Minecraft crashed twice

Minecraft crashed twice in the same place on the flat map type Tunellers dream. Can you tell me why is this hapenning? I am bulding a long track there to meet achievement. When I a certain place so me MC always crashes. Here are crash reports:
1.

---- Minecraft Crash Report ----
// On the bright side, I bought you a teddy bear!

Time: 3.11.12 18:10
Description: Exception in server tick loop

java.lang.OutOfMemoryError: GC overhead limit exceeded
	at java.lang.Long.valueOf(Unknown Source)
	at adw.a(SourceFile:18)
	at aaa.a(SourceFile:29)
	at zx.d(SourceFile:102)
	at id.c(ChunkProviderServer.java:104)
	at ic.<init>(PlayerInstance.java:26)
	at ib.a(PlayerManager.java:102)
	at ib.d(PlayerManager.java:258)
	at ge.d(SourceFile:147)
	at im.a(NetServerHandler.java:343)
	at dt.a(SourceFile:136)
	at bw.b(SourceFile:55)
	at im.d(NetServerHandler.java:78)
	at in.b(SourceFile:35)
	at bce.b(SourceFile:83)
	at net.minecraft.server.MinecraftServer.r(SourceFile:731)
	at net.minecraft.server.MinecraftServer.q(SourceFile:661)
	at bcb.q(IntegratedServer.java:124)
	at net.minecraft.server.MinecraftServer.run(SourceFile:595)
	at fp.run(SourceFile:818)

Relevant Details:
- Minecraft Version: 1.4.2
- Operating System: Windows 7 (amd64) version 6.1
- Java Version: 1.7.0_09, Oracle Corporation
- Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
- Memory: 119457704 bytes (113 MB) / 954466304 bytes (910 MB) up to 954466304 bytes (910 MB)
- JVM Flags: 2 total; -Xms512m -Xmx1024m
- AABB Pool Size: 2451 (137256 bytes; 0 MB) allocated, 2061 (115416 bytes; 0 MB) used
- ModLoader: Mods loaded: 2
ModLoader 1.4.2
mod_ReiMinimap v3.2_05 [1.4.2]

- Is Modded: Very likely
- Profiler Position: N/A (disabled)
- Vec3 Pool Size: 473 (26488 bytes; 0 MB) allocated, 428 (23968 bytes; 0 MB) used
- Player Count: 1 / 8; [ih['jpelda'/594350, l='New World', x=-817,13, y=237,00, z=0,35]]
- World New World Entities: 0 total; []
- World New World Players: 0 total; []
- World New World Chunk Stats: ServerChunkCache: 0 Drop: 0
- Type: Integrated Server

2.

---- Minecraft Crash Report ----
// Why did you do that?

Time: 3.11.12 18:31
Description: Exception in server tick loop

java.lang.OutOfMemoryError: Java heap space
	at dl.<init>(SourceFile:64)
	at ih.j_(SourceFile:147)
	at xe.a(SourceFile:1218)
	at ie.a(SourceFile:424)
	at xe.g(SourceFile:1197)
	at xe.h(SourceFile:1122)
	at ie.h(SourceFile:352)
	at net.minecraft.server.MinecraftServer.r(SourceFile:718)
	at net.minecraft.server.MinecraftServer.q(SourceFile:661)
	at bcb.q(IntegratedServer.java:124)
	at net.minecraft.server.MinecraftServer.run(SourceFile:595)
	at fp.run(SourceFile:818)

Relevant Details:
- Minecraft Version: 1.4.2
- Operating System: Windows 7 (amd64) version 6.1
- Java Version: 1.7.0_09, Oracle Corporation
- Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
- Memory: 119721504 bytes (114 MB) / 952172544 bytes (908 MB) up to 954466304 bytes (910 MB)
- JVM Flags: 2 total; -Xms512m -Xmx1024m
- AABB Pool Size: 1886 (105616 bytes; 0 MB) allocated, 242 (13552 bytes; 0 MB) used
- ModLoader: Mods loaded: 2
ModLoader 1.4.2
mod_ReiMinimap v3.2_05 [1.4.2]

- Is Modded: Very likely
- Profiler Position: N/A (disabled)
- Vec3 Pool Size: 337 (18872 bytes; 0 MB) allocated, 17 (952 bytes; 0 MB) used
- Player Count: 1 / 8; [ih['jpelda'/118033, l='New World', x=-647,82, y=238,00, z=0,57]]
- World New World Entities: 0 total; []
- World New World Players: 0 total; []
- World New World Chunk Stats: ServerChunkCache: 0 Drop: 0
- Type: Integrated Server

I am from the Czech republic, so the translation may not be completely correct. Thank you in advance for your answer.