How I Take Field Notes on Clastic Dikes

A page from one of my field books showing what data and information I collect at each outcrop. Twenty-one dikes were exposed at this site near Lewiston, ID. The largest was 50cm wide. I spent about 20 minutes at this site, scrambling around on the somewhat steep, loose slope at the base of the vertical exposure.

Field notes for a site that lacks clastic dikes. The Marengo railcut is located along the Columbia Plateau Trail route east of Lind, WA. Ancient flood deposits, calcic paleosols, and loess atop CRB bedrock are well exposed here. Marengo is an interesting site, in part because it lies at the margin of scabland scour, more fully developed to the west. Floodwaters here were vigorous, but the channels were not deep. Water coursed through low hills, many of which still preserve their loess mantles and mature soil profiles (i.e., advanced stage calcretes). Over time, floodwaters became focused along main routes and incised them, effectively simplifying the channel network. New floods quickly became confined to a system of bedrock gorges and coulees, each successive flood deepening the route. But here at Marengo, we get a glimpse into the ancient landscape, one swept by a few floods, but one that was not deeply incised or stripped to scabland. Here, loess and flood deposits exist together and interfinger. Places like Marengo suggest the Ice Age floodway evolved over time. Early floods followed a distributed network through an intact maze of loess hills. Later floods followed fewer routes once channels incised the bedrock.

Sheeted clastic dike intrudes a small-offset fault in Touchet Beds at Lewiston, ID. Note the downward-pinching spurs that branch from the dike at several points. This outcrop, located at Hells Gate Park along the Snake River, reveals the character of these structures as well as any in the region.

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