**1. Introduction**

Global Geoparks authorized by the United Nations Educational, Scientific and Cultural Organization (UNESCO) have expanded in number since inception in 2000 to more than 160 units in 45 different countries. Geology in one form or another is the educational focus of such parks, but with an emphasis on geotourism in support of conservation and the socioeconomic development of rural areas [1]. Remote by nature, many islands possess features of extraordinary significance worthy of development as geoparks. As an example, Iceland is iconic for its status as an island that straddles an expanding oceanic ridge. The island boasts of two UNESCO geoparks, but the potential for several additional parks is anticipated by local planners [2]. In Norway, the Trollfjell (Troll Mountain) Geopark is an example of a well-organized geopark in a remote part of that country near the Arctic Circle that includes many coastal islands with small communities. Stunning coastal scenery that enfolds world-class aspects of geology in the Trollfjell Geopark also includes elements of regional folklore that relate the landscape to its human occupation. Established in 2010, the National Norwegian Geological Monument on Leka Island within the greater geopark boosts the program beyond that envisioned by UNESCO [3]. The monument not only abets an increase in commerce through geotourism but also attracts working geologists and geomorphologists who otherwise may not have known about the

place. The dynamics foster a feed-back loop, through which a steady increase in knowledge adds to the overall significance of geoheritage. The present contribution on a unique boulder beach and the interpretation of its hydrodynamics is offered in that spirit.

The aim of this paper is to review unusual physical traits along one of the monument's well-marked trails at Støypet near the north shore of Leka Island. In Norwegian, the word *støypet* may be translated as "the foundry" in reference to the rare igneous rocks and chromite ore concentrated at that place [4,5], although there is no evidence that mineral extraction and smelting took place any time since human occupation began in the earliest Holocene. The unique aspect of Støypet as a geomorphologic and cultural site is the accumulations of a boulder beach dominated by "rolling stones" eroded from adjacent rocky shores composed of dunite and chromite. These ultramafic rocks originated in the deepest part of the earth's crust near the discontinuity with the upper mantle and register high values of specific gravity that give them greater mass. The implication is that wave heights affecting coastal surge were sufficiently powerful to remove joint-bound blocks from sea cliffs that resulted in unusually dense boulders. The same mathematical equations for estimation of wave heights as applied previously to other shores with boulders derived from more common source rocks of lesser mass, including limestone, rhyolite, andesite, and basalt [6–9], are newly applied at Støypet. In addition, the present-day steering winds and wave dynamics characteristic of Norway's Arctic Circle region [10] are reviewed in the context of prominent storms during recent decades in order to appraise the likely direction of wave impact responsible for the boulder beach at Støypet.

#### **2. Geographical and Geological Setting**

Open to the Norwegian Sea off the coast of northern Norway, Leka Island lies south of the Arctic Circle within the Trollfjell Geopark (Figure 1a). Together with smaller Madsøya, the two islands combine for a total area of 57 km<sup>2</sup> (Figure 1b), and support a population of about 500 inhabitants. From the mainland, the island is accessible by ferryboat across a 4-km wide strait. Norway's National Geological Monument offers an extensive system of hiking trails that are well laid out and include educational trail-side markers with texts in Norwegian, English, and German. Several themes combine to make the park an attractive experience, including local folklore. The island's most prominent landmark is a monolith at the side of Lekamøyhammaren Mountain (Figure 1b), said to embody a troll maiden. The monolith (Figure 2a) is a sea stack composed of the igneous rock gabbro, now isolated inland by postglacial rebound at an elevation 100 m above present-day sea level. Other effects of coastal erosion are evident around the sea stack, where former sea cliffs show deep wear in the form of surge channels cut and polished in gabbro basement rocks (Figure 2b). Detailed work by Høgaas and Sveia (2015) that covers the inner (southeastern) part of Leka Island outlines slightly earlier shore erosion by ice scour during the Younger Dryas interval of 12.8 to 11.5 thousand years ago [11].

Geologically, Leka Island is renowned for exposures of the igneous rocks dunite and harzburgite. These are attributed to formation around the boundary between the Earth's lower crust and upper mantle, known as the Mohorovicic Discontinuity [4]. Such rocks are accessible in few other places around the world where their occurrence is justly celebrated, most notably in Canada's Gros Morne National Park in western Newfoundland [12]. Enrichment in minerals belonging to the platinum group, including gold, platinum, and chromium, also occurs as a dense chromite ore associated with dunite on Leka Island [5]. Brought to the surface by tectonic events, these parent rocks were locally subject to Holocene coastal erosion. Thus, today's Leka Island represents a rare convergence of factors related to the geology of deep ocean crust and the geomorphology of recent marine coastal erosion.

On the island's north shore (Figure 1c), access to a Holocene valley crosses a seam of banded chromite framed by outcrops of dunite (Figure 3) that is described by geopark signage. It is noticeable that the chromite bands at this locality are dissected by joints and fractures typical of exfoliation under subaerial conditions comparable to those in former sea cliffs elsewhere in Støypet valley.

**Figure 1.** The west coast of Norway and Leka Island: (**a**) Norway, showing latitude of the Arctic Circle and region of the Trollfjell Geopark, (**b**) Leka Island and Madsøya with details on highland peaks and location of the Leka maiden monolith (asterisk), and (**c**) inset showing location the Støypet study site.

**Figure 2.** The raised shoreline now about 100 m above sea level at the base of Lekamøyhammaren Mountain (see Figure 1b): (**a**) Gabbro sea stack attributed by legend to the stony embodiment of the Leka troll maiden (author for scale) and (**b**) wave-polished surge channel eroded in gabbro close by the sea stack (figure for scale).

**Figure 3.** Rough exfoliation of chromite bands (dark rocks) and dunite (pale rocks) at the northeast (NE) end of Støypet valley 250 m from the island ring road on the access trail.
