the view upslope where the face landforms are obviously displayed. The flat is the nearly horizontal soil surface just downslope of the face. Sometimes a very thin calcite crust (“tufa”) remains accreted to the face element of the landform. The berm and face expose light-beige-colored limestone, whereas the flat is covered by darker reddish-brown aeolian soil with short xerophytic shrubs. Hand lens observation of the soil of the flat reveals abundant fine volcanic shards reminding us of the volcanics of San Francisco Peaks. Aeolian sorting, not direct air fall, appears to be the dominant process impacting the upper surface of the flat. There is pronounced color and texture contrast between the rocky, often tufa-covered, face and adjacent soil-covered flat. Landform lineation is impressive on highresolution satellite photos and low-altitude aerial photos because of the color and texture contrast. That texture and color contrast can be obscured if tufa-covered limestone clasts are strewn over the surface of the flat. Rarely, cross sectional views of the flat show coarse gravel overlying an erosional terrace underlying the flat. That gravel fill is likely a significant volume of the flat as can be seen from the borrow-pit excavation of the flat by Interstate Highway construction engineers (Figure 9). Typical spacing between berms is 20 m (+/- 5) over the ground. Height of the landform between adjacent berms averages 0.4 (+/- 0.2) meter. Figure 19 is a typical view of landforms occurring in steplike terraces ascending the limestone slope. These three landform elements can be deflected by topographic irregularities as if a shoreline of a lake rose over the curved surface of limestone forming coves and points. INTERPRETATION -- DIP SLOPE DEPOSITIONAL TERRACE MODEL The word “shoreline” can be defined as a narrow erosional and depositional zone formed where the earth’s exposed surface meets a body of water. The terrace, composed of a front slope (the “riser”) and a flatter elongate step (the “tread”), is the familiar shoreline landform (Bates and Jackson, 1984). Shorelines can be described as belonging to one of two types dominated by either erosion or deposition. Most familiar are high relief erosional shorelines that make classic impact on human perception. There are also less impressive low relief depositional shorelines. Further characterization recognizes the time sequence of shoreline landforms produced whether regression or transgression. Thus, shorelines can be classified into four types (inspired by Helland-Hansen and Martinsen, 1996): 1. High-relief erosional shorelines. a. Regressive erosional terrace. b. Transgressive erosional terrace. 2. Low-relief depositional shorelines. a. Regressive depositional terrace. b. Transgressive depositional terrace. Figure 20. The “transgressive depositional terrace” model for Hopi Lake shoreline landforms. The high Lake Bonneville shoreline model of Chen and Maloof (2017) is applied to the Wagon Box Draw terraces of Hopi Lake. Here we depict two successive terraces have been inscribed by rising water on a limestone dip slope possessing resistant and friable beds. These two terraces are about 40 meters wide through the slope but only 0.5 meter high. Shorelines S-1, S-2, S-3, and S-4 mark succeeding water levels during the rapid lake transgression over the limestone slope. Transgression causes milling of the limestone surface and leaves behind subsequent erosional and depositional landforms. Note block diagram employs extreme vertical exaggeration (>25 times actual) to bring landscape elements into horizontal proximity. AUSTIN, HOLROYD, FOLKS, AND LOPER Shoreline Transgressive Terraces 2023 ICC 357
RkJQdWJsaXNoZXIy MTM4ODY=