*3.1. Data Collection*

Leka Island was visited in July 2019, when the original data for this study were collected from an unconsolidated beach deposit dominated by chromite cobbles and boulders at Støypet. Individual clasts from three stations each limited to collection within a 2-m span were measured manually in three dimensions perpendicular to one another (long, intermediate, and short axes). The stations are confined to a southwest (SW) to northeast (NE) trending valley that contains the Støypet beach deposit. Differentiated from cobbles, the base definition for a boulder adapted in this exercise is that of Wentworth (1922) for an erosional clast equal or greater than 256 mm in diameter [13]. No upper limit for this category is defined in the geological literature [14]. Triangular plots were employed to show variations in clast shape, following the design of Sneed and Folk (1958) for river pebbles [15]. Comparative data on maximum cobble and boulder dimensions were fitted to bar graphs to show size variations in the long and short axes from one sample to the next. Fracture patterns in chromite layers exposed along the valley floor and walls can be examined in two dimensions only, but roughly rectangular outlines can be appraised for a comparison between the original source rocks and range of three-dimensional clast sizes found in the beach deposit. A chromite cobble from the top of Støypet was collected for laboratory determination of density in order to yield weight and volume assigned as a function of equal displacement when submerged in a beaker of water. The commercial "Tour Map" for Leka from 2017 [16] was scanned and adapted in preparation of a detailed base map representing Støypet topography around the study site close to the north shore of Leka Island (see Figure 1c).

#### *3.2. Hydraulic Model*

Dependent on the calculation of density for low-grade chromite, a hydraulic model may be applied to predict the force needed to remove cobbles and boulders from a rocky shoreline with joint-bound blocks as a function of wave impact. Chromite is an igneous rock that forms in the deepest part of the Earth's crust with variable thicknesses due to zonal banding. These factors mitigate the size and shape of blocks loosened by storm waves once the crustal rocks are brought to the surface. Herein, two formulas were applied to estimate the size of storm waves against joint-bound blocks derived, respectively, from Equation (36) in the work of Nott [17] and from an alternative approach using the velocity equations of Nandasena et al. (2011) [18] as applied by Pepe et al. (2018) [19]:

$$Hs = \frac{\left(\frac{\rho\_s - \rho\_w}{\rho\_W}\right)a}{\mathcal{C}\_l} \tag{1}$$

$$H\_{\rm S} = \frac{2 \cdot \left(\frac{\rho\_{\rm s} - \rho\_w}{\rho\_w}\right) \cdot c \cdot (\cos \theta + \mu\_s \times \sin \theta)}{\mathcal{C}\_I} / 100 \tag{2}$$

where *Hs* = height of the storm wave in meters at breaking point, ρ*<sup>s</sup>* = density of the boulder (tons/m<sup>3</sup> or g/cm3), ρ*<sup>w</sup>* = density of water at 1.02 g/mL, *a* = length of boulder on the long axis in cm, θ is the angle of the bed slope at the pre-transport location (1◦ for joint-bounded boulders), μ*<sup>s</sup>* is the coefficient of static friction (= 0.7), *C*<sup>l</sup> is the lift coefficient (= 0.178), and *c* is length of boulder on the short axis in cm. Equation (1) is sensitive only to the length of a boulder on the long axis, whereas Equation (2) is more sensitive to the length of a boulder on the short axis. Therefore, some differences are expected in the estimates of *HS*. It is noted that Equation (2) (above) was shown incorrectly in a previous paper dealing with basalt boulder beds from Santa Maria Island in the Azores [9], although the accompanying calculations were performed according to the proper formula.

#### **4. Results**

#### *4.1. Base Map*

The base map for the study site at Støypet defines a narrow valley that crosses a topographic saddle between prominent highlands at Steinstind and Hagafjellet, respectively, 190 and 345 m above present-day sea level (Figure 4). The valley is accessed from two endpoints on the Leka Island ring road and follows a well-marked geopark trail for a distance of 2 km. At the topographic saddle between Steinstind and Hagafjellet, the NE to SW trending valley is 50 m wide at an elevation just under 100 m above present sea level (Figure 5a).

The park trail leading from the NE trailhead (Figure 4) climbs a smooth gradient through the deposit to the top located in mid-valley (Figure 5). The view to the northeast across the slope includes the enclosing valley walls with interbedded chromite–dunite layers (Figure 6). Based on horizontal distance in proportion to vertical rise, the slope from the NE direction amounts to 5◦. The slope on the opposite side that descends to the SW is similar in vertical drop over horizontal distance, but is broken by a series of cobble-boulder ridges that make it difficult to project a simple gradient.

**Figure 4.** Topographic base map for the study area and access trail at Støypet in the National Norwegian Geological Monument on Leka Island. The shaded area near the center represents the limits of an unconsolidated cobble/boulder deposit at the pass between Steinstind and Hagafjellet. Black dots mark the location of three sample sites. See Figure 1c for orientation with respect to the rest of the island. See Figure 1c for location on Leka Island.

**Figure 5.** Details at the top of Støypet within a southwest to northeast (SW-NE) trending valley: (**a**) view to the northwest across the cobble-boulder field perpendicular to the trend of the valley and (**b**) close-up of cobbles and boulders dominated by chromite (darker rocks) with the clast at the center (marked by tape-measure case) having a diameter of 26 cm across the long axis.

**Figure 6.** View from the mid-valley beach deposit over the slope descending to the NE.
