Dataset: Fine Scale Substrate Characteristics of the Pacific Coast from June of 2022 to July of 2023

Quadrats 

** We used half meter quadrats locate fine scale sampling locations.*** Quadrats were made by creating a half-meter square of PVC pipe and threading the pipes with nylon rope to create a grid every 10 cm. Samples were taken for each rock category within the quadrat. 

Rock Strength 
We quantified the strength (friability) of intertidal rock substrates using a mechanical sclerometer configured as a scratch tester. This approach resembles Mohs hardness testing, however instead of switching between materials of varying hardness, we used a constant scratching tool—a 1 mm diameter tungsten carbide tip—and increased the applied force until the tip produced a visible scratch. For each trial, we aimed to create a scratch approximately 2 cm in length, although surface conditions sometimes required shorter passes. We repeated each test at progressively higher forces to identify the minimum force necessary to initiate a scratch.

We measured friability on a fine scale at 14 sites along the Pacific coastline of California and Washington (see map). Sites were selected based on three criteria: accessibility, presence on geologic maps, and prior sampling of marine community composition, which allowed us to link our measurements with regional monitoring programs (aka MARINe). At each site, we targeted mid-intertidal bench habitats with low to moderate slopes and sampled during low tide to maximize surface dryness. We identified fine scale sampling locations using half-meter length (0.25m^2) quadrats. Quadrats were placed in a "T" shape, with the coordinates of the site at the intersection of the T (see T Shaped Survey Drawing and Quadrat Organization Diagram). The T shape was orthogonal to the orientation of the approaching waves - in other words the cross bar was parallel to shore while the stem was pointing towards the waves. There were four quadrats on each spoke - four to the left side of the T, four on the stem, and four on the right side of the T. Each quadrat was surveyed for visible rock categories. Each identified rock category was sampled three times (where possible) to obtain an average strength value for that rock category in that quadrat.  

In the field, we assigned each surface to a broad rock categories. Although we consulted geologic maps when available, these maps typically describe formations above the high-tide line and often fail to resolve intertidal substrates, which can differ substantially and are frequently labeled as undifferentiated “bedrock.” To maintain consistency, we classified rocks into the following categories: black, striated black, grey, green, striated green, conglomerate, metamorphic, sandstone, granodiorite, mudstone, siltstone, embedded cobblestone, and rounded cobble. Rocks were then described in more detail in the notes. 

Analog Sclerometer
We used the Elcometer 3092 analog Sclerometer Hardness Tester in this study. This instrument is designed to evaluate the hardness of manufactured coatings and surfaces, such as those applied to automobile bodies for protection. As an inspection device, the sclerometer specifically measures the force required to scratch or abrade a surface. The instrument resembles a large pen and determines coating hardness by drawing a tungsten carbide tip across the surface under a controlled, pre-set load.

To operate the sclerometer, we first set a target force by compressing an internal spring to a predetermined length. As we increased compression, the applied force increased accordingly (F_spring = k_stiffness × x_compression). A tightening screw attached to the collar held the spring in place. We then tested the selected force by pressing the tip against a surface and moving it in a short, straight pass (~2 cm over 1 s), simulating a scratch. We followed an iterative procedure, adjusting the applied force and repeating passes until we identified the minimum force (N) required to produce visible abrasion.

In addition to the pen-like sclerometer body, the instrument includes a case, a 0.75-mm diameter tungsten carbide tip, and three interchangeable springs—grey, red, and blue—with stiffness values corresponding to force ranges of 0 to 3 N, 10 N, and 20 N, respectively. For harder materials, the system also offers an optional green spring rated from 0 to 30 N. The instrument supports additional tips, including 0.5- and 1.0-mm diameter tungsten carbide tips and a diamond-capped tip. Larger diameter tips apply lower pressure, while the diamond-capped tip enables testing of extremely hard surfaces, such as crystalline materials.