Using Rapids cast on 52(64)72 sts and divide them onto four needles, 16(20)22 sts on needles I and IV and 10(12)14 sts on needles II and III. The beginning of round is between needles I and IV. Work twisted ribbing in the round for 3(3)4 cm.
Join Lichen and work 4 rounds in stockinette st.
Join Rapids and work the first colourwork stripe: alternate 1 st Lichen with 1 st Rapids for 6(8)8 rounds. Use larger needles for the colourwork sections if needed. Then knit 6(8)8 rounds using Lichen. Alternate colourwork stripes [= 6(8)8 rounds] and stockinette st [= 6(8)8 rounds] until you have completed 4(5)5 colourwork stripes. After the last one, work stockinette st using Lichen.
Note: When the leg measures 8(15)15 cm, begin decreases on needles I and IV. Decrease round: k1, k2tog, work to last 3 sts, skp (= slip 1 knitwise, k1, pass slipped st over), k1. Repeat the decreases once more every 8 rounds and 4(6)6 times every 4 rounds. 10(12)14 sts on each needle. Keep striping until the leg measures 24(34)35 cm.
Begin heel flap: using Lichen knit the sts on needle I onto needle IV [= 20(24)28 sts]. Leave the other sts on hold. Turn work and begin reinforced pattern using Rapids:
Row 1: (WS) slip 1 with yarn in back, purl to end. Turn work.
Row 2: (RS) *slip 1 with yarn in back, k1*, repeat *–* to end. Turn work.
Work rows 1–2 10(12)13 times in total [= 20(24)26 rows].
Work the WS row once more and begin turning the heel: continue the reinforced pattern as established. Work the RS row until 7(9)10 sts remain. Skp, turn work. Slip 1, p6(6)8, p2tog, turn work. Slip 1, work to last 6(8)9 sts, skp. Continue in this manner, decreasing at the ends with 8(8)10 sts in the middle. When only the middle sts remain, divide the sts onto two needles, 4(4)5 sts each. Knit the 4(4)5 right-hand sts. You are now at the midpoint of the heel flap (= beginning of round).
Using needle I pick up 10(12)14 sts from the left end of the heel flap + 1 st from between the flap and needle II. Using Lichen knit the 4(4)5 heel flap sts and knit the picked-up sts through the back loop. Knit the sts on needles II and III. Using the needle with 4(4)5 sts pick up 10(12)14 sts from the right end + 1 st from between needle III and the heel flap. Knit the picked-up sts through the back loop and k4(4)5.
With these 50(58)68 sts work stockinette st and begin gusset decreases: at the end of needle I, k2tog; at the beginning of needle IV, skp. Repeat the decreases on every 2nd round until 10(12)14 sts remain on each needle.
Note: After you have completed the gusset decreases knit two colourwork stripes with a Lichen stripe in between. Then use Lichen for the rest of the sock.
Continue until the sole measures 15(19)25 cm or the sock covers your fifth toe.
Begin toe decreases: At the end of needles I and III, k2tog, k1; at the beginning of needles II and IV, k1, skp. Decrease as established on every 2nd round until 5(6)7 sts remain on each needle. Then work the decreases on every round.
When 8 sts remain, break yarn, pull it through the sts and securely weave in.
Lightly steam the socks.