THE IN FLU ENCE OF SUB-QUA TER NARY BASE MENT ON THE RE LIEF IN THE EAST ERN PART OF THE POM ER A NIAN LAKELAND, POLAND

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1 Annales Societatis Geologorum Poloniae (2011), vol. 81: THE IN FLU ENCE OF SUB-QUA TER NARY BASE MENT ON THE RE LIEF IN THE EAST ERN PART OF THE POM ER A NIAN LAKELAND, POLAND Krzysztof PETELSKI In sti tute of Ge og ra phy, Pom er a nian Acad emy in S³upsk, Partyzantów 27, S³upsk, Po land, kpetelski@box43.pl Petelski, K., The in flu ence of sub-qua ter nary base ment on the re lief in the east ern part of the Pom er a nian Lakeland, Po land. Annales Societatis Geologorum Poloniae, 81: Abstract: Geo log i cal map ping works, con ducted on 1:50,000 litho-petrographic map sheets of the South Bal tic coast and the east ern part of the Pomerania Lakeland, show a clear re la tion ship be tween large land forms of this area, like: subglacial chan nels, Wie yca Hill, the ice lobe that ac cu mu lated Gardno mo raine de pos its, and sub-qua ter nary re lief. These land forms came into ex is tence as a re sult of re ac ti va tion of pre-ex ist ing fault zones in the sub-caino zo ic base ment, due to the ice mass load ing dur ing suc ces sive Pleis to cene ice-sheet ad vances. Subglacial chan nels de vel oped and thick tills and ice-dammed lake de pos its were ac cu mu lated in the re gion. The subglacial chan nels and morainic belts are now dom i nat ing land forms in the Pom er a nian Lakeland. Key words: re lief of the top of sub-qua ter nary de pos its, subglacial chan nels, Kaszuby Lakeland, Bytów Lakeland, Gardno Low land, Po land. Manu script re ceived 15 February 2010, ac cepted 28 April, 2011 INTRODUCTION Geo log i cal map ping works, con ducted by the au thor for over thirty years in the South Bal tic coast and the east ern part of the Pom er a nian Lakeland, from the u³awy Wiœlane bor der to Bytów, S³upia River val ley, S³upsk and Lake Gardno (Fig. 1), made it pos si ble to draw some con clu sions about the li thol ogy of Qua ter nary sed i ments. They show a clear re la tion ship be tween the pres ent-day to pog ra phy and sub-qua ter nary re lief of this area. My hy poth e sis is that the sub-qua ter nary re lief is not the one in her ited from Neo gene and preglacial times, but it formed dur ing the suc ces sive Pleistocene ice-sheet advances. RESEARCH METHODS I con ducted geo log i cal map ping works on 12 sheets of the 1:50,000 De tailed Geo log i cal Map of Po land (DGMP) to examine the geological architecture of Quaternary sedi - ments (Petelski, 1998, 1999, 2000a, b, 2001, 2002, 2003a,b, 2005, 2006, 2008, 2009). Geo log i cal struc ture of Qua ter - nary de pos its was stud ied by spe cial re search bore holes. The or i gin of gla cial de pos its and stra tig ra phy of tills was in ter preted based on lithological and petrographic in ves ti - ga tions (Lisicki, 2003). Anal y sis of the bore holes drilled for the DGMP and all ar chi val bore holes ever drilled in this area en abled for the con struc tion of re lief maps of the top of sub-quaternary strata. Geophysical resistivity surveys were per formed along geo log i cal cross-sec tions. They con firmed the pres ence of sub-qua ter nary land forms de scribed in the pa per. Zones of glaciotectonic dis tur bances in the study area were iden ti fied and ana lysed as well. GEOLOGICAL SETTING OF THE SUB-QUA TER NARY BASE MENT VS. THE RE LIEF OF THE EAST ERN PART OF THE POMERANIAN LAKELAND The east ern part of the Pom er a nian Lakeland, span ning the Kaszuby and Bytów lakelands, is a highly el e vated area (over 200 m a.s.l.) of morainic pla teaus with some cul mi na - tions at tain ing m a.s.l. There oc curs the high est point of the Eu ro pean Low lands Wie yca (329 m a.s.l.). The sur faces of the morainic pla teaus are com posed of tills. The area is transected by four sys tems of large subglacial chan nels; from the east these are: the D¹browskie and Patulskie lakes val ley, Raduñskie lakes val ley, Gowidliñskie Lake val ley and Jasieñ Lake val ley. A few tens of metres high prom i nent es carp ments bound the val leys. The val leys bot toms or wa ter lev els of the val leys lakes lie m lower than the sur faces of the sur round ing morainic pla - teaus. Tak ing into ac count the depths of the lakes, the dif - fer ences in el e va tion be tween the pla teau sur face and the val ley bot tom range from 80 to 100 m.

2 106 K. PETELSKI Fig. 1. Lo ca tion of ar eas dis cussed in the pa per: 1 lo ca tion of the sheets of the De tailed Geo log i cal Map of Po land, scale 1:50,000, pre pared by the au thor, 2 lo ca tion of geo log i cal cross-sections, 3 lo ca tion of lake val leys pre sented in the pa per, 4 lo ca tion of in di vid - ual fig ures The morainic pla teaus of the Kaszuby and Bytów lakelands de scend south wards to the el e va tion of m a.s.l. They ad join here a zone of outwash plains de pos ited dur ing the Pom er a nian Phase of the Vistulian (Weichselian) Gla ci ation. To wards the north, the pla teau ar eas de scend and pass into coastal morainic pla teaus of Damnica and arnowiec. The coastal pla teaus are sit u ated at much lower el e va - tions (100 m a.s.l.) than the morainic ar eas of the lakelands. The morainic pla teaus are com monly re ferred to as lakeland ridges. This term is jus ti fied not only by the high el e va tions of these ar eas but also by their subsurface geo - log i cal struc ture. The sub-qua ter nary bed rock of this re gion lies at higher el e va tions than it does in the ar eas sit u ated to the south and north of the lakeland ridge. The top of Mio - cene strata that un der lie the Qua ter nary suc ces sion in the lakeland ridge is lo cated at el e va tions rang ing from 100 m a.s.l. in the Bytów Lakeland to m a.s.l. in the Kaszuby Lakeland (Fig. 2). The top of Mio cene strata de - scends north wards. In the South Bal tic coast, it lies at el e va - tions rang ing from 20 m b.s.l. to 60 m b.s.l. To the south of the lakeland ridge, in the outwash plains area, the top of Mio cene strata de scends to m a.s.l. In pre vi ous pa pers (Zaborski, 1933; Oko³owicz, 1956; Piasecki, 1958, 1962; Pachucki, 1961; Sylwestrzak, 1961, 1972, 1973, 1978; Galon, 1967, 1968; Roszko, 1968; Beniuszys, 1968; Szukalski, 1968; Augustowski, 1969) con - cern ing the re lief and geo log i cal struc ture of the east ern part of the Pom er a nian Lakeland, li thol ogy of Qua ter nary sed i - ments and their re la tion ship to the subsurface geo log i cal struc ture of the area was ne glected. The ex cep tions are the works de voted to the Gdañsk re gion, pre sented by Mojski (1979, 2002), who in di cated a clear re la tion ship be tween the pres ent-day topography of the area and the re lief and geo log i cal struc ture of Neo gene strata.

3 THE IN FLU ENCE OF SUB-QUA TER NARY BASE MENT ON THE RE LIEF 107 Fig. 2. Re lief of the top of sub-qua ter nary strata in the Kaszuby and Bytów lakelands; lo ca tion of geo log i cal cross-sec tions from Fig - ures 3 and 4: 1 edges in re lief of the top of sub-qua ter nary strata, M Mio cene sands, Ol Oligocene sands and silts, E Eocene sands Geo log i cal works con ducted by my self on 12 sheets of the DGMP re vealed a re la tion ship be tween large subglacial chan nels and the sub-qua ter nary base ment re lief. The Raduñskie Lake tun nel val ley and the D¹browskie and Patulskie lake tun nel val ley (Fig. 3) oc cur in the area where the deep Raduñska and Patulska subglacial chan nels (fault-bounded val leys) are in cised in the top of the Mio - cene. The val leys bot toms are sit u ated at m b.s.l. el e va tion and lie m lower than the sur round ing ar - eas where the top of the Mio cene is lo cated at 60 m a.s.l. The trend of these subglacial chan nels and their geo log i cal struc ture is doc u mented by 30 ar chi val bore holes and re - search bore holes drilled for the pur poses of the DGMP. The Qua ter nary sec tion drilled in the Patulska val ley is over 308 m thick (a bore hole at Szymbark, sheet Egiertowo of the DGMP). The bot toms of both val leys are lined with de pos its of the old est (Narewian; Menapian) gla ci ation. The Raduñska val ley is filled with gla cial de pos its, in clud ing ice-dammed lake silts and clays, Narewian Menapian, Sanian Elsterian 2, Odranian Saalian 1 and Wartanian Saalian 2 till ho ri zons, and glaciofluvial sands. Flu vial sed i ments of the Lubawa (Saalian 1/Saalian 2) Inter gla cial com prise only a 4 m-thick se ries of the en tire 266-m-thick depositional succession filling the valley. The Patulska val ley, in con trast to the Raduñska val ley, is filled mainly by very thick tills of the Odranian (Saalian 1) Gla ci ation (56.0 m) and Wartanian (Saalian 2) Gla ci - ation (in clud ing Lower Stadial (76.0 m) and Mid dle Stadial (20.0 m)), as well as by glaciofluvial sands. Flu vial sed i - ments in this 308-m-thick sec tion are merely 8.0 m in thick - ness, and are rep re sented by the Mazovian (Holsteinian) Interglacial sands. The fill ing of both the val leys proves that they were sites of subglacial de po si tion through out the en tire Qua ter - nary pe riod. No signs of ero sion is ob served in the val leys bot toms, thus they were not river val leys dur ing inter gla cial pe ri ods. Deep val leys cut into the sub-qua ter nary base ment formed and de vel oped in the Qua ter nary. Dur ing Neo gene and preglacial (Early Pleis to cene) times, vari a tions in the re lief of the top of Mio cene strata were the same as to day. Such high vari a tions in the el e va tion of the top of the Mio - cene would have had to be re duced by gla cial ero sion and ac cu mu la tion. At the site of the pres ent-day deep Raduñska and Patulska val leys, there were prob a bly shal low top o - graphic de pres sions, most likely river val leys. Dur ing the Narewian (Menapian) ice-sheet ad vance, the val leys be - came filled with tills and glaciofluvial sands. The cov er ing of the area by the ad vanc ing ice sheet re - sulted in re ac ti va tion of pre-ex ist ing fault zones within the sub-ce no zoic base ment. Af ter the ice re treat, iso static re - bound of the Earth s crust oc curred in the area that was pre vi - ously de formed due to ice load ing. The up lift con tin ued with dif fer ent speed along the pre-ex ist ing fault zones, re sult ing in the for ma tion of a horst. The am pli tude of ver ti cal move - ments along the fault lines at tained a few tens of metres. The same sit u a tion oc curred dur ing the suc ces sive ice-sheet ad vances. Al ways at the same sites, along the fault

4 108 K. PETELSKI Fig. 3. Geo log i cal cross-sec tion through the Raduñskie lakes val ley and the D¹browskie and Patulskie lakes val ley. Cr Cre ta ceous, Ol Oligocene, M Mio cene; Glaciations: A Narevian (Menapian), N Nidanian (Elsterian 1), S Sanian 1 (Elsterian 2), G Sanian 2 (Elsterian 2), O Odranian (Saalian 1), W Wartanian (Saalian 2), B Vistulian (Weichselian); 1 peat, 2 sands, 3 clays, 4 tills, 5 Neo gene and Palaeogene sands, 6 Cre ta ceous chalk and marl zones, subglacial chan nels de vel oped dur ing the fol low ing glaciations. Thick till se ries were de pos ited in these val leys, which were sub se quently de formed by glaciotectonic pro - cesses. The trend of the val leys co in cides with that of photolineaments iden ti fied by Ba yñski et al. (1984) and Graniczny et al. (1987, 1995) in di cat ing that these land - forms are as so ci ated with the tec tonic struc ture of the base - ment.the Wie yca Hill is lo cated upon a fault zone at the bound ary be tween the Patulska val ley and the area of the up lifted top of Mio cene strata (up to 60 m a.s.l. in el e va tion; Fig. 3). The hill is a large cre vasse-fill landform at tain ing the el e va tion of over 250 m a.s.l., whose in di vid ual peaks ex ceed 300 m a.s.l. and the sum mit is at 329 m a.s.l. (the high est hill of the Eu ro pean Low lands). The thick ness of sand-gravel-boul der sed i ments com pos ing the Wie yca Hill ex ceeds 53 m. The for ma tion of such a large cre vasse-fill landform was pos si ble ow ing to its lo ca tion upon a fault zone where a set of cre vasses de vel oped in the ice sheet when its front was to the south of the study area and the ice mass was rel a - tively thick. To the east of Wie yca, the NE SW-trending Marszewo val ley is sit u ated. It is deeply in cised (60 70 m) into the sur round ing Przywidz Pla teau that rises to the el e va tion of over 200 m a.s.l. To the east, be tween Kolbudy and Czerniewo, the pla teau is bor dered by a high (70 80 m) edge slop ing to wards the u³awy Wiœlane Low land. The trend of the Marszewo val ley and the pla teau-bound ing edge is closely re lated to the an cient land forms ob served in the sub-quaternary basement. The N S-trending an cient edge is 100 m high. It sep a - rates the area of el e vated top of Mio cene strata (40 m a.s.l.) in the Kaszuby Lakeland from the area where the top of the Mio cene is sit u ated at the el e va tion be low 60 m b.s.l. in the u³awy Wiœlane re gion. At the foot of the edge, there is a thick com plex (60 m) of Narewian (Menapian), Nidanian (Elsterian 1), Sanian 1 and Sanian 2 (Elsterian 2) tills. The an cient Marszewo val ley is in cised to a depth of 80 m be low the top of Mio cene strata that lies at 40 m a.s.l. (Fig. 4). Its bot tom is lined with the Augustovian (Bavelian s.l.) Inter - gla cial flu vial sands and grav els over lain by Nidanian Gla - ci ation de pos its (tills interlayered by ice-dammed lake sed i - ments, 53 m in thick ness). The val leys of the Raduñskie lakes and D¹browskie and Patulskie lakes, as well as the Marszewo val ley and the edge of the Kaszuby Lakeland be tween Kolbudy and Czerniewo clearly fol low the di rec tion of pre-ex ist ing land forms ob - served in the sub-qua ter nary base ment. Sed i ments of the old est gla ci ation (Menapian) and South-Pol ish (Elsterian) glaciations of all these land forms fill the val leys (re lief de - pres sions) in the top of Neo gene strata. In the ar eas where the top of sub-qua ter nary base ment is sit u ated at high el e va tions, sed i ments of the old est and South-Polish glaciations are absent. Glacial deposition started here with the Mid dle-pol ish (Saalian) Glatiation (Odranian and Wartanian) sed i ments that smoothed the rough re lief of older gla ci ation pe ri ods. Geo log i cal crosssec tions through the Raduñskie, D¹browskie and Patulskie lake val leys (Fig. 3), the Marszewo val ley, and the Kaszuby Lakeland be tween Kolbudy and Czerniewo (Fig. 4) dif fer in the num ber of iden ti fied till ho ri zons, which was proved based on litho-petrographic in ves ti ga tions con ducted for the DGMP. A sim i lar type of geo log i cal struc ture is ob served in the east ern part of the Bytów Lakeland where the course of the Jasieñ Lake val ley is re lated to the trend of an an cient val ley ob served in the sub-qua ter nary base ment. A more in ter est - ing is sue is the prob lem of the bound ary be tween the Odra and Vistula ice lobes of the Pom er a nian Phase of the Main Stadial of the Vistulian (Weichselian) Gla ci ation. The bound ary is drawn across the Bawernica morainic pla teau ris ing above 200 m a.s.l. (Keilhack, 1898; Oko³owicz,

5 THE IN FLU ENCE OF SUB-QUA TER NARY BASE MENT ON THE RE LIEF 109 Fig. 4. Geo log i cal cross-sec tion through the Marszewo val ley. For ex pla na tions see Fig ; Sylwestrzak, 1972; Petelski, 1998), where in di vid ual peaks ex ceed 225 m a.s.l. The Bawernica pla teau co in cides with the area where the top of Neo gene strata lies at very high el e va tions (over 100 m a.s.l.). An cient subglacial chan - nels oc cur on both sides of the el e vated sub-qua ter nary bed rock. Their bot toms are at depths of m a.s.l. These an cient chan nels are as so ci ated with large subglacial chan - nels ob served at the sur face: the Jasieñ Lake subglacial channel run ning to the west of the Bawernica morainic pla - teau, and the Gowidliñskie Lake subglacial chan nels oc cur - ring to the east of the pla teau. In the cen tral part of the Bytów Lakeland, north of Miastko, the morainic pla teau is dis sected by deep val leys and its el e va tion ex ceeds 200 m a.s.l. The val ley bot toms lie m be low the pla teau sur face. The courses of the val - leys co in cide with the trends of an cient val leys in the sub-quaternary basement (Petelski et al., 2009). The an cient val leys of the Bytów Lakeland, like those in the Kaszuby Lakeland, are filled with sed i ments of the old est gla ci ation and the South Pol ish glaciations. No de - pos its of these ages have been found in ar eas of the el e vated top of Neo gene strata. Sed i ments of the Odranian, Wartanian and Vistulian glaciations cover both the study area and the an cient de pres sions in the sub-qua ter nary sur face, smooth ing the rough re lief of the sub-qua ter nary bed rock. SOUTH BAL TIC COASTLAND I con ducted geo log i cal in ves ti ga tions in the Gardno Low land, Reda eba ice-mar ginal val ley and arnowiec Lake re gion of the Bal tic coastland (Petelski, 1975, 1976, 1978, 1985, 1990; Petelski & Jurys, 1989). Since the be gin ning of the 20th cen tury, many dis cus - sions have been made on the for ma tion of the Reda eba ice-mar ginal val ley. How ever, Petelski and Sadurski (1987a, b, 1988) in di cated a re la tion ship be tween these land forms and an cient val leys that ex isted in the present-day ice-mar - ginal val ley through out the en tire Qua ter nary pe riod. They also pointed out to the re la tion ship be tween these land forms and the arnowiec Lake tun nel val ley, which was a subgla cial chan nel dur ing the Qua ter nary. The for ma tion and sig nif i cance of the arnowiec Lake tun nel val ley at the site where a fault cuts the Palaeozoic base ment have been con - firmed by many au thors (cf. Dadlez, 1990; Znosko, 1998). A dif fer ent sit u a tion ex isted in the Gardno Low land where one of the best-doc u mented lobal pat terns in Po land ex ists. Al though the area was de scribed al ready in the 20th cen tury lit er a ture (Borne, 1857; Jentzsch, 1914; Bülow, 1924, 1930, 1932; Hartnack, 1926; Halicki, 1947; Giedrojæ- Juraha, 1949; Rosa, 1963, 1964, 1968; Bartkowski, 1965; Roszko, 1968; Sylwestrzak, 1972, 1973, 1979; Petelski, 1975, 1976, 1978, 1985; Petelski & Jurys, 1989; Mojski & Or³owski, 1978; Tobolski, 1984, 1989; Rotnicki, 1987, 1994, 1999; Rotnicki & Borówka, 1990, 1994; Wojciechowski, 1988, 1990; Florek & Or³owski, 1991; Dobrzyñski et al., 1991; Jasiewicz, 1998, 1999; Czerniawska, 1999, 2004), re con struc tion of a rel a tively de tailed im age of both the li thol ogy of the Qua ter nary se ries and the re lief of the top of Neo gene strata was pos si ble in only (Petelski, 2005, 2006; Fig. 5). Geo log i cal stud ies car ried out in this area over the last years have shown a clear re la tion ship be tween the re lief of the area and the re lief of the sub-qua - ternary basement. The dom i nant landform of the sub-qua ter nary base - ment is an ex ten sive val ley. It be gins south of S³upsk and then runs across S³upsk to W³ynkówko, where it turns northeastwards to Smo³dzino and then to wards the north be - neath the Bal tic Sea floor. The bot tom of the val ley is un - even; it slopes at dif fer ent an gles. In S³upsk, it is lo cated at a depth of more than 140 m b.s.l. (Fig. 6), be tween W³ynkówko and Smo³dzino be low 120 m b.s.l. (Fig. 7), whereas north of Smo³dzino it reaches a depth of 257 m b.s.l. The width of the landform var ies be tween 2 and 3 km.

6 110 K. PETELSKI Fig. 5. Re lief of sub-qua ter nary strata in the Gardno Low land and S³upsk re gion; lo ca tion of geo log i cal cross-sec tions from Fig - ures 6 and 7. 1 geo log i cal cross-sec tion lines, 2 edges of morainic pla teau, 3 morainic pla teau, 4 Gardno fron tal mo - raines, 5 edges in re lief of the top of sub-qua ter nary strata The val ley is sur rounded by ar eas where the top of Neo gene strata is sit u ated at el e va tions of 0 to 20 m a.s.l. The only re - gion where the top of Neo gene strata oc curs at lower el e va - tions (40 60 m b.s.l.) is the north ern part of the study area. The ba sic el e ments of the pres ent-day re lief of the study area co in cide with the trend of both the an cient de pres sions ob served in the top of the Neo gene and the ac com pa ny ing glaciotectonic de for ma tion zones. The po si tion of the ice lobe that ac cu mu lated Gardno mo raine de pos its was con - trolled by a de pres sion in the top of Neo gene strata, which ac com pa nies the above-men tioned val ley. The course of the north ern edge of the Damnica morainic pla teau co in cides with the trend of the south ern edge of an an cient val ley run - ning be tween W³ynkówko and Smo³dzino. The subglacial chan nel, con tain ing the S³upia River be tween Kawkowo and W³ynkówko, runs along an an cient val ley ob served in the top of Neo gene strata. As in the case of the Kaszuby and Bytów lakelands where the an cient val leys are filled with sed i ments of the old est gla ci ation and the South-Pol ish glaciations, the val - ley run ning from S³upsk through W³ynkówko and Smo³dzino, and far ther north wards un der the Bal tic Sea floor, is filled with Narewian, Nidanian, Sanian 1 and Sanian 2 de - pos its. No flu vial sed i ments have been found here. In the val ley sec tion that crosses the S³upsk re gion, the sed i ments are rep re sented by Narewian, Sanian 1 and Sanian 2 tills inter- bed ded by glaciofluvial sands and grav els. They are strongly glaciotectonically de formed and rep re sent an ex - am ple of the val ley-side glaciotectonics (Dadlez & Jaroszewski, 1994). Be tween W³ynkówko and Smo³dzino, the val ley is filled with a thick (88 m) com plex of till ho ri zons (also strongly glaciotectonically de formed) de pos ited dur - ing the Narewian, Nidanian, Sanian and Odranian glaciations, with no in ter ven ing sed i ments ob served. To the north of Smo³dzino, the bot tom of the val ley is at 263 m b.s.l el e - va tion. It is filled with thick till se ries of the fol low ing glaciations: Narewian (72 m), Nidanian (24 m) and Sanian (59 m). The tills are sep a rated by glaciofluvial sands, whose thick ness is small as com pared to the till thick ness. In the area out side the an cient val ley, where the top of the Neo - gene is at higher el e va tions, the gla cial suc ces sion com - mences with the Odranian or Wartanian sed i ments which lev elled the vari able re lief of the top of the Neo gene. Geo - log i cal cross-sec tions pre sented in Fig ures 6 and 7 dif fer in the num ber of iden ti fied tills, as doc u mented by litho-petrographic in ves ti ga tions of the DGMP. Geo log i cal struc ture and tec tonic set ting of the sub-qua ter nary base ment were thor oughly ex plored in the South Bal tic area by geo phys i cal meth ods in clud ing high-res o lu tion seis mic sur veys (Kramarska et al., 1999; Krzywiec et al., 2003). There is a clear re la tion ship be tween the Cre ta ceous Caino zo ic lithologies and the ori en ta tion of fault zones in the Palaeozoic base - ment. There fore, the oc cur rence of glaciotectonic de for ma - tion zones in the S³upsk re gion (Figs 6, 7) is sup posed to be con strained by the block-type tec ton ics of the Cre ta ceous succession.

7 THE IN FLU ENCE OF SUB-QUA TER NARY BASE MENT ON THE RE LIEF 111 Fig. 6. Geo log i cal cross-sec tion through the Damnica Pla teau and S³upia River val ley. For ex pla na tions see Fig. 3 Fig. 7. Geo log i cal cross-sec tion through the Gardno Low land and Damnica Pla teau. For ex pla na tions see Fig. 3 CONCLUSIONS The or i gin of the an cient val leys from the South Bal tic coastland and Kaszuby and Bytów lakelands is the same and con sists in re ac ti va tion of fault zones pre-ex ist ing in the sub-caino zo ic base ment due to the ice mass load ing dur ing suc ces sive ice-sheet ad vances. These ar eas were sub se - quently the sites of de vel op ment of subglacial chan nels where thick till ho ri zons and ice-dammed lake sed i ments accumulated. The trend of the val leys co in cides with photolineaments iden ti fied by Ba yñski et al. (1984), in di cat ing that these land forms are re lated to the tec tonic struc ture of the base - ment. Ac cord ing to Dadlez (1976, 1989, 1990) and Znosko (1998), the arnowiec Lake tun nel val ley is lo cated on a fault zone within the Palaeozoic basement.

8 112 K. PETELSKI The Gardno val ley, in par tic u lar its north ern sec tion whose bot tom is at 263 m b.s.l. el e va tion, can be com pared with an cient val leys de scribed from the floor of the South Bal tic Sea (Kramarska et al., 1999) where high-res o lu tion re flec tion seis mic data re vealed their re la tion to the fault zones of the Palaeozoic basement. Iso static move ments of the Earth s crust along the re ju - ve nated fault zones caused re ac ti va tion of the pre-ex ist ing subglacial channels during successive Pleistocene glaciations. It is worth not ing, how ever, that the pro cess was more in tense in Mid-Pleis to cene times, prob a bly due to neotectonic move ments of the Kuyavian and Mazovian phases (Baraniecka, 1975) that were su per im posed on the iso static crustal move ments. The vari a tions in the geo log i cal struc - ture style (dis con tin u ous de for ma tion in the Raduñskie lake tun nel val ley, D¹browskie and Patulskie lake tun nel val ley, and in the zone of strong glaciotectonic de for ma tion in the S³upsk tun nel val ley; quiet de po si tion in the Marszewo tun - nel val ley) were caused by vari able in ten sity of iso static crustal move ments in the study area. Large and deep tun nel val leys, filled with strongly de formed sed i ments, developed in areas of intense vertical crustal motions. All the subglacial chan nels are filled with sed i ments of the old est gla ci ation and the South-Pol ish glaciations. These sed i ments are ab sent from the sur round ing ar eas where the top of Neo gene strata was be ing up lifted and the sed i ments have been re moved. The Up per Pleis to cene de pos its cover the whole of the study area, lev el ling dif fer en ti ated re lief of the top of Neogene strata. The bound ary be tween the Odra and Vistula ice lobes of the Pom er a nian Phase of the Main Stadial of the Vistulian Gla ci ation runs across the Bawernica morainic pla teau. This is an area of highly el e vated top of Neo gene strata (above 100 m a.s.l.). The up lift ing move ments of this area fa voured sep a ra tion of the ice into two ice lobes: the Odra and Vistula lobes. The in flu ence of the re lief of the sub-qua ter nary base - ment on the pres ent-day to pog ra phy of this area is much stronger than pre vi ously thought. The lo ca tions of large subglacial chan nels and high top o graphic edges, the po si - tion of the high est cul mi na tion of the Pom er a nian Lakeland (Wie yca) and the bound ary be tween the Odra and Vistula ice lobes are all re lated to the re lief of the top of Neo gene strata. The re lief formed as a re sult of ver ti cal move ments of the Earth s crust during the Pleistocene. 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