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

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 at 38 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. For general surveys, we performed at least three scratch tests per rock type. For more detailed surveys, we increased sampling effort to a minimum of twelve measurements per rock type.

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.