SALT RIVER SITE

Plane polarized light is just visible light under the microscope. Crossed (X) polarized light allows you to see mineral characteristics such as twinning in the feldspar and alteration. At 2.5X magnification, the distance across the slide is 4.8 mm and at 10 X magnification it is about 1.2 mm.

190172: This sherd has a slightly reduced (gray black) core region and what appears to be either a very thin slip or differential grain distribution on the outside of the sherd. Composed of very fine to fine grain quartz and feldspar (no twinning) with <1% mafic mineral grains.

2.5X magnification. Cross polarized light. Note the very fine grain size.

190204a: This sherd is composed of very fine to medium grain rock fragments, mineral grains and grog. Two types of rock fragments are found: edge rounded shale (of mudstone) composed of mainly subangular quartz grains and angular, igneous (quartz + k-spar + mafic mineral) fragments. Monocrystalline and polycrystalline quartz and sericitized (altered) plagioclase mineral grains range from very fine to medium in grain size. A few medium to fine grained shell fragments (probably mollusk or gastropod) and foraminifera tests (shells) are found. Lastly, this sample has several rounded, fine to medium grained grog fragments that show the characteristic deep red color as a result of being refired.

2.5X magnification. Plane polarized light.

10X magnification. Cross polarized light. Shell fragment (several, largest in lower right) and rounded shale (or mudstone) rock fragment (top center).

10X magnification. Cross polarized light. Angular rock fragment composed of quartz + feldspar + pyroxene) and rounded grog fragment (though could be a clay clot since does not have many inclusions of quartz).

190204b: This sherd has a dark brown-black color (reduced) and is dominated by coarse grained, angular igneous rock fragments (quartz + plagioclase + k-spar) with medium to fine grained plagioclase mineral grains and very little quartz. There are a few mafic mineral grains (~ 1%) and the texture of the these grains, as well as the quartz and feldspar suggest that they are derived from the breakdown of material such as the rock fragments found here.

2.5X magnification. Plane polarized light. Note the overall sherd colour and the amount of angular rock and mineral fragments.

2.5X magnification. Cross polarized light. Same sherd as above, note the twinning of the feldspars (both polysynthetic and Carlsbad twin laws).

191043: This sherd was formed in oxidizing conditions and is dominated by angular igneous rock fragments (quartz + feldspar) and rounded grog fragments which have angular quartz and feldspar mineral grains. Angular to subangular, coarse to fine grained quartz and feldspar mineral grains comprise the remainder of the temper.

2.5X magnification. Cross polarized light. Note the angular rock fragments and the rounded grog.

10X magnification. Plane polarized light. Rock fragment composed of quartz + feldspar.

10X magnification. Cross polarized light. Same rock fragment, note what appears to be Carlsbad twinning (and sericite alteration) on the feldspar.

10X magnification. Plane polarized light. Grog fragment (subrounded) with angular quartz and feldspar grains. Note the separation void that surrounds the grog and that the rock fragments (above) did not show this feature.