Dan Bailey's Geo Postcard

Greywacke and Shale Road Cut

The above photo is of a road cut along State Route 73 to the east of the Southern Alps. This road cut consists of greywackes and shales of the Torlesse Supergroup. These rocks are sedimentary rocks resulting from the lithification rock fragments and sand in a clay matrix that made up the original New Zealand geosyncline. This sedimentary rock was then subsequently uplifted, twisted and turned, and brought to the surface by the Rangitata and Kaikoura orogenies.

The rocks in this road cut showed several interesting sedimentation features. This rock outcrop showed graded bedding. The graded bedding arose from variable velocity underwater or marine mudflows that deposited sand, silt, and clays in layers that were graded from the coarsest/densest grains to the finest/lightest clay sediments. This mechanism of deposition gives rise to sole markings, where the densest/largest debris is dragged along the bottom of a flow gouging the freshly deposited mud layer below it. These gouges or scours are then infilled by coarser sediments, yielding a “cast” of the original marking. Once the sediments are lithified and the shale layer is eroded away, the sole mark becomes visible. The specific type of sole marking shown in the above photo is a symmetric grove cast, which can be used to determine the trend in flow direction.

The graded bedding can be seen with the dark fine clay grains (almost black in the lower right) above the lighter brown coarser sand grains (in the middle) with another dark clay layer below in the upper left. The prominent ridge seen in the coarse brown layer is a grove cast indicating the trend in direction of the mudflow that deposited these sediments. The whole rock formation was uplifted from its horizontal orientation of deposition to its current nearly vertical orientation on the road cut.

Kura Tawhiti Conservation Area

The above photograph was taken at the Kura Tawhiti Conservation Area also along State Route 73 just east of the Southern Alps. The photo shows a rock formation that is known as a tor. A tor forms when a continuous layer of rock is eroded and weathered, leaving the most resistant rocks protruding from the landscape. Tors are usually made of granite or weather resistant metamorphic rocks. What makes this rock outcrop so interesting is that it is made out of limestone, which is particularly soft and susceptible to chemical weathering.

This rock started as a continuous layer of sedimentary limestone. The layer became exposed during the last mountain building event and was chemically and freeze-thaw weathered in the rounded/scooped manner characteristic of limestone. This rock outcrop must be relatively young for two reasons. First, limestone is a soft rock that is susceptible to chemical weathering and would be completely eroded away if it were exposed for too long. Second, glacial activity during the last glacial maximum would have destroyed this susceptible surface formation. Therefore, is must be younger than about 10,000-12,000 years old. The rock outcrop is now a popular attraction for tourism and bouldering (type of rock climbing characterized by short, technical climbs without a rope).