Friday, April 11, 2014

Friday Rocks #8: Eureka Valley Sand Dunes

Photo by Tim Sherry
Wow, almost forgot about today's Friday Rocks! because thesisjail, so this one is quick.

The Eureka Valley Sand Dunes in Eastern California are "booming" sand dunes. Under the right humidity conditions and if enough sand avalanches downslope, soundwaves from the falling sand are amplified and making booming sound.

Friday, April 4, 2014

Friday Rocks 7: Dunite with Chromite

Photo by Timothy Sherry

Dunite is an ultramafic igneous rock often found in Ophiolite sequences. The silver-grey layers in the brown dunite are deposits of chromite. Lenscap diameter is 72 mm. Eastern Quebec.

Follow me and all the Friday Rocks on Instagram.

Friday, March 28, 2014

Friday Rocks 6: Sierra Nevada Roof Pendant

Photo by Tim Sherry
A roof pendant is a unit of country rock that sits on top of an intrusive igneous body. Here the dark rock at the top of the ridge was baked in a process called contact metamorphism by the Sierra Nevada igneous batholith. I couldn't find an exact name of this particular roof pendant. If you can name it, let us know in the comments!

Friday, March 21, 2014

Friday Rocks 5: Baby Duplex

photo by Tim Sherry

Here we have a thin sandy-mudstone bed (~20 cm thick, sorry for no scale) at MontaƱa de Oro State Park, California. The two diagonal leaning fractures are small thrust faults that bound a block of the original bed creating what geologists call a "horse". On the left side of the photo a small backthrust is present. The faults are not continuous across the siltstone beds above and below this layer. The sedimentary bedding in this area is typically planar, but some flexure is present as seen in the photo below.

photo by Tim Sherry
Here the siltstone is deformed by the sandy-mudstone load casts (thanks Greg!).

Friday, March 14, 2014

Friday Rocks 4: Stylolites in Limestone

Photo by Tim Sherry
The thin red-brown wavy band in this limestone is a stylolite. Stylolites are formed via a process called pressure solution and is the result of a volume decrease in the rock during diagenesis. Material, usually in the presence of a fluid, is dissolved and transported away, leaving a wavy band of material. Here the stylolite is roughly parallel to horizontal laminae in the limestone. Photo taken in the Poleta Fold Belt, Eastern California.