Page 4 - sarahbrewer_ch1geology

Conglomerate: A sedimentary rock, in this case sandstone, which was formed with inclusions of other
rocks. These rocks can be sedimentary, igneous, or metamorphic, and can range in size from small
pebbles to large boulders. Notice in the Park that you will see small to large-sized granite rocks situated
within the sandstone layers. It’s interesting to think that these rocks are much older than the “matrix”,
or main sandstone rock around them- these rocks were already fully formed as the sandstone was just
being deposited!

An example of a conglomerate later located near the center of the park. The green arrow notes the conglomerate layer,
which contains various complete rocks, including small granite boulder. Note that this layer is largely intact and is of a
stronger material than the layers above and below it. For comparison, the yellow arrow notes a more fine-grained layer
that has eroded away more quickly due to its relative softness compared to the surrounding layers (see Differential
Erosion). Photograph by Sarah Brewer, 2012.

Desert Varnish: On the top of some rock outcrops are blackish deposits of clay minerals (with
manganese, which provides a black color, and iron which provides red), which have been weathered out
of the rock by the heat of the sun. Surfaces of pebbles, boulders, and rock outcroppings that are exposed
to the sun may become darkened by this film of oxidized chemicals. Colorful formations such as these
are called “desert varnish” because of their dark, paint-like appearance.

12