the Keyser or Tonoloway Formation along rt. 30 in Bedford County, Pennsylvania (scale bar in centimeters). The last photo shows Skolithos (viewed either from the top or the bottom) in a boulder from the Devonian Catskill Formation from a quarry in Susquehanna County, Pennsylvania.


A cross-section of a bryozoan, cut, polished, etched, and scanned at high resolution. The specimen was found in a stream bed in Ohio, close to Cincinnati. It is probably Ordovician in age, and is probably of the genus Hallopora.


Dikes are roughly vertical sheets of rock that intrude into overlying rocks from below and crystallize or lithify there.

The top photo shows me next to a felsic dike in the Sacaton Mountains of Arizona, taken March 25, 1998. The Sacaton Mountains are inselbergs of granite in the Basin and Range physiographic province of the American West.

The second photo shows two diabase dikes that have intruded through limestone of the Ordovician Epler Formation and are now exposed in the wall of the Burkholder Quarry, operated by the Martin Limestone Company, in New Holland, Pennsylvania. See more pictures of these dikes here.

The third photo shows me next to a much larger diabase dike cutting through some limestone near Winkleman, Arizona. Here is a second view of the same dike. GPS 33.004017, 110.765158

The next photo shows the Triassic Stony Ridge Dike along the Pennsylvania Turnpike (MM 229.2), also diabase, cutting through the Ordovician St. Paul group of limestone. The next photo is a close-up of the contact between the diabase and limestone.

These dikes are igneous dikes, which form from liquid rock cooling in the place into which they have intruded. But there is another, much rarer kind, called a sedimentary or clastic dike, which forms when water trapped within the pore spaces of deeply buried sediment suddenly escapes from the sediment along faults or joints in the sediment, and the weight of the overlying sediment causes it to move rapidly upward, carrying sediment with it. When most of the pressure is released, the water stops moving and the sediment is left in place along the fault or joint, leaving a clastic dike. The second to last photo shows a clastic dike in the White River Badlands of South Dakota, taken on June 5, 1997, during field camp. The bottom photo shows a small clastic dike in the Chinle Formation in the Island In the Sky District of Canyonlands National Park, taken May 9, 2002.

Copyright 2015 James L. Stuby. All Rights Reserved.