STOP 15: Morphology and internal structure of a ribbed moraine and drumlin remnant in the environs of Baldone, Central Latvian Lowland

Abstract

Mašēni and Aizvējas are two sand pits located on a composite angular-shaped hummock with a relative height of up to 40 m and an extent of approximately 1.5 × 1.7 km (Fig. 15.1). The NE part resembles a remnant of a drumlin, oriented parallel to the regional ice flow direction (NNW-SSE). The Aizvējas sand pit (24°20'12"E, 56°46'17"N) is located on this drumlin-like ridge, whereas the Mašēni sand pit (24°20'19"E, 56°45'32"N) is located on the ribbed moraine ridge. The overall morphology of the Zemgale ribbed moraines is characterized by two sets of ridges that are perpendicular to each other, caused by ribbed moraine superimposition on drumlins (Zelčs 1993; Zelčs and Dreimanis 1998), but more complex landforms are found in the NE part of the Zemgale ribbed moraine field in the environs of Baldone (Lamsters and Ošs 2012). The Zemgale ribbed moraine area lies in the Upmale hummocky plain between the Iecava drumlin field in the west and the marginal formations of the North Lithuanian glacial phase, the so-called Valle ridge (Āboltiņš 1970), in the east. A number of ribbed moraines are also found on the Ropaži Plain, west of the Madliena drumlin field (Lamsters 2012). In the Upmale hummocky plain the bedrock surface is composed of Upper Devonian Frasnian sedimentary rocks. Beneath the ribbed moraine under consideration the bedrock consists of dolomite and marl with interlayers of clay. Gypsum layers are also found closer to Baldone (Naumenko and Jankins 1999). In general the bedrock surface lowers in the W and NW direction from 40 m to -5 m a.s.l. The Pleistocene cover is on average from a few metres up to 20 m thick, although it is thicker in some individual hummocks in the surroundings of Baldone, reaching a maximum of 45 m.