The interplay between climate, sea-levels and erosion carved in Baltic rauks - complicated love story from Gotland

Gotland and its sister island Faro are one of the hidden gems of Baltic Sea rocky coastal landscapes. The most intriguing, both in terms of morphology and evolution, landforms found along local coasts are rauks or raukars. Those limestone (remnants of Silurian reefs) stacks and pinnacles not only tower above present-day rocky shore platforms and beaches but also appear across numerous uplifted marine terraces and inland slopes. Rauks in often occur in groups or fields called raukfält. Surprisingly, those peculiar landforms have been to some extent neglected by geomorphologists and awaited in-depth study of their micro-relief and potential of the well-preserved marine notches to reconstruct past sea-level and environmental conditions operating on rocky coasts over the Holocene. It is important to note that marine notches are commonly treated as one of the best sea-level indicators, as they form close to the local tidal range.

In our paper, we present the results of the most recent mapping of raukars fields in key localities in northern Gotland and Faro including Langhammars, Digerhuvud, Gamla Hamn, Lergrav and Asundens. We measured the shape and elevation of marine notches above present-sea level using Terrestrial Laser Scanning and characterized the degree of weathering of rock surface of raukars bases and well-preserved notches using Schmidt hammer rock tests. In addition to morphometric and geotechnical tests, we monitored modern rates of downwearing of shore platform constitution base of rauks and tops of the local cliffs and rocky escarpments. We discuss the results of selected field surveys with well-established climatic reconstructions and the recently constructed Holocene shore displacement curves of Gotland. We hypothesize that the shape of detected notches not only indicate positions of former sea-levels but also could give us an insight into the differences in the efficiency of erosional and weathering processes operating on rauk surface (notch formation) during periods of different salinity and water temperature, characteristic for Holocene evolution of Baltic Sea.

This is the contribution to the National Science Centre in Poland project ‘RAUK- forgotten witness of Holocene sea-level changes and development of Baltic rocky coasts’ (UMO-2016/21/D/ST10/01976).